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Genetics AND Evolution A- Level Notes BY AKM
Human biology (Cavendish University Uganda)
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GENETICS, VARIATIONS, POPULATION GENETICS AND
EVOLUTION
SECTION 1:
GENETICS
Genetics is the study of heredity and variations among organisms. Heredity/inheritance is
transmission of characteristics from parents to offspring (from one generation to the next).
Genetics accounts for the stability of inheritance (e.g why you are a human as are your parents)
and variations between offspring from one generation to the next.
TASK: Explain the importance of the knowledge of genetics in today’s society (at least six points
COMMON TERMS USED IN GENETICS
Gene; A gene is the basic unit of inheritance that determines the organisms’ characteristics.
Alleles; Are alternative forms of the same gene responsible for determining contrasting
characteristics. Most genes have two alleles but some have more. Usually, one allele has more
power of expression in the phenotype. For study purposes, genes are represented by letters (e.g
A, a can be alleles for normal skin colour)
Gene locus; Is the position of a gene/an allele on a DNA molecule.
Dominant gene/allele; Is a gene/allele which expresses itself in the phenotype of an organism
even in the presence of an alternative allele. A dominant allele is always represented by a capital
letter when performing a genetic cross.
Recessive gene/allele; is a gene/allele which only influences the appearance of the phenotype
only in presence of another identical allele (i.e if it exists in a homozygous recessive genotype).
A recessive allele is always represented by a small letter in a genetic cross.
Genotype; is the genetic makeup/constitution of an organism with respect to the gene in
consideration. With respect to a particular gene, there are only three possible genotypes:
Homozygous genotype; one where the two alleles at a gene locus are identical. e.g.
TT, AA, tt, aa. This can be in one of the two forms;
i)
Homozygous dominant genotype; where both alleles at a gene locus are
dominant (AA, TT)
ii)
Homozygous recessive genotype; where both alleles at a gene locus are
recessive (aa,tt)
iii)
Heterozygous genotype; a condition in which the two alleles at a gene locus
are different. E.g. Tt, Aa, Rr etc.
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Phenotype; Refers to the observable characteristics of an organism (usually resulting from
interaction of the genotype and the environment in which it leaves)
Pure breeding (true breeding); where organisms produces the same phenotype generation after
generation (Note that this only possible if the individuals being crossed are homozygous for that
particular characteristic).
Test cross is a cross between an organism showing a phenotype of a recessive allele and an
organism showing a phenotype of a dominant gene. The purpose of which is to distinguish
between the homozygous dominant genotype and a heterozygous genotype since they lead to the
same phenotype.
Back cross. This is the mating of an offspring with a recessive phenotype with one of its parents
so as to identify the genotype of the parent.
Linked-genes; these are genes carried on the same chromosome
Sex-linked genes; these are genes determining other characteristics other than sex but are carried
on sex chromosomes. E.g the gene for colourblindness, gene for haemophilia etc
Sex-limited genes; these are genes which determine characteristics found in only one sex e.g e.g
the gene for growth of breasts (limited to females), beards(limited to males) etc. Note that these
genes are found in both sexes but are expressed in only one sex. They remain turned off in the
other sex.
Multiple alleles; these are more than two forms of the same allele determining the same
characteristic.
Although Mendel had no knowledge of genes and chromosomes, his experiments showed that
inheritance is particulate
MENDEL’S GENETIC EXPERIMENTS AND
MONOHYBRID INHERITANCE
Mendel started his work on inheritance by studying the inheritance of just one pair of contrasting
characteristic. This is called called monohybrid inheritance.
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DFN: Monohybrid inheritance is the inheritance of a single pair of contrasting characteristics.
His experimental organism was a garden pea plant called scientifically called Pisum sativum.
Mendel chose the garden pea plant for his genetic experiments because of the following reasons:





The plants were easy to cultivate
All their offspring were fertile
They have a short life cycle that they reproduced so quickly
The plants also had many contrasting characters with no intermediates
Their reproductive structures were enclosed in petals which allowed for production of
pure breeding plants due to self-pollination over many generations
Reasons for Mendel’s success in his genetic experiments:
i.
ii.
iii.
iv.
He carried out preliminary investigations to become familiar with the experimental
organism
He carefully planned for all his experiments and he focused on only ne variable at a
time. This simplified his observations and led to proper conclusions
He was much carefully in carrying out experiments hence eliminating ambiguity of
results
He kept accurate records for all his experiments
MENDEL’S EXPERIMENTS
I
n one of his experiments, Mendel crossed a pure breeding tall pea plants with a pure
breeding dwarf pea plant. The resulting seeds were planted and gave rise to tall plants. No
short plants were produced in this generation which was called the first filial generation
(F1 generation).
Mendel then selfed (self-pollinated) theF1 tall plants. To his surprise, the F2 generation was a
mixture of tall and dwarf pea plants. He counted and worked out the ratio of the tall plants to the
dwarf plants which was approximately 3:1.
When Mendel carried out similar monohybrid crosses, he obtained the following results:
Character
investigated
Form of seeds
Cross
Colour of cotyledon
Colour of seed coat
Form of pods
Number
of
F2
offspring
Smooth vs wrinkled
5474 smooth, 1850
wrinkled
Yellow vs green
6022 yellow, 2001
green
Grey-brown vs white 705 grey-brown, 224
white
Inflated
vs 882 inflated, 299
Ratio
2.96: 1
3.01:1
3.15:1
2.95:1
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Position of flowers
constricted
Axial vs terminal
Length of stem
Long vs short
constricted
651
axial,
207 3.14:1
terminal
787long, 277 short
2.84:1
 The table shows that the phenotypic ratio of the F2 offspring is approximately 3:1 for all
the monohybrid crosses that Mendel carried out. It is called Mendel’s F2 monohybrid
phenotypic ratio.
 In all his crosses, Mendel found out that one of the contrasting features of a pair was not
represented in the F1 generation, but the feature reappeared in the F2 generation where it
was consistently outnumbered by 3 to 1 by the contrasting feature.
Significance of Mendel’s findings in the above monohybrid crosses




Since the original parents were pure breeding, each parental phenotype must have been
due to two factors.
The F1 generation possessed one factor from each parent which were carried by the
gametes.
The F1 offspring were not intermediate between the two parental phenotypes which
shows that there was no blending/mixing of the features (factors).
It also shows that as only one of the factor expressed itself in the F1, this factor was
dominant to the other (which is said to be recessive). In the cross of the tall and dwarf pea
plants, tallness is dominant to dwarfism.
In interpreting his results, Mendel concluded that organisms’ characteristics were due to internal
factors which were passed from one generation to the next via gametes. Each parent possess two
factors but only one factor can be carried in each gamete. On the basis of this, Mendel
formulated his first law of inheritance, called the law /principal of segregation which states that:
The characteristics of an organism are determined by internal factors which occur in pairs but
only one factor can be represented in each gamete.
Modern explanation of Mendel’s first law in terms of meiosis.
 What Mendel called internal factors are today called genes.
During prophase I, homologous chromosomes associate; and consequently align as
bivalents at the metaphase plate/equator position during metaphase I; During anaphase
I, the homologous chromosomes separate; and move with their alleles to opposite poles;
Subsequent cell division results into formation of two gamete cells each containing one of
the two alleles of each gene; therefore, the alleles occur in pairs on homologous
Illustration:
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Genetic representation of the Monohybrid cross
By convention, the initial letter of the dominant characteristic is used as the symbol for the gene
and its capital form represents the dominant allele while the lower case represents the recessive
allele.
For example: the full genetic explanation of Mendel’s cross between a tall and a dwarf pea plant
is shown below:
Let
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Genotypic ratio
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: All Tt,
Phenotypic ratio
: All tall
To obtain F2 generation, F1 hybrids were selfed as shown below
Genotypic ratios: 1TT : 2Tt : 1tt;
Phenotypic ratios:
3tall : 1 Short
WORKED EXAMPLES
1. In a garden pea plant there are two forms of seed shape, roundvseed shape and wrinkled seed
shape. When a pure breeding round seed plant was crossed with a a wrinkled sees pea plant,
all the offsprings obtained where had round seeds. When the offsprings were selfed, both
round seed and wrinkled seed offspring were obtained.
a.
b. generation
c. What are the phenotypic and genotypic ratios of the F2 generation
d. Explain how you would determine the genotype of F1 tall pea plants formed
e. Suppose 700 pea plants where produced in the F2 generation
i.
How many were tall?
ii.
How many were short?
2. Suppose a man who is a tongue roller marries a woman who is a non-tongue roller and all the
children obtained in F1 are tongue rollers.
(a) Work out the phenotypic and genotypic ratio as obtained in F2 generation.
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(b) What is the probability that the 4th born is a non-tongue roller?
DIHYBRID INHERITENCE
(NOTES WERE ACCIDENTALLY DELETED)
Mendel’s laws are in agreement with the events of MEIOSIS
Mendel’s second law can be explained/accounted for on the chromosomal basis by meiosis.
The genes concerned in dihybrid inheritance are carried on different chromosomes e.g, in the
example above, the alleles of the gene for tallness are located on one pair of chromosomes and
the alleles of the gene for flower colour are also located on a different pair. During metaphase I,
homologous chromosomes line up sided by side before separating but they do so in such a way
that the allele of one pair position themselves entirely independent of the positioning/orientation
of alleles of any other pair i.e the arrangement of the different pairs of chromosomes is
completely random. Subsequent separation during anaphase I leads to a variety of allele
combinations in gametes (genetic diversity).
Illustrations: (notes book)
Dihybrid (second law)
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NB: For the haploid number of chromosomes = n, the total number of possible combinations in
gametes is given by 2n
Independent assortment can be demonstrated in other organisms like the fruit fly (Drosophila). In
Drosophila, wing length can be long or vestigial and body colour can be grey or ebony. The grey
colour is dominant to the ebony colour and long wing light is dominant over vestigial. A cross
between grey bodied fly possessing long wings and an ebony bodied fly with vestigial wings
gives only grey bodied long winged F1 offspring. Selfing these F1 flies yields four types of flies:
Long-winged- grey-bodied
Long winged- ebony bodied
Vestigial winged-grey bodied
Vestigial winged-ebony bodied.
These occur in a ratio of 9:3:3:1.
QN: Using genetic crosses, explain the above results
NOTE:
The conclusion that can be made is that the genes determining wing length and body colour are
located on different chromsomes; their alleles assort and separate independently of each other
during metaphase I and anaphase I respectively
EXERCISE
Explain three advantages of using Drosophila in modern genetic experiments (DG Mackean pg
613)
Summarize Mendel’s hypotheses (ref. BS pg 813)
Give a comparison between (similarities) between the events occurring during meiosis and
fertilization and Mendel’s hypothesis. Ref BS pg 814
The dihybrid test cross
Just like in monohybrid inheritance, organisms with the recessive characteristics must be
homozygous for that condition, but organism showing the dominant characteristic may be either
homozygous or heterozygous. A test cross can be done between the organisms showing the
recessive characteristic and one showing the dominant characteristic. If the offspring produce a
1:1:1:1 phenotypic ratio, then the unknown genotype is a heterozygote.
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Trial question.
To establish the genotype of a male long-winged grey bodied fruit fly, the fly was crossed with a
vestigial winged ebony bodied female fly. The following results were obtained:
Long-winged- grey-bodied-25%
Long winged- ebony bodied-25%
Vestigial winged-grey bodied-25%
Vestigial winged-ebony bodied-25%
Carry out genetic crosses to deduce the genotype of the male fruit fly.
SPACE
EXCEPTIONS TO MENDEL’S LAWS (NON-MENDELIAN INHERITENCE PATTERNS)
-
Mendel only studied characteristic determined by single genes that has two alleles, Some
of which was dominant and the other recessive. The following phenomena deviate from
Mendel’s patternsof inheritance:
INCOMPLETE DOMINANCE AND CODOMINANCE
GENE/ALLELE LINKAGE
MULTIPLE ALLELES
EPISTASIS
PLEIOTROPY
Lethal alleles
POLYGENIC INHERITENCE (POLYGENIC TRAITS)
1. LINKAGE
This is existence of two or more genes on the same chromosome.
Linked genes are genes carried on the same chromosome. Genes found on the same chromosome
form a linkage group and are expected to pass into
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the same gamete and therefore inherited together. They therefore do not show independent
assortment(i.e do not segregate in accordance to Medel’s
law of independent assortment which is shown by unlinked genes) and do not produce the typical
F2 dihybrid 9:3:3:1 ratio. A variety of rations can be obtained depending on the degree of
linkage.
Examples of incidences of linkage:
1. It is known that genes for flower colour and fruit colour in tomatoes are on the same
chromosome. Plants with yellow flowers bear red fruits (dominant characteristics), those
with white flowers bear yellow fruits (recessive characteristics). If
the two plants are crossed, the results theoretically expected to be as follow:
Let R=allele for red fruit; r= allele for yellow fruit; Y =allele for yellow flowers and
y=allele for white flowers.
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Selfing F1 = F2
The above phenotypic ratio is a typical dihybrid F2 phenotypic ration in case of complete
linkage of the two genes. How ever this ratio is rare/never achieved because total linkage
is rare. Most breeding experiments involving linkage produce some phenotypes
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resembling parental phenotypes in approximately equal proportions and some new
combinations of characteristics. These are called RECOMBINANTs because they are
new combinations of the parental characters. The two or more genes are said to be linked
if recombinants occur less frequently than the parental phenotypes.
For instance, the above cross is done practically, the following results are typical for
example if 100 F2 plants are produced:
Yellow flowers and red fruit -68
Yellow flowers red yellow fruit- 7
White flowers and red fruits – 7
White flowers and yellow fruits- 18
The phenotypic ratio in this case is approximately 10:1:1:3.This is neither typical of a
dihybrid F2 phenotypic ratio in absence of linkake (i.e 9:3:3:1) nor typical of an F2
dihybrid ratio in presence of linkage (i.e 3:1)
Cross (in BS)
Explanation of results
The only possible explanation for failure to obtain the typical F2 dihybrid phenotypic
ratio of 3:1 (in presence of linkage)and formation of recombinants is the occurrence of
crossing over between non-sister chromatids of homologous chromosomes during
prophase I of meiosis. Crossing over involves exchange of section of DNA between nonsister chromatids hence can separate alleles which where originally linked. In this way,
new combinations of genes arise in gamete hence producing recombinant phenotypes;
This explanation was provided by Thomas H. Morgan using his exprimentson
Drosophila. He crossed a pure breeding grey bodied long winged drosophila with a pure
breeding black bodied vestigial winged drosophila, as expected, all the f1 flies were grey
bodied with long wings. Again if the two genes for wing length and body colour to be
considered totally linked, then a 3:1 phenotypic ratio was expected in F2 as shown by the
genetic crosses below:
(students)
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Phenotypic ratio is 3 grey long winged: 1 black vestigial winged.
 However, practically, he never obtained the above ratio indicating that the two genes
were not totally linked.
To confirm his assumption that the genes for wing length and body colour are not completely
linked, Thomas H. Morgan carried out several test crosses between grey bodied-long winged
drosophila heterozygous for both traits with a black vestigial winged drosophila.
Morgan postulated that if the two genes were not linked, they should show independent
assortment (normal Mendelian inheritance) and give phenotypes in a typical dihybrid test cross
ration of 1:1:1:1.
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If genes were completely linked, parental phenotypes would be obtained in a ratio of 1:1 as
shown below.
 MORGAN’s results did not prove any of his assumptions to be correct. He instead
obtained approximately equal numbers of the parental phenotypes with significantly few
recombinant phenotypes also in approximately equal numbers as summarized from the
cross below.
41.5% grey, long winged
41.5% black, vestigial winged
8.5% grey, vestigial winged
8.5% black, long winged
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Morgan explained this results in terms of crossing over during meiosis as illustrated above.
Sample question:
A homozygous purple-flowered short stemmed plant was crossed with a homozygous redflowered long stemmed plant and all the F 1plants had purple flowers and short stems. When the
F1generation was taken through a test cross, the following progeny was produced
53 purple flowered short stemmed
47 purple flowered long stemmed
49Red flowered short stemmed
45 red flowered long stems. Explain the results fully.
Crossing over and cross over values
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As already mentioned, crossing over involves the exchange alleles/ of sections of DNA between
homologous chromosomes. It is a natural form of gene reshuffling and leads to separation of
linked genes and formation/creation of new combinations of alleles/genes in gametes. This
results into production of offspring with combinations of traits that differ from those of their
parents. Such offspring are called recombinants.
Dfns
Genetic recombination/reshuffling is the formation of new gene combinations in gametes as a
result of crossing over of genes during prophase 1.
Recombinants are offspring which have combinations of characteristics that are different from
those of their parents (show new combinations of characteristic)
Qn Explain how genetic recombination increases the amount of genetic variation.
The frequency of genetic recombinationis known as cross over value and is calculated as a
percentage from the expression below:
Example
In a test cross carried out on a grey long
winged drosophila, the following results
were obtained
Phenotype
Number
offsprings
Grey, long winged
965
Black, vestigial
944
Black, long winged 206
Grey,
vestigial 185
winged
Solution:
of
= 17%
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SIGNIFICANCE OF THE CROSS OVER VALUE
The COV also indicates the number of crossovers which have occured during gamete formation.
The cross over value reflects the relative positions of genes on chromosomes. The furthest apart
linked genes are on the chromosome, the greater the cross over value i.e the greater the chances
of crossing over occurring between them). Estimation of the relative positions of genes on
chromosomes facilitates the process of chromosome mapping (locating and positioning genes on
a chromosome.
Example:
the distance separating these four genes is shown below;
P-Q
= 24%
R-P = 14%
R-S = 8%
S-P = 6%
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Draw a line that represents the chromosome and follow the following procedure:
a. Insert the positions of the genes with the smallest cross over value first in the middle of the
chromosome map i.e S-P =6%
b. Examine the next largest cross over value (i.e R-S=8%) and insert both possible positions of
R
c. Repeat the procedure for the next largest COV (i.e R-P= 14%)- note that it is obvious that R
cannot be on the right hand position.
d. Repeat the procedure for the COV for P-Q = 24%. Also note that the relative position of Q
cannot be ascertained so it can be placed on either sides.
Example
In maize, the genes for coloured seed and full seed are dominant to the genes for colourless and
shrunken seed. Pure breeding strains of double dominant variety were crossed with a double
recessive variety and a test cross of the f1 generation produced the following results
Coloured full
380
Colourless shrunken
396
Coloured shrunken
14
Colourless full
10
Calculate the distance between the genes for coloured seed and seed shape
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Sex determination and sex linkage
SEX CHROMOSOMES
The sex chromosomes are called heterosomes because they are non-identical and are designated
X and Y. The X chromosome is rod shaped and much bigger than the Y chromosome which is
hook shaped.
The Y chromosome carries genes responsible for secondary male sex characteristics,
differentiation of testes and development of genital organs in humans. Actually in some
organisms, the Y chromosome is absent and is believed not to carry genes necessary for survival
of the organism and is described as genetically inert.
SEX determination and sex LINKAGE:
Sex chromosomes are chromosomes which carry the gene that determines sex of an organism.
They are an exception to the general rule that homologous chromosomes should be identical in
appearance. The sex chromosome pair is usually different in the two sexes and hence called
heterosomes. Body (determine other characteristics of an organisms other than sex)
chromosomes are identical in appearance in both sexes and are called autosomes. The sex
chromosomes are known an X and Y chromosomes. The genotype of the female in most
organisms including human, is XX and that of the male is XY. This means that females produce
only one type of gamete (homogametic) and males produce two types of gametes
(heterogametic). However, in some organisms like birds, moths and buttterflies, the reverse is
true, the females are XY while the males are XX.in other organisms like grasshoppers, the Y
chromosome is completely absent such that the male genotype is XO.
In production of gametes in humans, all the egg cells have an X chromosome, whereas 50% of
the sperms carry an X chromosome and 50% carry a Y chromosome. Tis means that at
fertilisation, the egg cell may be fertilized by an X sperm, producing producing a zygote with an
XX genotype (female)in the zygote. With the same probability, the egg cell may be fertilized by
a Y sperm producing an XY genotype in the zygote (male. Therefore, sex in this case is
determined by which sperm fertilises the egg i.e by the male organism.
The Y chromosome carrries a gene (testis determining factor-TDF) which controls the
differentiation of testis from the undifferentiated testis. In absence of this gene (y chromosome),
the embryonic gonads develop into ovaries.
A cross showing the determination of sex
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Genotypic ratios: 1XX: 1XY
Phenotypic ratios: 1female: 1male
This shows that there is a 50% chance of any child being a male or female
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Reviosn qn: UNEB 530/2 2017 no. 4.
SEX LINKAGE
This is the inheritance of genes carried on the sex chomosomes and are inherited together with
those determining sex. Sex linked genes are genes carried on the sex (X) chromosome and
determine characteristics which are inherited together with sex. Sex linked traits (characters) are
traits determined by genes carried on sex chromosomes. However, these genes determine body
characteristics and have nothing to do with sex. majority of the sex linked genes are carried on
the X-chromosome. Features linked on the X chromosome may arise in either sex.
Examples of sex-linked characteristics
Inheritance of eye colour in Drosophila
In one of his experiment, Morgan mated a wild type (pure breeding) red-eyed female with a
mutant (white eyed) male. All the F1 hybrids were red eyed. He went on to interbreed the F1
males and females to obtain an F 2 generation which consisted of red eyed and white eyed
offsprings in a ratio of 3:1 respectively. However, all female were red eyed and all the white eyed
flies were males though some males were red eyed.
Explanation; all the F1 were red eyed; implying that this allele is dominant over that for white.
Since in the F2 all the white eyed were males, this indicates that the gene for eye colour is located
on the X chromosome and there is no corresponding locus on the Y chromosome; otherwise
some females would also be white eyed. (the white eye recessive allele is
linked to the X
chromosome and it is absent on the Y chromosome)
Genetic crosses
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Obtaining F2 generation;
Phenotypic ratios: 3 red eyed: 1 white eyed
Phenotypic ratio in terms of sex: 2 red eyed females: 1 red eyed male: 1white eyed.
Note that all the white eyed are males yet some red eyed are males
ASSIGNMENT: Do qn 24.10 BS pg 821
Sex linked traits in humans
They include;
 Inheritance of red-green colour blindness
 Inheritance of haemophilia
 Inheritance of premature balding
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 Inheritence of the gene for muscular dystrophy (DMD)
HAEMOPHILIA (BLEEDERS’ DISEASE)
Haemophilia is a disease/ disorder in which the blood of the affected person has a markedly
reduced ability to clot, due to a deficiency of one of the blood clotting factors.
If untreated, it is results into
 internal bleeding into joints and muscles
 uncontrolled bleeding (haemorrhage in case of injury (even if minor)
It is treated by administering the deficient clotting factors to the victims.
Just like other sex-linked traits, haemophilia is carried on the X chromosome and it is recessive
to the allele for normal blood clotting.
The allele being recessive, haemophiliac females must inherit two copies of the defective allele
while males need only one (since it does not exist on the Y-chromosome). The heterozygous
females show normal blood clotting and are described as carriers. The males lack the alternative
allele and the recessive allele is automatically expressed phenotypically. (males cannot be
carriers and the would be carriers are automatically affected. This explains why the condition is
more prevalent in males than in females. The following are possible genotypes and
corresponding phenotypes that can occur:
For example: the following cross shows the results of a marriage between a normal but carrier
woman and a normal man.
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It can be noted that there is a 50% chance of a daughter being a carrier and a 50% chance of a
son being haemophiliac. Sons can only inherit haemophilia (and other sex linked traits) from
their mothers but not fathers as they only inherit the father’s Y chromosome and not the X
chromosome that carries sex linked genes. Girls can inherit from both parents.
Research qn: Study the pedigree/family tree showing how haemophila arose in royal family of
England (starting queen Victoria (carrier vs Prince Albert (normal). Make some conclusions
about the inheritance of haemophilia in this royal family.
Note: A pedigree chart or family tree is a diagram that shows the occurrence and phenotypes of a
particular gene from ancestors to the next generations.
To analyze a pedigree chart, the dominant and recessive genes must be known first. The symbols
used on the diagram must also be known as indicated below:
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ASSIGNMENT:Number 24.11 BS page 822
RED –GREEN COLOUR BLINDNESS
It is a condition/disorder in which a person cannot distinguish the colours that others see as
green, yellow, orange and red. It is due to a defect in one of the three groups of colour sensitive
cone cells in the retina.it cannot be treated.
The gene that leads to red-green colour blindness is recessive and is carried on the X
chromosome. This means that all males whose X chromosome carries the recessive allele are
affected.
A female can be a carrier but have normal eye vision. For a female to be colour blind, she must
be homozygous for the recessive colur blindness allele. The condition is therefore also more
common in males (affects about 8%) and less common in females (affects only 0.4%).
Example
Using suitable genetic symbols work out the genotypes and phenotypes of children from a
marriage between a normal man and a colour blind woman.
SEX -LIMITED CHARACTERISTICS
These are characteristics which normally occur in one sex althouth the genes controlling them
are found in both sexes. Sex-limited genes are genes that can be carried on any chromosome but
determine characteristics which normally appear in only one sex. They are usually carried on
autosomes (body chromosomes)and their phenotypic expression is largely influenced by the level
of sex hormones in the body. They may be turned on in one sex (by sex hormones), and turned
off in the other( by absence of a certain sex hormone. Sex limited genes are responsible for
secondary sex characteristics as well as primary sex characteristics. They are responsible for
sexual dimorphism (phenotypic differences between males and females)
Examples include;
 Facial hair/ beard development, deep voice in males
 Breast development, lactation, widening of hip bones, high pitched sound in females etc
Note: Appearance of a characteristic that is limited to one sex in the other sex is usually due to
hormonal imbalance.
SEX-INFLUENCED CHARACTERISTICS
These are characteristics which are controlled by genes whose expression depends the sex of
the individual. Although these characteristics can be seen in both sexes, the degree or frequency
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varies according to sex. The gene responsible will show dominance depending on sex meaning
that in one sex, it will only require one for the characteristic to be expressed. However, in the
other sex, the gene shows recessiveness and can only be phenotypically expressed if both alleles
are present.
Examples:
 Baldness: males show this pattern more than females because a male is bald if has only
one allele (responsible for the condition), whereas a woman must receive two alleles to
be bald.
 Length of the index finger; short index finger is due to a gene that is dominant in males
but recessive in females.
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2. Gene interactions
2.1
Degree of dominance
Mendel only studied genes that had two alleles, one of which was completely dominant and the
other recessive. However, this is not always true as illustrated in the cases below:
2.11 Partial dominance
This is when alleles fail to show complete dominance or recessiveness such that in the
heterozygous state, their phenotypes blend (mix) to produce an intermediate phenotype.
For example; When red flowered snapdragon plant is crossed with a white flowered snapdragon
plant, all the F1 hybrids have pink flowers. When these F1 plants are selfed, the original parental
phenotypes (colours) reappear alongside the intermediate colour in a ratio of 1 red: 2
pink:1white.
Note: Since none of the alleles is completely dominant over the other, the both alleles are
represented with capital letters.
F1 phenotypes :
All pink
Interbreeding F1= F2
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Genotypic ratios: 1RR: 2RW: 1WW
Phenotypic ratios: 1 Red: 2pink: 1 White
Other examples of incomplete dominance include:
Characteristic
Mirabilis Japalla (4-oclock plant)
Angora rabbit hair length
Plumage colour in Andalusian fowls
Allelomorphic
characteristic
Red and White
Long and short
Black and splashed white
Heterozygous phenotype
Pink
Intermediate
Blue
Trial question:
In a genetic experiment, a pure breeding black Andalusian male fowl was crossed with a pure
breeding splashed white female. All the F1 offspring where blue feathered. When the F1 fowls
where selfed, the F2 offspring consisted of black, blue and white splashed feather fowls in a ratio
of 1:2:1. Using genetic symbols carry out genetic crosses to explain these observations.
2.12
CODOMINANCE
This is when none of the alleles is dominant over the other(the two alleles have equal powers to
be expressed in the phenotype) and in the heterozygous state, both alleles are expressed but
separately/independently in the phenotype. Note that the phenotypes of both alleles are expressed
in the heterozygote but without blending.
Example;
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During the inheritance of fur/coat colour in short-horned cattle, when red and white cattle are
mated, all the F1 hybrids have white fur thickly interspersed with red fur. This phenotype is
referred to as the roan condition. It is also found in horses, goats, dogs etc
For genetic crosses on codominance, both alleles are given capital letters (since they are equally
dominant) and are raised to capital ‘C’; to
distinguish this pattern of inheritance from
incomplete dominance.
Illustration of a genetic cross between a pure breeding red fur cow and a pure breeding white fur
bull.
F1 generation
Selfing F1 to get F2
Other examples of codominance include;
 the allele for blood group A and that for blood group B are codominant such that, the
heterozygotes are neither blood group A nor blood group B but rather blood group AB.
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 Inheritance of the sickle-cell trait where about 50 percent of the victim’s red blood cells
have normal haemoglobin and also about 50 percent have abnormal haemoglobin.
2.2
MULTIPLE ALLELES
This is when a single characteristic is controlled by three or more alleles of the same gene; but
only two can occupy the gene locus on a homologous chromosome pair.
For example: Inheritance of blood groups;
The gene for human blood group is known to occur in three allelomorphic forms; A, B and o.
Alleles A and B are codominant while o is recessive to both. The three alleles produce six
possible genotypes and four phenotypes as shown below:
Blood group
Possible genotypes
A
IAIo,IAIA
B
IBIo IBIB
AB
IAIB
O
IoIo
The gene locus is represented by I (isohaemagglutinogen) and hence, the alleles are usually
raised to I.
Worked example:
If a father and a mother are both heterozygous for blood group A and B. Show the possible
genotypes of their children. If they bear non-identical twins, what is the probability that both
twins are of blood group A.
Solution
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Offspring phenotypes: Blood groups AB, A, B and O
Probability for a child with blood group A = ¼
Probability for both twins with blood group A = ¼ * ¼ =1/16 = 0.0625
Trial question:
1. Work out the possible blood groups of the offsprings produced if a man of blood group A
marries a woman of blood group AB
2. 1997 pp1 no. 41
2.3 EPISTASIS
This is a form of gene interaction where one gene allele at one locus suppresses the phenotypic
expression of another gene/allele at a different locus. The suppressing gene is referred to as an
epistatic gene (inhibiting gene) while the suppressed gene is called a hypostatic gene.
The genes for presence of fur colour in mice and type of fur colour show epiststatic interaction.
The dominant form of the gene for presence of colour is responsible for coloured fur while its
recessive form results into no colour deposition and the phenotype is white (albino). If colour is
present, the colour type is determined by another gene (at another locus) whose dominant allele
produces grey fur (agouti) while the recessive allele produces black fur. Any of the two colours
can be present only and only if their respective alleles are accompanied by the gene for coloured
fur. Absence of this gene will result into albinos even if the genes for grey or black are present.
The gene for coloured fur is epistatic to the gene responsible for colour type of fur
(hypostatic).This interaction produces three possible phenotypes as summerised below.
Phenotype
Grey (agouti)
Black
Albino (white)
Possible genotypes
CCGG, CCGg, CcGG, CcGg
CCgg, Ccgg
ccGG, ccGg, ccgg
Worked example
Work out a genetic cross to show the phenotypes and the phenotypic ration of a cross between
i)
ii)
A pure breeding grey fur male mouse and a pure breeding albino female
mouse(homozydous for the black allele)
A double heterozygous agouti (grey)male mouse and a double heterozygous
agouti (grey) female mice
Trial question
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In oat plants, the inheritance of color is controlled by the gene with two alleles, the dominant
results into colour formation while the recessive results into no colour formation (white or
albino). The other gene is responsible for the kind of colour, grey being recessive to one for
black. Identify the nature of gene interaction and show the F1 and F2 outcomes starting with true
breeding parental stocks.
F1 phenotypes: All black
Obtaining F2;
Trial qn 2
Ref. Bs qn 24.15 page 825.
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2.6 LETHAL GENES/alleles
A lethal gene/allele is one that causes death of the organisms that carries them usually at
embryonic stage. Lethal alleles may be recessive or dominant. Lethal alleles can cause death of
an organism prenatally (at embryonic stage) or any time after birth, though they commonly
manifest early in development.
An example is clearly illustrated in the inheritance of fur colour in mice. Wild mice are known to
have grey coloured fur (a condition called agouti) or yellow fur. A cross between two
heterozygous yellow mice produces yellow and agouti offspring in a ratio of 2:1 respectively.
The expected 1:2:1 Mendelian genotypic ratio i.e. 1 homozygous yellow :2 heterozygous
yellow :1 homozygous agouti which would lead to a normal phenotypic ratio of 3 yellow :
1agouti is not obtained. The results always give a phenotypic ratio of 2 yellow to 1 agouti.
Further investigations reveal that one of the genotypes that would increase the ratio to 3: 1 causes
death of the offspring at embryonic stage.
The allele for yellow fur is dominant over that for agouti and all living yellow mice are
heterozygous for fur colour. The 2:1 ratio of phenotypes is due to the death of the yellow mice
that are homozygous for yellow fur colour before birth. In this case the allele is lethal when in
homozygous (dominant) state
Phenotypic ration: 2 yellow:1 agoute
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 Another example is a recessive lethal allele that occurs in the Manx cat . Manx cats
possess a heterozygous mutation resulting in a shortened or missing tail. Crosses of two
heterozygous Manx cats result in 2 offspring displaying the heterozygous shortened tail
phenotype, and 1 offspring of normal tail length that is homozygous for a normal allele.
Homozygous offspring for the mutant allele cannot survive birth and are therefore not
seen in these crosses.
 Dominant lethal genes that need only one copy to be present in an organism to be fatal.
These alleles are not commonly found in populations because they usually result in the
death of an organism before it can transmit its lethal allele on to its offspring. An example
in humans of a dominant lethal allele is Huntington's disease, a rare neurodegenerative
disorder that ultimately results in death. A person exhibits Huntington's disease when they
carry a single copy of the dominant copy of the gene.
2.7. Pleiotropy
Is a situation in which one gene affects two or more unrelated phenotypic characteristics. It arises
for example when a gene codes for an enzyme that affects more than one phenotypic
characteristic. e.g cystic fibrosis is due to a pleiotropic gene but its victims also have breathing
and digestion problems.
2.8. The gene complex (two or more genes occurring at different loci controlling a single
characteristic; for a gene complex.
Many characteristics in plants and animals are produced by an interaction of several genes
located on different loci; forming a gene complex. A single characteristic may be produced by
the interaction of two or more genes occurring at different loci. A good example is shown by the
inheritance of comb shape in domestic fowl.
In this case, two genes on different chromosomes (loci) interact to produce four (4) distinct
phenotypes of combs. Pea and rose combs are each produced by presence of the dominant forms
of their respective genes (P and R respectively) but in absence of the other dominant gene. If the
dominant alleles of both genes are present, the phenotype becomes walnut comb. The walnut
and single combs are produced by the interaction of the genes at both loci as summerised below:
Name of comb
Pea comb
Rose comb
Walnut comb
Single comb
Production
Dominant allele P but without dominant
allele R
Dominant allele R but without dominant
allele P
Dominant alleles for both P and R
Only by homozygous double recessive
condition
Possible genotypes
PPrr, Pprr
ppRR, ppRr
PPRR, PpRR, PPRr, PpRr
pprr
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Starting with pure breeding parents, the following are the expected results for F 1 and F2
generations.
Obtaining F2
All walnut combed
Phenotypic ratios: 9 walnut: 3Pea: 3rose: 1single
practice question
In poultry, the allele for white feathers (W) is dominant over the allele for black feathers (w). The
alleles P, for pea comb and R, for rose comb produce their respective phenotypes. If they are
present together, the comb shape is modified to walnut and if their recessive alleles are present in
homozygous recessive condition, a single comb I produced. A cross between a black rose comb
cock and a white walnut hen produced the following phenotypes:
3white walnut: 3black walnut: 3white rose: 3black rose: 1white pea: 1black pea: 1white single:
1black single. Identify the possible parental genotypes and show clearly how they give rise to the
above phenotypes.
2.9 Complementary genes
Are two or more genes which when present together, produce a phenotype that is qualitatively
distinct from the separate effect of any of them. Example:
In the sweat pea plant (Lathyrus odoratus), when two white flowered varieties are crossed, all the
F1 offspring are found to bear purple flowers. Selfing of these progeny produce purple and
white-flowered plants in a ratio of 9:7. This arises because:
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
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The characteristic is controlled by two different gene loci on different chromosomes
One gene (P), controls the production of colourless pigment precursor.
The other gene (C) controls the conversion of the precursor into a purple pigment.
Therefore, unless the dominant alleles P and C are both present, the flowers will be white either
because no precursor was formed or because it was formed but not converted to the active form
(purple pigment).
Task: Carry out genetic crosses to illustrate the observations and explanations in the above
example. (you can visit Understanding biology by DG Mackean pg 622-623 for more
information)
SECTION 2:
VARIATION
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Variation is refers to existence of differences in characteristics of organisms of the same species.
The differences can be in their physiological or physical or behavioral characteristics or in two or
all the three aspects.
Causes of variation / Sources of variation
The phenotypic appearance of an organism is determined by its genetic composition and the
influence of the environment. However, the genetic composition plays the biggest role.
Therefore, differences among members of a species (variation) are due to:
-
Difference in genetic composition called genetic variation
Difference in environmental factors.
The genetic composition of an organism dictate how the organism should appear phenotypically.
However, the appearance can be modified by environmental factors. For example, if the
genotype of a pea plant dictates that it should be tall, it will only attain full height if adequate
light and water and conducive soil factors are available. Another example is the action of light on
light-skin colour (which may make it darker). A deficiency in one of these factors may prevent
the gene for height from exerting its full effect. This proves that both hereditary/genetic factors
(‘nature’) and environmental factors (‘nurture’) interact to varying degrees to influence the final
appearance of an organism.
Causes/sources of genetic variation (genetic differences)
They include;
(a) Gene reshuffling; which is the random mixing of genes/DNA leading to production of
new gene combination in gametes or offspring. Gene reshuffling occurs during crossingover, independent assortment and fertilization.
(i)
Crossing over: alleles are exchanged between non-sister chromatids of homologous
chromosomes; during prophase I and separates linked genes;/ hence creating new
combinations of genes/alleles in the resulting gametes; This provides a source of
genetic variation in gametes.
(ii)
Random assortment of chromosomes; During metaphase I of meiosis, homologous
chromosomes are distributed/align randomly at the cell equator and subsequently
segregate independently during anaphase I. This leads to further mixing of genes
resulting into enormous genetic diversity among gametes.
(iii)
Random fertilization: fusion of gametes is completely random; different gametes may
carry different alleles of the same genes; thus fusion of gametes produces new
combinations of alleles in the zygote (offspring).
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Note: Random mating (the choices of mates is random) further increases genetic variation.;
different individuals carry different combinations of genes and random mating creates new
combinations of genes.
(b) Mutation; usually random and create new alleles/ alters the nature of a gene in an
organism; leading new phenotypes in a population; the changes in DNA due to mutations
are inheritable meaning that genetic variations due to mutations can be maintained in a
population.
Types of variation
There are two types;
Continuous variation
 Characteristics of individuals in a
population show a smooth gradation
from one extreme to another; the
differences are not clear-cut
Discontinuous variation
 Characteristics of individuals in a
population show sudden transitions
between two or more extremes; the
differences between individuals are
clear-cut
Intermediate types exist between extremes
Characters are controlled by many genes i.e
are polygenic
Characters produce normal distribution
curvs/bell-shaped curves
No intermediates between extremes
Characters are usually controlled by one gene;
which may have two or more alleles
Characters are best represented on
histograms; do not produce normal
distribution curves.
Examples;
 blood groups, ear lobe shape. Tongue
rolling, sex etc
Examples
 Height , weight/body mass,
intelligence,
MUTATIONS
1. Mutations
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A mutation is a change in the structure or amount of DNA of an organism.



Mutations produce changes in genotypes which are then expressed in the phenotypes
hence producing sudden and distinct differences among individuals.
Most mutations occur in body (somatic) cells-somatic mutations but these cannot be
passed to the next generation.
Only mutations which occur during formation of gametes (and therefore carried by
gametes) can be inherited.
Role of mutations in evolution of species
Mutations in germcells /gametes lead to emergence of new inheritable characteristics/variation
among organisms in a population which may be advantageous or disadvantageous. When the
enviroment exerts a selection pressure, organisms with advantageous variations are favoured and
pass on the good characteristics to the next generation; those with unfavourable characteristics
are weeded out/eliminated. Over several generations, a population of new species with
advantageous variations arise.
Causes of mutations (1998 paper 1 no. 42)
Any agent (substance/form of energy )that causes mutation is called a mutagen:




Spontaneous mutations result from errors in the replication of DNA. (are not induced).
Exposure to high energy radiations: these include UV light, X-rays and Gamma rays.
Exposure to high energy particles such as alpha particles, beta particles and neutrons
Exposure to certain chemicals such as mustered gas, colchicines, formaldehyde, nitrous
acid etc. Colchicine particularly inhibit spindle formation and so causes polyploidy.
Note that organisms with a short life cycle have a greater rate of mutation since their meiosis
occurs more frequently.
Types of mutations


Gene/point mutations
Chromosomal mutations/chromosomal aberrations
Chromosome mutations
These are mutations which result into change in number of structure of chromosomes.
Therefore, two forms of chromosomal mutations can be recognized:
a) Change in chromosome number: which involves;
- Loss or gain of a single chromosome. This condition is called aneuploidy.
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gain of a complete set of chromosomes. This condition is called euploidy/polyploidy.
Aneuploidy
Is a condition in which half the daughter cells produced by cell division have an extra
chromosome e.g (n+1), (2n+1) etc , while the other half have a chromosome missing. E.g (n-1),
(2n-1) etc. Aneuploidy results form non-disjunction;
Non- disjunction is the failure of homologous chromosomes to separate during anaphase I of
meiosis such that both members of a bivalent unit go to one pole and the other pole does not
receive a chromosome. This results into conditions like
-
Trisomy- (2n+1) in humans: non-disjuction occurs in chromosome number 21 (hence also
called trisomy-21)
Monosomy (2n-1)
Examples of genetic disorders resulting from non-disjunction
Euploidy (polyploidy)
Polyploidy is a condition in which a cell contains extra complete sets of chromosomes. i.e
instead of having a haploid set in the sex cells or a diploid set in the body cells, they have several
complete sets.

Three sets- cell is said to be triploid; four sets cell is said to be tetraploid etc
Polyploidy may arise in one of the following ways:
-
Failure of spindles to form hence preventing chromosomes separating during anaphase.
This can be artificially induced by chemicals like colchicines (to introduce hybrid vigour)
If diploid gametes are produced and the self-fertilize, producing a tetraploid.
If a normal haploid gamete fuses with a diploid gamete, producing a triploid.
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When a whole set or sets of chromosomes double after fertilization.
Two forms of polyploidy exist:
-
Autopolyploidy: a form of polyploidy where the normal set(s) of chromosomes and the
extra sets of chromosomes come from the same species.
Allopolyploidy : a form of polyploidy where the normal set(s) and the extra sets of
chromosomes come from different species. Gametes having ythis condition cannot fuse
successifully because the chromosomes are not homologous resulting into hybrid
sterility.
Significance of polyploidy
Polyploidy is associated with advantageous features such as increased size, hardness and
resistance to diseases. This is called hybrid vigour. Increase in sets of chromososmes leads to
increase in size of the nucleus and hence the entire cell. Polyploidy is thus applied in crop
husbandry to obtain desired characteristics.
Role of polyploidy in evolution;
Polyploidy does not add new genes to a gene pool but gives rise to new combination of genes in
cells/gene mixing; creating genetic variation which lead to hybrid vigour; such as increased
resistance to diseases. Such good characteristics are selected for by environmental selection
pressure at expense of the others; leading to speciation.
b) Change in chromosome structure
These arise from mistakes that occur during crossing over and include:
i)
ii)
iii)
iv)
Deletion; a portion of chromosome is lost; often lethal since it involves loss of genes
Inversion; a portion of a chromosome breaks off, rotates through 180 and then rejoins
the chromosome; reverses the sequence of genes at that portion; overall genotype
remains unchanged but the phenotype is changed due too change in sequence of
genes.
Translocation; a portion of chromosome breaks off, and either re-joins the same
chromosome at the other end or joins another non-homologous chromosome.
Duplication; a portion of a chromosome is doubled, resulting in repetition of a gene
sequence.
Gene mutations (point mutations)
A gene mutation is a change in the base sequence of DNA thus altering the genetic code/ a
change in the structure of DNA which occurs at a single locus.
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The change in the base sequence of a gene is transmitted to mRNA during transcription and may
result in change in amino acid sequence of a polypeptide hence a wrong protein.
Gene mutations take the following forms:
i)
ii)
iii)
iv)
v)
Deletion; a nucleotide is removed from the sequence of nucleotides of a gene. E.g
cystic fibrosis is due to a mutant allele that arises by deletion of three adjacent DNA
nucleotides coding for the amino acid phenylalanine.
Duplication; a particular nucleotide is repeated in the nucleotide sequence of a gene.
Addition/insertion; an extra nucleotide become inserted in the original sequence of
nucleotides
Inversion; a certain nucleotide sequence become separated from the chain and rejoins
in the original position but while inverted, changing the original nucleotide sequence.
Substitution- one of the nucleotide is replaced by another of a different organic base.
Genetic diseases due to this include sickle cell anaemia and haemophilia.
Describe how a gene defect can cause a genetic disease/disorder (08 marks)
A gene mutation occurs (small errors in the base sequence of DNA); Changes the base sequence
of a particular gene/cistron on DNA; e.g through substitution, deletion etc of a nucleotide; A
wrong sequence of bases/nucleotides on DNA is transcribed into a (mutant) mRNA during
transcription; The base sequence on the messenger RNA determines the sequence of amino acids
in a polypeptide ; thus the altered/wrong base sequence of mRNA results in an incorrect amino
acid sequence in the synthesized polypeptide; during translation; and thus an incorrect/abnormal
final structure after modification. The protein will therefore change in function or fail to perform
its function completely; For example, if the protein is an enzyme then the enzyme may fail to
work; thus a metabolic pathway may become blocked (eg. phenylketonuria) or an unwanted
substance may accumulate;
Genetic disease/disorders due to point mutations include;
i)
ii)
iii)
iv)
v)
Sickle cell anaemia
Cystic fibrosis
Haemophilia
Phenyketonuria (PKU)
Huntington’s disease
Presentation question: (8 minutes each)
Describe how the following genetic disorders arise (you can use illustrations/genetic crosses to
emphasize your points). State how each manifests itself and its effects to a victim.
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i)
ii)
iii)
iv)
v)
vi)
vii)
viii)
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Sickle cell anaemia ( fred )
Cystic fibrosis; (francis)
Phenylketonuria; (Ouma)
Red-green colour blindness; (etum)
Haemophilia (Kasirye)
Klinefelter’s syndrome (Millonah)
Turner’s syndrome (Agaba)
Trisomy 21 (down’s syndrome)( Mwanja)
SECTION 3:
POPULATION GENETICS
Population genetics is the study of the frequency of genes and genotypes in a Mendelian
population. By Mendelian population is meant a population in which genes have two alleles, one
being completely dominant over the other. In genetic, a population is an interbreeding group
oforganisms,
A gene pool: this is the total of all the alleles and genotypes of all genes of all the individuals in
a sexually reproducing population. Within the gene pool, the number of times any one allele
occurs is referred to as its frequency.
Within a gene pool, the frequency of alleles may remain constant or change over time. If the
frequency of all alleles in a gene pool does not change over time/remains stable, then the gene
pool/population is said to be in a state of genetic equilibrium. However, if the frequency of
some (or all) the alleles changes, then the population is said to be evolvSing. Therefore, change
in gene frequency is an indicator that a population is evolving.
Three kinds of frequencies can be measured;



Phenotype frequencies; are the easiest to determine, because we can see and count
organisms showing a particular phenotype in a population. E.g If a population of 1000 cats
has 840 black cats and 160 white cats then frequencies of the black and white phenotypes
are 0.84 (840/1000) or 84% and 0.16 (160/1000) or 16% respectively.
Genotype frequencies are the proportions of the three possible genotypes in a gene pool
of a Mendelian population i. e homozygous dominant, homozygous recessive and
heterozygous genotype (BB, bb and Bb for the example of black and white cats). Its not
to measure genotype frequencies because we can’t see the genotypes, but we can use the
Hardy-Weinberg equation to calculate them.
Allele frequencies are the proportions of the two alleles for a particular gene i.e the
dominant and the recessive alleles in a population. We can also use the Hardy-Weinberg
equation to calculate allele frequencies.
Factors which lead to change in allele and genotype frequency in a gene pool.
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They are also called the forces of evolutionary change because it is the change in allele
frequency that results into evolution (microevolution) over time. If these factors do not change,
the allele frequency of all genes remain constant and the population is said to be in a state of
genetic equilibrium.
i)
Natural selection
Natural selection is a mechanism of evolution in which organisms which are better adapted to
their environment survive and reproduce, and transmit their good adaptations/characteristics to
the next generation/offspring while those which are less adapted fail to survive and to reproduce
leading to elimination of their unfavourable characteristics from the population. e.g
Natural selection e.g in form of differential/selecttive predation therefore eliminates individuals
with inferior genotypes/genes and promotes survival and breeding of individuals with better
genotypes/genes; as a result the frequency of such alleles with a selective advantage increase
while the frequency of those which are inferior decrease and over time, this results into
evolutionary change.
ii)
Mutations
Mutations cause sudden changes in alleles/introduce new alleles in a population which increases
its genetic variation. Some mutations are harmful because they lead to phenotypes/
characteristics which reduce the fitness of an organism to survive in the environment hence the
organism can easily be eliminated by natural selection. Some mutations are beneficials and lead
to increased fitness of the organism to survive and reproduce in the environment. Only mutations
in germ cells/gametes are inheritable and hence are the most important producing evolutionary
change.
iii)
Gene flow
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Gene flow is the transfer of alleles from one population to another as a result of interbreeding
between members of the two populations. It is a results of migration of organisms from one
geographically separated group into another. The random introduction of new alleles into the
recipient population and their removal from the donor population affects the allele frequency of
both populations and leads to increased genetic variation. The frequency of gene flow between
populations depends upon their geographical proximity, and the ease with which organisms or
gametes can pass between the two populations. The closer two populations are, the more chances
of continuous interbreeding between them. Even when populations are not so close, flight and
different agents of dispersal can facilitate gene flow e.g dispersal of pollen, by wind.
iv)
Non-random mating
Occurs when two individuals of the same sex do not have equal chances of mating in the
population usually due to sexual selection. As a result, some individuals within a population
have a higher chance to reproduce and pass their favorable genes to the next generation.
Organisms with less favorable characteristics have a decreased chance of reproduction and the
frequency of their alleles being passed to the next generation is reduced. This can change the
genotype and allele frequencies in a population;
v)
Genetic drift
Is the random change in the frequency of alleles of a small population by chance events rather
than by natural selection; Chance events can cause loss of rare alleles from the gene pool; which
results into decrease in genetic diversity/variability of the population; for example accidental
death of an organism which is the only possessor of a particular allele before reproduction may
result into elimination of that particular allele from the gene pool; Chance events may also cause
an allele to drift to a higher frequency as the frequency of its alternative at the same locus
become lower; These changes in allele frequency can result into formation of new species over
time;
Genetic drift affects small ppulations more than large populations because some alleles are more
likely to be rare in small population than in large population; leading to reduced genetic variation
in small population.
Trial qn:
(a) What is meant by the term genetic drift?
(b) The figure below shows computer simulations of changes in allele frequency of two
populations of different generations, Y and Z.
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Explain the changes in the allele frequency of the populations of the two generations (07 marks)
(c) How does genetic drift affect the amount of genetic variations within very small
populations? (03 marks)
 Founder effect
Is a form of genetic drift which occurs when a small population becomes separated from the
parent population in such a way that the frequency of its alleles is not representative of the
frequency of alleles of its parent population; such that the population that emerges from the few
founders is has limited genetic diversity. The separation may be physical (by a physical barrier or
social.
In the pioneer population, some alleles of the parent population may be completely absent and
others may be disproportionally represented which creates a small gene pool with limited genetic
diversity. Due to continuous breeding a large population arises from the few pioneer individuals
with allele frequencies different from those of the original parent population.
Illustration;
 Genetic bottle neck
It is also a form of genetic drift which occurs when there is a sudden sharp decline in a
population size due to natural or man-made disasters such as earth quakes, tsunamis, epidemics,
habitant destruction etc; The small population that remains will have limited genetic diversity
because some alleles may be completely lost and others disproportionally represented. A large
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population is then established by the few individuals through continuous breeding that has
frequency of alleles different from that of the population before the disaster.
Heterozygosity as a reservoir of genetic variation
In all populations, dominant alleles express themselves more than the recessive alleles e.g in
human populations, brown eyes occur more frequently than blue eyes. It might be expected
therefore that at a time, the dominant alleles would dominant to a point where recessive
alleles completely disappear. However, this does not happen and the proportion of
dominant and recessive alleles of a particular gene remain the same.
Explanation; Because a large proportion of the recessive alleles in a population exist in the
heterozygous genotype and heterozygotes make up the largest percentage of the population ; In
the heterozygous state , they are not expressed in the phenotype (masked by the dominant allele)
hence even when they are harmful, they will not cause death of the organism. Some even confer
a selective advantage to the phenotype of the heterozygote which further increases their chances
of remaining in the population e.g the sickle- cell trait allele.
 Genetic load is the existence of disadvantageous/harmful alleles in the heterozygous
genotype within a population. Some recessive alleles which are harmful in the
homozygous genotype may be carried in the heterozygous genotype and confer a
selective advantage on the phenotype in certain environmental conditions. This is called
heterozygous advantage/heterozygous superiority. For inheritance of the sickle cell
trait in regions where malaria is endemic.
The Hardy-Weinberg Principle
It is used to determine the frequency of alleles and genotypes in a Mendelian population. It
states that;
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“The frequency of all alleles and genotypes in a population remains constant from
generation to generation provided the population is sufficiently large, mating is random, no
mutations occur, no natural selection occur and no geneflow/migrations take place”
 This principle was independently discovered by an English mathematician G.H
Hardy and a German physician W. Weinberg.
 It can be mathematicaly derived from the understanding of behavior of alleles
during meiosis and fertilization as follows:
Consider a gene which has a dominant allele B and a recessive allele b.
-
Let p be the frequency of the dominant allele B, and
q the frequency of the recessive allele b.
The frequency of the two alleles must add up to unity (1)- since there is only two types in
a Mendelian population i.e
p + q = 1.0 (100%)
In a diploid population, allele B and b occur in three combinations formed during random
fertilization as shown blow:
This Punnett square gives us the frequencies of the different genotypes in the population when
the organisms reproduce. The genotype BB has a frequency p2, the genotype bb has a frequency
q2, and the genotype Bb has a frequency 2pq. The sum of the genotype frequencies must add up
to one:
-
AA + 2Bb + bb = 1.0 (100%)
p 2 + 2pq + q 2 = 1(100%)
This is the Hardy-Weinberg equation (principle), and is based on the following assumptions:
1.
2.
3.
4.
5.
The population is sufficiently large, so is no genetic drift
Mating is random
There is no gene flow due to immigrations, so no new alleles are introduced
There are no mutations, so no new alleles are created
There is no selection (natural or artificial), so no alleles are favoured or eliminated (all
genotypes are equally fertile)
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The Hardy-Weinberg principle provides a means of:
-
detecting whether evolution is taking place
quantifying the rate of evolutionary change.
Worked examples:
1. A mental defect X is caused by a recessive allele. If the number of babies born with
the defect is 1 in 25 000 births, determine the
i)
ii)
iii)
the frequency of the recessive allele
the frequency of the dominant allele
the number of babies who are carriers.
2. Cystic fibrosis occurs in the population with a frequency of 1 in 2200. Calculate the
frequency of the carrier genotype.
3. The prevalence of phenylketonuria in Europe is 1 in 10 000 people. Determine the
frequency of the allele for phenylketonuria give that it is recessive and hence
determine the proportion of individuals who are carriers.
Questions
Tongue- rolling is caused by a dominant allele. In a certain human population of size 20000, 84%
of people can roll their tongues. Use the Hardy-Weinberg formula to determine how many people
are heterozygous and how many are homozygous for tongue rolling.
“It is possible to eliminate all dominant alleles of a particular gene from a population.” Justify
this statement? (04 arks)
Dominant alleles are always expressed in the phenotype once they appear in a genotype; which
exposes them to the forces of natural selection; if the dominant gene is harmful/lethal, all the
individuals possessing it will be eliminated by natural selection; hence eliminating all the
dominant alleles
SECTION 4:
ORIGIN OF LIFE AND EVOLUTION
THEORIES OF ORIGIN OF LIFE (STUDENTS’ RESEARCH AND PRESENTATION)
Describe briefly what the following theories state about the origin of life.
I.
Special creation theory;
Proposes that life was created by a supernatural being at a particular time in the past. It is
supported by by most of the world’s religions and civilizations. This theory cannot be proved or
disproved scientifically since because it draws its insights from divine revelation and faith while
science relies on observation and experiments to seek truth. (further reading BS pg 880)
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II.
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Spontaneous generation (abiogenesis); (JACOB)
This was a belief that some organisms were generated spontaneously from non-living matter. For
example, it was believed before the 18th century that maggots originated from decaying matter,
worms from the soil etc. This theory was experimentally disproved by Louis Pasteur in the 18th
century and dropped. It was succeeded by new ideas that life comes from pre-existing life
(biogenesis)
III.
Steady- state theory; ( ABBA)
This theory states that the earth had no origin , has always been able to support life, has changed
little and that species had no origin. (read more about the support and criticism against this
theory BS pg 881)
IV.
Cosmozoan / panspermia theory; ( PHOEBE)
This theory states that life an extra-terrestrial origin. i.e could have arisen one or several times
from other planets (Further reading BS page 881)
V.
Biochemical evolution; (OTTO)
This theory suggests that chemical reactions between the gaseous contents of the early
atmosphere (methane, ammonia, water vapour and hydrogen) led to the formation of amino acids
from which proteins were formed. Molecular changes in proteins led to formation of other
organic compound which together formed a primordial soup from which the first cell was made.
According to this theory, the first organisms was a heterotroph from which an autotroph evolved.
With clear explain the meaning of the following term.
Evolution (victor )
Microevolution (Madline)
Macroevolution (Madline)
Co-evolution (iffat)
EVOLUTION
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In a broad sense evolution refers to a process by which living organisms have changed from the
simplest forms to the most complex forms that are existing today. Involves modifications in the
existing organisms and the inheritance of these modifications. Evolution is not concerned with
origin of life but how life has changed from simple to complex and how it has diversified.
Related terms
Microevolution; - Evolution on a small scale (species level) that involve change in
allele frequency of a population leading to formation of a new species.
Microevolution is due to mutations, selections, gene flow and genetic drift, resulting
into a new species.
Macroevolution
i)
ii)
Refers to large scale evolution above the species level /at higher taxonomic group such as
genera families.
iii)
Co-evolution
This is the evolution of two or more ecologically related non-interbreeding species in which one
evolves certain features in response to how the other has evolved. i. e two species reciprocally
affect each others evolution by exerting selection pressure on it.
Examples of coevolution
-
Evolution of flowering plants and insects which pollinate their flowers
Evolution of prey and predator species.
EVIDENCES FOR EVOLUTION
Evidence of evolution is obtained from
iv)
v)
vi)
vii)
viii)
ix)
x)
Palaeontology
geographical distribution
classification
comparative anatomy
adaptive radiation
comparative embryology
comparative biochemistry
(1998 nov/dec and 1999)
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I.
Fossil study (Palaeontology- study of fossils )
Fossils are the remains of dead plants and animals preserved in by a natural process for
example in rock or ice.
They take various forms which include; bones, shells, footprints, exoskeleton of
arthropods, teeth , wood tissue in plants etc; Fossils were formed mainly in sedimentary
rocks
In some places, sedimentary rock has been laid down in layers called strata and the depth
of each stratum gives the relative age of any fossils it contains. More accurate estimates of
ages of fossils in different strata can be achieved using radio-isotope dating e.g carbon
dating, uranium dating etc. When fossils from different strata are put in chronological
order after being dated, they show progressive increase in complexity and show existence of
transitional forms of a species hence giving an idea of how one group of organisms may
have evolved into another. Fossil records indicate that invertebrates were the first animals,
followed by fish, amphibians, birds and then mammals. Complete fossil records of some
organisms like the horse exist, from the ancestor to the present day descendants. Fossil
records also reveal evidence of extinction of some species for example dinosaurs.
Advantage: Fossils provide the only direct evidence of history of evolution.
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Limitations of fossil evidence
 Fossil evidence is unreliable since only hard parts of organisms fossilize
 Dead organisms decompose rapidly and usually do not form fossils
 Soft-bodied organisms do not fossilize making fossil evidence incomplete
 Only a small fraction of organisms exist in fossil records i.e fossils of most
organisms are so rare that it is impossible to trace their evolutionary
pathways
 Conditions for fossilization are relatively rare
Explain the meaning of the following evolutionary concepts
 Punctuated equilibria
 Salutatory evolution
II.
Geographical distribution
 Biogeography is the study of the distribution pattern of species across the
planet.
 It is believed that at one time, the earth had a single large land mass called
Pangaea but due to forces in the earth’s crust, it broke apart over time and
the different parts formed the present day continents. This is called
continental drift (the splitting and movement of land masses from their
original position to new geographical positions).
 Continental drift influenced the process of speciation by causing
geographical isolation which led to adaptive radiation. This led to organisms
being naturally confined to certain parts of the world e.g
 Australia has many endemic species (Marsupials) that evolved independently
following its geographical isolation from south America whose related species are
now extinct. However, some widely separated regions may have related species
due to similarity in climatic conditions. This is called parallel evolution (where
two populations that have become separated evolve similar adaptations
independently if they live in similar environmental conditions). For example, the
three remaining species of lung fish are found in tropical areas of America, Africa
and Australia.
 Also regions of the world with similar climates and habitats do not necessarily
have the same flora and fauna; for example south America has faunas like new
world monkeys, llamas, janguals, pamas etc while Africa has new world
monkeys, apes, African elephants, lions etc yet they similar climates and habitats.
The explanation for this is that the two land masses were once joined and shared
common flora and fauna but drifted apart (continental drift) which isolated their
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fauna and flora. As a result, the organisms on each land mass proceeded to evolve
along their own lines due to difference in selection pressures.(underwent adaptive
radiation).
III.
Comparative anatomy
This establishes evolutionary relationship on the basis of similarities and differences in
structures of organisms;
Some groups of organisms have structures which have the same basic plan
but perform different functions, called homologous structures. This shows
that such organisms have a common origin (common ancestor) but their
structures have undergone adaptive radiation to survive in different
environments. for example; The pentadactyl limb system found in all
terrestrial vertebrates; mouth parts of insects etc
- Some groups of organisms have structures that differ in basic plan/structural
details but perform a common function that enables the organisms to survive
in the same environment. Such organs are called analogous organs and
provide evidence for convergent evolution.
- Some organisms have structures which are extremely reduced and have no
apparent function but fully existed and performed a normal function in their
ancestors; and are homologous to a functional structure in other related
groups. Such are called vestigial organs and are used to establish
evolutionary relationship between organisms possessing them and those in
which they are fully functional. They give evidence of common origin of such
different groups of animals. Examples; appendix in man, pelvic girdle of the
whale, coccyx (vestigial tail) in man; erector pili muscle etc. Vestigial organs
IV.
Classification: During classification, plants and animals are placed into different
taxonomic groups such as species, genus, family, order etc. Organisms are put
together in a particular taxonomic group depending on similarities and differences.
The resemblance of organisms/ groups of organisms is due to their genetic affinities
which proves a common ancestral origin while the differences between them are due
to descent with modification from a common ancestor.
Explain why natural classification should be based on homology but not analogy (05
marks) ref. FA pg 575
-
V.
Cell biology; Cells of most organisms are remarkably alike in their fine structure
and biochemistry/physiology; organelles like mitochondria, ER and ribosomes are
of universal occurrence just like biomolecules like ATP, nucleic acids, ATP,
cytochromes etc. All cells are made of carbon, hydrogen, oxygen and nitrogen and
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are made up of Mainly Water. These prove that all living things have had a common
ancestry.
VI.
Comparative embryology;
Studies which compares the stages of development of embryos of different groups of
organisms show that most vertebrates go through the same stages of embryonic
development; which shows that these organisms have a common ancestor. This is further
stressed by Ernst Haeckl’s principle (1843-1919) that states that ‘ ontogeny recapitulates
phylogeny’ i.e the embryonic developmental stages through which an organism passes
repeat the evolutionary history of the group to which it belongs.
VII) Comparative biochemistry and physiology; Biochemical analysis suggest a common
origin for all living things; all living organisms have DNA as their genetic material; with a
genetic code which is almost universal; the process of protein synthesis using RNA and
ribosomes is also similar in prokaryotes and eukaryotes; Comparison DNA sequences show
that it is 99.9% certain that chimps are humans’ closest relatives; ATP is the universal
source of energy;
Physiological processes such as respiration and photosynthesis involve the same steps and
similar reactions in all organisms in which they occur; comparison of blood proteins of
different animals in serological tests prove common origin since many have related blood
proteins; analysis of blood of different animals show that blood pigments especially
haemoglobin are widely distributed. All these show common ancestry of the different
groups.
Homology and divergent evolution:
Homology is the similarity in structures of organisms that are believed to have originated
from a common ancestor. Homologous organs are structures found in different groups of
organisms with the same basic plan but differ in function. The explanation for the
similarity is that the different groups of organisms possessing such structures descended
from a common ancestor (have a common origin) but because of the need to occupy
different ecological niches/survive in different environmental conditions, the structures
became modified/specialized in a process called adaptive radiation.
Adaptive radiation is a process by which different forms of organisms diversify from an
ancestral species by developing new features/characteristics that enable them to survive in
different environmental conditions/habitats. This is a form of divergent evolution.
Divergent evolution is the process by which different groups of organisms evolve from a
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common ancestor by developing specializations to occupy different ecological niches.
Homologous structures provide evidence for divergent evolution.
Examples of homology/homologous organs /adaptive radiation/divergent evolution
I.
II.
III.
Evolution of mouth parts in insects
Evolution of the middle ear bones in different groups of vertebrates.
Adaptive radiation of the pentadactyl limb system (five-fingered limb): This is the
basic plan of limbs of tetrapods which consists of a single long bone joined to two
other long bones which are in turn joined to several small bones terminating in
small digits.
ILLUSTRATION
The limb system has been modified differently in different tetrapods to perform different
functions in the environment.
In some of the tetrapods, the limb system is modified in the following ways: Check qn 1994
pp1
 In rats, carpals are displaced, phallanges are elongated and pointed for
digging of burrows and running fast.
 In monkeys, the fore limbs have elongated phalanges to a grasping hand for
swinging from branch to branch. Humerus, radius and ulna are elongated
for increased agility.
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
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In bats, four of the digits of the fore limb are greatly elongated to provides an
extended attachment of skin so as to increase the surface area for the wing
used for flight; The fifth digit is short and hooked to enable it to hang and
suspend its body in space.
Relate structure to function of the limb systems of the following mammals (ref. FA pg569571)
1.
2.
3.
4.
(iv)
Horse
Pig
Whale
Mole rat
Evolution of Darwin’s Finches by adaptive radiation (see evolution by Natural
selection)
Analogous structures and convergent evolution
Analogous structures are structures possessed by different groups of organisms with a
common function but differ in their basic structural plan. They are a result of convergent
evolution and hence provide evidence for it. They differ in their microscopic details and
embryonic development.
Convergent evolution is the process in which organisms that have different ancestral
origins independently evolve similar structural features to enable them survive in the same
environment.
Examples of analogous structures/ convergent evolution include;
- Evolution of flight (wings) in birds, insects and bats. In all these groups, the
wings have the same function (flight) but differ in anatomic details.
- Streamlined shapes of marine fish, birds(like penguins), mammals (such as
dolphins, sea lions etc).
Trial qn: Explain how the following provide evidence for evolution.
I.
II.
III.
Vestigial organs
Adaptive radiation
Homologous structures
THEORIES OF EVOLUTION
There are three prominent theories;
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i)
ii)
iii)
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Larmack’s theory (Larmackism)
Darwin’s theory (Darwinism)
Neo-darwinism (modern theory)
A. Larmarckism (Inheritence of acquired characteristics)
In 1801, Jean- Baptist de Lamarck put forward a theory to account for the mechanism of
evolution which is based on the following ideas:
i)
ii)
iii)
iv)
Living organisms or their component parts tend to increase in size/complexity.
When an organism develops a need for a particular structure due to demand by the
environment, the need induces its appearance.
The use and disuse of parts i.e structures which are subjected to constant use become
well developed while those that are not used tend to degenerate and eventaully
disappear;
The beneficial characteristics acquired can be passed on to the offspring.
Lamarckism is the evolutionary idea that an organism can acquire certain characteristics during
its life time due to need and pass them on to its offspring. As such evolutionary change could be
achieved by the transmission of acquired characters which would accumulate over time in a
population. For example;



According to Lamarck the elephants used to have short trunks. When there was no food
and water that they could reach with short trunks, they stretched their trucks to reach the
water and branches and their offspring inherited long trunks.
Lamarck believed that giraffes stretched their necks to reach food. Their offspring and
later generations inherited the resulting long necks.
Flightless birds: Development of flightless birds like ostrich from flying ancestors due to
continued disuse of wings as these were found in well protected areas with plenty of
food.
Etc
How speciation arises according to Lamarck; In every generation, new characters are acquired
and transmitted to the next generation. The new characters accumulate over time and after a
number of generations, a new species is formed. To Lamarck, an existing individual is the sum
total of the characters acquired by a number of previous generations.
Criticism of Lamarckism:
 Mendel’s laws of inheritance object the idea of inheritance of acquired characters of
Lamarckism.
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 In 1892 A.D, it was proved (by a German biologist, August Weismann) that
environmental factors affect only somatic cells and not the germ cells yet the link
between the generations is only through the germ cells (that produce gametes). The
somatic cells are not transmitted to the next generation so the acquired characters must be
lost with the death of an organism so these should have no role in evolution.
Experiments and observations which disqualify Lamarckism include (optional in your notes):





Weismann mutilated the tails of mice for about 22 generations and allowed them to breed,
but tailless mice were never born.
Pavlov, a Russian physiologist, trained mice to come for food on hearing a bell. He
reported that this training is not inherited and was necessary in every generation.
Similarly, boring of pinna of external ear and nose in Indian women; tight waist, of
European ladies circumcising (removal of prepuce) in certain people; small sized feet of
Chinese women etc are not transmitted from one generation to another generator.
Eyes which are being used continuously and constantly develop defects instead of being
improved. Similarly, heart size does not increase generation after generation though it is
used continuously.
Presence of weak muscles in the son of a wrestler was also not explained by Lamarck.
Finally, there are a number of examples in which there is reduction in the size of organs
e.g. among Angiosperms, shrubs and herbs have evolved from the trees.
So, Lamarckism was rejected.
Qn: Explain the significance of Lamarckism to the later theories of evolution
B. Evolution through Natural selection by Darwin and Wallace
Natural selection is a mechanism of evolution in which organisms which are better adapted to
their environment survive and reproduce, and transmit their good adaptations/characteristics to
the next generation/offspring while those which are less adapted fail to survive and to reproduce
leading to elimination of their unfavourable characteristics from the population. The theory of
evolution by natural selection was proposed by Charles Darwin (1809- 1882 A.D.). Darwin
made an extensive study of nature for over 20 years, especially in 1831-1836 when he went on a
voyage on the famous ship “H.M.S. Beagle” and explored South America, the Galapagos Islands
and other islands. He collected the observations on many animal species and plant species, their
distribution and the relationship between living and extinct animals.
Darwin’s observations and deductions which led to his theory
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1. Over production of offspring: All organisms produce large numbers of offspring
and if all survived, would lead to a geometric increase in the size of the population.
2. Constancy of numbers; Despite the tendency to increase the numbers due to over
production of offspring, most populations maintain relatively constant numbers.
3. Limited resources: two main limiting factors on the tremendous increase of a
population are; limited food and space. These do not allow a populations to grow
indefinitely
4. Struggle for existence: Due to rapid multiplication of populations but limited food
and space, there starts an everlasting competition between individuals having
similar requirements. In this competition, every living organism desires to have an
upper hand over others. This competition between living organisms for the basic
needs of life like food, space, mate etc., is called struggle for existence which is of
three types:
 Intraspecific: Between the members of same species e.g. two dogs struggling for a
piece of meat.
 Interspecific: Between the members of different species e.g. between predator and
prey.
 Environmental or Extra specific: Between living organisms and adverse
environmental factors like heat, cold, drought, flood, earthquakes, light etc.
- Out of these three forms of struggle, the intraspecific struggle is the strongest type of
struggle as the needs of the individuals of same species are most similar e.g., sexual
selection in which a cock with a more beautiful comb and plumage has better
chances to win a hen than a cock with less developed comb.
- Similarly, cannabilism is another example of intraspecific competition as in this;
individuals eat upon the members of same species.
- In this death and life struggle, the majority of individuals die before reaching the
sexual maturity and only a few individuals survive and reach the reproductive stage.
So struggle for existence acts as an effective check on an ever-increasing population
of each species.
5. Variation among offspring; sexually produced offspring of any species show
individual variations so that no two offspring are identical except for monozygotic
(identical) twins.
Darwin stated that on the basis of their effect on the survival chances of living organisms,
the variations may be neutral, harmful or useful. He proposed that living organisms tend to
adapt to changing environment due to useful continuous variations {e.g., increased speed in
the prey; increased water conservation in plants; etc.), as these will have a competitive
advantage.
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6. Survival of the fittest by natural selection; Because of variation, among the offspring
there will be some better adapted to with stand the prevailing conditions. That is, some will
be better adapted one generation to another. So the useful variations go on accumulating
and after a number of generations, the variations become so prominent that the individual
turns into a new species. So according to Darwinism, evolution is a gradual process and
speciation occurs by gradual changes in the existing (fitter) to survive in the struggle for
existence and therefore more likely to survive long enough to breed hence passing on their
beneficial genes to their offspring.
7. Inheritance of useful variations (Like produces like) : Those that survive to breed are
likely to produce offspring similar to themselves. The useful variations (advantageous
traits) that made them to survive in the struggle for existence are likely to be passed on to
the next generation.
How new species arise according to Darwin (speciation); useful variations appear in every
generation and are inherited from species.
In summary, Darwin’s theory of natural selection states that ‘New species of organisms arise
from existing ones by natural selection which operates on variations; organisms with favourable
characteristics are preserved and allowed to reproduce while those with unfavourable
characteristics are eliminated in the struggle for existence.
From the above account the role of natural selection in the process of evolution is to
eliminate organisms in a population with unfavourable variations and promote survival of
those with favourable variations; which reproduce and pass their good traits to the
offspring; over several generations, it allows accumulation of good variations in the
population which results into emergence of new species when the parent population and the
new population become reproductively isolated.
By eliminating the poorly adapted individuals in a population, natural selection regulates
population size; which enables the more adapted organisms to flourish in conditions of adequate
resources.
 Factors in the environment that operate to keep populations in check are called
‘selection pressures’ or ‘environmental resistances’. They include:
 competition for resources such as food and space to live;
 disease;
 predation;
 lack of water, oxygen, or light;
 changes in temperature;
 Within any population there will usually be range of phenotypes. The most frequent
phenotype will be the one that is best fitted to that particular environment but there
will usually be much smaller numbers of less successful phenotypes. If the
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environment changes these phenotypes may become much fitter and therefore
increase in frequency.
Role of the environment in natural selection
The environment exerts a selection pressure which allows natural selection to occur. In case of
change in environmental conditions, organisms that are more adapted to the new environment
survive and reproduce while others fail to survive and are eliminated from the environment. The
environmental selection pressure may eliminate intermediates of a phenotypic characteristic and
maintain the extremes, leading to disruptive selection and formation of two sub-species. The
environmental selection pressure may also act on one extreme leading to directional selection
and formation of a new species;
Limitations in Darwin’s theory of evolution.
-
Does not explain the origin of life but only focuses on how already existing species
change.
It does not give an explanation to the source of variations and the mode of transmission
of variations which are key for natural selection to operate.
REFER TO BS pg 887 and summarize the misconceptions which surround the Darwin’s theory
of evolution by natural selection.
C. Neo-darwinism (synthesis /modern theory )
This is a restatement of the theory of natural selection in terms of Mendelian and post Mendelian
genetics. This theory gives an explanation for the sources of variations among organisms. It
explains that the main source of genetic variation are mutations and gene reshuffling during
meiosis (Crossing over and independent assortment). According to this theory, once genetic
differences are established in organisms, they are likely to be expressed to cause differences in
phenotypes. Some phenotypes may be better adapted than others to survival and reproduction in
particular environmental conditions. When such environmental conditions arise, natural selection
operates and causes changes in allele frequency through differential reproduction. The frequency
of alleles which give survival advantage increase while the frequency of alleles which lead to
survival disadvantage decrease. Over time, this leads to formation of new species.
Misconception!!!
Discussion question: “Populations evolve but not individuals” Discuss. Gps of 3’s
 Biological fitness means the ability of some organisms to survive long enough to
produce offspring that will also survive and produce offspring which results into
maintaining of their good characters in the population.
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Types of selection
Generally, selection is a process by which organisms that are better adapted to their
environment survive and breed, while those which are less adaptd fail to do so. It is either
natural or artificial.
Directional Selection; one which occurs when only one extreme phenotype is favoured over the
other extreme (e.g. shortest). This happens when the environment changes in such a way that it
favours only a particular extreme of a phenotype. The fittest phenotype may initially be rare but
its frequency increases over time as environment changes. e.g.
 Selective predation.
 Evolution of antibiotic resistant bacteria is due to directional selection.
 Rise of herbicide resistant plants
• Disruptive (or Diverging) Selection; one which occurs when two or more extremes of a
phenotype are favoured over intermediate phenotypes. For example, in a population of finches,
birds with large and small beaks feed on large and small seeds respectively and both do well, but
birds with intermediate beaks have no advantage, and are selected against.
• Stabilising (or Normalising) Selection. This occurs when the intermediate phenotype is
selected over extreme phenotypes. It tends to occur when the environment doesn't change much.
This mode of selection reduces variation and tends to maintain the intermediate for a particular
phenotypic character. For example- the birth weights of most human babies lie in the range of34 kg ; babies who are either much smaller or much larger suffer higher rates of mortality.
 Basing on the above account, natural selection is responsible for both changing of a
species (through disruptive and directional selection) as well as maintaining the
constancy of a species (stabilizing selection).
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Task: Explain the role of natural selection in adaptive radiation.
Examples of natural selection
1. Evolution of Darwin’s Finches
 On his Journey to the Galapogas islands, Darwin noted that the finches on the islands had
beaks which were different in shape and size from those on the mainland. He proposed
the following explanation:
 The ancestor of the 13 finch species migrated to the islands from the nearest main land.
The mainland had only one type of finch which whose beak was adapted to crushing
seeds. On arrival on the islands, they occupied the vacant niches on the island and at first,
there was plenty of food and so there was no competition.
 As the population increased, stiff competition arose. Random mutations produced finches
with small beaks which gave them a selective advantage to feed on small insects and
finches with large beaks which gave them a selective advantage to crack open tough
seeds or feed on fruits. Over many years, the two population became different species due
to continuous selection. A similar process of adaptive radiation led to development of
several finches each adapted to a particular food.
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2. Changes in frequency of the specked white/normal and black/melanic forms of the peppered
Moth (Biston betularia) before during and after the industrial revolution in Europe.
Before the Industrial Revolution in Europe, specked/ pale form was most common. its lighter
colour meant that it was well camouflaged on the pale bark of lichen-covered trees. And hence it
was not easily seen by predatory birds. It was therefore less eaten. However, by 1900 in
Manchester, the black form dominated. By the 1980s, after legislation had drastically reduced air
pollution, the frequency of the normal /specked white form had increased again.
 It is believed that both extreme forms - the pale/speckled white form and the
black/melanic forms existed before the Industrial Revolution. The melanic form arose as
a result of random mutations in the formerly white speckled population of the peppered
moth.
During the period of industrial revolution, industrial soot blackened the trees and killed lichens
on which the white speckled form used to camouflage. They were now easily seen by birds and
hence heavily predated on. This led to drastic decline in their frequency. The selective advantage
shifted to the black forms. They were now well camouflaged in the dark environment. Reduced
predation led to the rapid increase in their frequency.
Natural selection in form of differential predation became a factor that determined the frequency
of the peppered moth. The nature of change in the environment determined the direction of
predation.
Qn. Name the type of natural selection taking place.
Industrial melanism is the evolution of dark body colours in animal species that live in habitats
blackened by pollution/ industrial soot.
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Polymorphism
This is the existence of two or more forms of the same species within the same population. For
example, the two main forms of the peppered moth (pale form and the melanic form) discussed
above show polymorphism.
Two types of polymorphism are recognized:
i)





ii)
Balanced / stable polymorphism: which occurs when different forms exist in the
same population in a stable environment. As a result, the different forms have equal
selective advantage and disadvantage and therefore their allele frequencies will
remain almost constant (exhibit equilibrium) Examples:
The existence of A,B, AB and O blood groups in humans.
Existence of different castes in a bee colony i.e workers, drones and the queen.
Red-green colour blindness in humans
Sickle – cell condition in humans
Existence of a number of different forms of the common land snail (Cepaea nemoralis);
which are distinguished by the colour of their shells (e.g yellow, pink, brown, red etc)
and the presence of variable number of black bands on the shell. Refer to BS for further
reading.
Transient/unstable polymorphism: which arises when different forms/morphs exist
in a population undergoing a strong selection pressure. As a result, the frequency of
the polymorphic alleles is determined by the intensity of the selection pressure.
Usually, one form is favoured by the selection pressure and may gradually replace the
other. For example existence of the melanic and non-melanic forms of the peppered
moth.
Question: Of what significance is polymorphism in the process of evolution?
The small differences between the morphs may be associated with more important
differences such as viability, reproductive efficiency, ability to camouflage etc which may
confer a selective advantage to one morph when natural selection operates; leading to its
survival in the environment.
3. Natural selection and the prevalence of the sickle cell trait
Sickle cell anaemia is caused by a point mutation in DNA. The base Thiamine (T) is mistakenly
replaced by the base Adenine (A), this changes just one amino acid when haemoglobin is
formed. In one part of the haemoglobin molecule (0ne of the beta chains) the amino acid valine
appears instead of glutamic acid. This change in just one amino acid in the whole of the
haemoglobin molecule changes the electrical charge of the haemoglobin. In turn, this means that
when oxygen concentrations in the blood are low the shape of the haemoglobin distorts - the red
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blood cells become sickle shaped. The cells clump together and can block the capillaries. This is
called sickle cell anaemia.
Individuals who are homozygous recessive - i.e. those who have received the mutant gene from
both parents have a very low prevalence but the incidence of the heterozygotes is very high in
some parts of the world where malaria is edemic.
The homozygous recessive individuals have a very low prevalence because they suffer from
sickle cell anaemia which gives them a survival disadvantage in the environment. They are
selected against and usually die before puberty. In heterozygotes, the sickle cell allele is not
expressed and hence the masked from the forces of natural selection; giving the heterozygotes a
selective advantage over sicklers. The heterozygotes also have a survival advantage over the
homozygous dominant individuals. They are more resistant to malaria because they have a
big proportion of sickle-shaped red blood cells which are e much less likely to be invaded
by plasmodia (the parasites that cause malaria). This is called heterozygous advantage..
5. The development of resistance in many bacterial species to antibiotics, for example,
the tuberculosis bacterium is now very difficult to treat because it is resistant to nearly all
antibiotics.
How antibiotic resistance arises in bacteria?
 Random gene mutations may occur in a few members of a population of bacteria; which
give them an advantage of survival when exposed to an antibiotic. The antibiotic
becomes a selection pressure; that eliminates the sensitive bacteria in the population
while leaving behind the well adapted. Thus the antibiotic resistant bacteria will
reproduce and give rise to many resistant forms which are also likely to survive on
exposure to the same antibiotic. Over exposure leads to a new population of mainly
resistant bacteria since each exposure eliminated the sensitive ones and leaves behind the
resistant ones.
Therefore, the bacterial resistance to antibiotics is enhanced by:
 Overexposure to antibiotics caused by over-prescription of antibiotics or overuse of
antibiotics in farm animals
 Improper use/ misuse of drugs
DO QN 1 2005 PP2
6. The development of resistance in many species of insects to insecticides. For example,
resistance to DDT by malaria parasites/mosquitoes, the development of resistance to Bt
toxin (genetically engineered into crop plants) by insect pests.
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Describe how does this resistance arises?
7. The development of tolerance to toxic metals and their oxides by certain plants. For
example, some grass species growing around disused tin and lead mines are now so
tolerant to tin and lead salts that they can even grow over the heavily contaminated spoil
heaps from the mines.
8. The development of new strains of virus. For example, influenza virus frequently mutates
to produce strains to which humans have little or no immunity. ‘Ancestral’ viruses
mutated to produce strains such as the HIV/AIDS virus and the SARS virus – these are
important human pathogens. There are now many different strains of AIDS virus around
the world and the virus is still mutating to form even more. This has made it difficult to
produce a permanent effective virus for many viral diseases.
OTHER TYPES OF SELECTION
1. Artificial selection; which is the selection of animals or plants with desired
characteristics by man and allow them to breed to give rise to better offspring; and
preventing of those lacking desired qualities from breeding. This is usually done through
through extermination or segregation or sterilization. It is a carefully planned breeding
programme whose purpose is to increase the quality of the breeds and the yield.
In the process, humans exert a directional selection pressure which leads to changes in allele and
genotype frequencies within the population. This is one way through which man influences the
process of evolution.
Qn. Explain how man can influence the process of natural selection. (05 marks)
Two types of breeding are recognized in artificial selection:
a) Inbreeding: which is the mating of genetically related organisms in order to maintain a
particular desirable characteristic in organisms. The extreme forms of inbreeding
include:
 Self-fertilization for example as a result of self-pollination in plants
 Crossing the offspring of the same parent
 Back – crossing the offspring with one of the parent.
Disadvantages of inbreeding



Prolonged inbreeding leads to reduction in fertility
It reduces genetic variability by increasing the number of homozygous genotypes in a
population. This reduces the chances of survival of an organism
It results into reduced vigour in form of low and poor yields
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QN: Suggest any advantages of inbreeding
b) Out-breeding; which is the mating of genetically unrelated/distant organisms; to produce
hybrids with characteristics which are superior to either of the parent organisms. The
progeny are called hybrids. Hybrids show characteristics which are superior to either of
the parents which include:
 Increased fruit size and number in plants
 Increased resistance to diseases
 Rapid growth and maturity
 Increased fertility
 A hybrid is an offspring of a cross between genetically unrelated organisms of a
population/species.
 hybrid vigour refers to characteristics of hybrids which are superior to those of either of
the two parents.
2. Sexual selection; refers to selection which favours mating of only individuals of one sex
with certain characteristics which are preferred by individuals of the other sex. This
allows successful individuals to reproduce and pass their good traits to the offspring
while the traits of the unsuccessiful individuals are not. Sexual selection is useful in
explaining aspects of sexual dimorphism (i.e the differences in appearance between males
and females of the same species, such as in colour, shape, size and structure).
3. Kin selection; a type of selection which favours behaviours that lead to survival of one’s
relatives at the expense of oneself. For example a person jumping into a river to save a
relative but instead he gets drowned and the relative survives. Kin selection is seen in
worker bees which defend the hive by stinging the intruder but usually die in the process.
4. Group selection; a form of selection which favours behaviours that lead to the survival
of the species or a sub-division of the species at the expense of one member of the
species. e.g when people risk their lives for their countries in wars.
Note: In kin selection and group selection, an organism engages in a behavior that promotes
another’s survival at the expense of its own survival. This is called altruistic behavior.
SPECIATION
Speciation is a process by which new species arise from pre-existing species.
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A species is a group of organisms that are capable of interbreeding to form fertile offspring
and are reproductively isolated from other such populations/cannot interbreed with other
species to form fertile offspring;
Organisms belonging to a given species do not usually exist as a single large population in
the same locality. They usually exist in small inter-breeding groups which are
geographically separated, each with its own gene pool. These are called demes.
Before a new species can develop a barrier must form that restricts breeding and gene flow
between populations (demes). Such a barrier, which prevents gene exchange, is called an
isolating mechanism
Once such a barrier is in place then the isolated populations can continue to vary
independently by the usual mechanisms. They eventually may become so different from
each other that they can no longer interbreed successfully - they have become separate
species.
Forms of speciation
Allopatric: Is a type of speciation that occur in populations that are geographically isolated
which makes it difficult for gene flow to occur.
Sympatric: speciation where the populations occupy the same geographical locality but gene
flow is restricted by reproductive isolation mechanisms.
Isolation mechanisms
In the modern theory of evolution/neodarwinism, the role of isolation mechanisms in
evolution/ speciation is highlighted.
 An isolation mechanism is a barrier that prevents successiful interbreeding between
members of the same population/species.
Types of isolation mechanisms
Geographical isolation mechanism
Reproductive isolation mechanism
i) Pre-zygotic (these are isolation mechanisms which prevent fertilization from taking place)
ii) Post-zygotic/genetic ( fertilisation may occur but any offspring produced is either not viable
or or sterile due to genetic incompatibilyt)
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1. Geographical isolation
This refers to physical separation of groups of a population by physical barriers such as mountain
ranges, water bodies etc, hence preventing gene flow from occurring between them.
In the evolution of Darwin’s finches the finches became established on several islands and since
each island population was separated from the others by sea (finches will not fly across wide
stretches of water), they were geographically isolated and thus diverged and evolved into distinct
species (allopatric speciation)
2. Reproductive isolation mechanisms (Pre-zygotic)
Reproductive isolation mechanisms are those which prevent inter-breeding among organisms of
the same population due to lack of attractiveness between opposite sex, physical noncorrespondence of genetalia, non-effectiveness of courtship behaviours etc. They include:
i)
Ecological isolation: This occurs when species inhabit the same geographical area
but occupy different habitats or ecological niches within the area. E.g In the
Galapagos islands, the finches within one population on an island diverge and become
adapted enabling them to occupy different ecological niches
ii)
Seasonal isolation: This occurs when populations exist in the same area but are
sexually
mature at different times of the year.
iii)
Behavioural isolation: This occurs when animals exhibit species-specific courtship
patterns isolation arises when such courtship patterns become less effective in
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iv)
v)
BY AKM 2018 – O787011683
attracting a mate to sexual activity. Mating only occurs if the courtship display by one
sex results in acceptance by the other sex. Many species of fish, bird and insect
exhibit highly specific courtship colourations, movements and dances to attract the
opposite sex. If such court
Mechanical isolation: This occurs in animals where differences in the shape of
genitalia prevent mating between closely related species. e.g The male palps of
spiders, which are used to insert sperm into the epigyne (female genital opening) are
extremely complex in shape and can only fit the complex epigyne of the specific
female by a ‘lock and key’ mechanism.
Physiological incompatibility: This is common in grasses and clovers. The stigma
produces genetically- determined proteins which inhibit or retard the germination and
growth of foreign pollen, even from closely related species.
3. Post-zygotic barriers (genetic isolation)
Genetic isolation occurs when mating is possible but fertilization does not take place and if it
takes place, the hybrid is sterile or inviable due to fundamental differences in genetic
constitution. It may be in form of:
i)
Hybrid inviability: Although hybrids are formed, they are usually weak and
malformed and die before they can reproduce.
ii)
Hybrid sterility: In this case the hybrid may be vigorous and grow to adult size but
will be sterile because meiosis will fail to produce gametes. This is because the
different parent species have different chromosome shapes (and possibly different
chromosome numbers). Thus pairing of homologous chromosomes (synapsis) cannot
occur in meiosis. E.g
 The sterile mule (2n = 63) results from a cross between a horse (2n = 60) and a donkey
(2n = 66).
 A horse and a zebra can interbreed to form a sterile zebroid.
Question ( 2000. No. 4). Describe how new species of organisms may arise
(process of speciation) (20 marks)
In allopatric specia琀椀on, a popula琀椀on becomes split into two or more separate
groups/demes; by a geographical barrier; e.g amountain range, water body etc;
with each group/deme having its own gene pool; as a result of geographical
isola琀椀on, no inter-breeding between the separate groups takes place and gene
昀氀ow stops; because of di昀昀erence in enviromental condi琀椀ons on either sides of
the barrier, each deme independently experiences gene di昀昀eren琀椀a琀椀on through
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muta琀椀ons, gene琀椀c dri昀琀 and selec琀椀ons; and diversify gene琀椀cally hence evolving
along its separate line to a dis琀椀nct species; such that if the two dis琀椀nct
popula琀椀ons are brought together again, they cannot interbreed successifully;
due to fundamental gene琀椀c di昀昀erences (gene琀椀c isola琀椀on) and reproduc琀椀ve
isola琀椀on.
Sympatric specia琀椀on occurs when organisms occupy the same geographical
area; but gene 昀氀ow is restricted between par琀椀cular groups by reproduc琀椀ve
isola琀椀on mechanisms; resul琀椀ng into failure to interbreed successifully;
Reproduc琀椀ve isola琀椀on may be due to;
- lack of a琀琀rac琀椀on between males and females; cou琀椀ship behavior of one
group failing to s琀椀mulate the other to sexual ac琀椀vity; called behavioural
isola琀椀on;
- incompa琀椀bility of genetalia hence copula琀椀on cannot take place; called
mechanical isola琀椀on.
- sub- popula琀椀ons maturing at di昀昀erent 琀椀mes of the year and therefore
breeding seasons do not coincide; called seasonal isola琀椀on.
- popula琀椀ons occupying di昀昀erent ecological niches called ecological/habitat
isola琀椀on
- Physiological incompa琀椀bility.
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Ex琀椀nc琀椀on of species
Ex琀椀nc琀椀on is the cessa琀椀on of existence of a species or group of taxa in the environment. It can
also mean the total loss of a gene from a gene pool.
A species may become ex琀椀nct when it fails to adapt to a change in the environment and
therefore selected against by the environmental selec琀椀on pressure. Factors can drive a species
to ex琀椀nc琀椀on include:







Habitant destruc琀椀on and fragmenta琀椀on
Overhun琀椀ng/poaching and indiscrimina琀椀ve 昀椀shing
Pollu琀椀on
Widespread use of pes琀椀cides
Agricultural intensi昀椀ca琀椀on
Preda琀椀on /introduc琀椀on of alien species
Natural calami琀椀es such as dras琀椀c clima琀椀c condi琀椀ons which may cause massive death of
the representa琀椀ve members.
Explain how each of the above factors can drive a species to ex琀椀nc琀椀on.
Assignment
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Explain the role of the following in the process of evolu琀椀on/specia琀椀on.
(i)
Natural selec琀椀on (05 marks)
Natural selec琀椀on eliminates weak/poorly adapted members of a popula琀椀on; and
allows those with good adapta琀椀ons to survive; reproduce and pass their good
adapta琀椀ons to a new genera琀椀on; Over several genera琀椀ons, it allows accumula琀椀on of
good varia琀椀ons in the popula琀椀on; which results into emergence of new species;
when the parent popula琀椀on and the new popula琀椀on become reproduc琀椀vely
isolated;
(ii)
Reproduc琀椀ve isola琀椀on (05 marks)
- Prevents interbreeding between members of a popula琀椀on; hence restric琀椀ng gene 昀氀ow
between them; over 琀椀me, the isolated groups accumulate su昀케cient gene琀椀c di昀昀erences;
through muta琀椀ons and selec琀椀ons; which make them become dis琀椀nct species;
(iii)
Geographical isola琀椀on;
(05 marks)
Two or more groups of a popula琀椀on/ demes; become separated by a physical
boundary; which prevents interbreeding between members of the two groups;
restric琀椀ng gene 昀氀ow; large gene琀椀c di昀昀erences arise in the two separate popula琀椀ons;
due to muta琀椀ons, gene琀椀c dri昀琀 and selec琀椀ons occurring independently; making each
popula琀椀on to evolve along its own path resul琀椀ng into new species which cannot
interbreed even when reunited.
(iv)
Polyploidy
Polyploidy increases the amount of cell DNA, gives rise to new combination of genes in
offsprings (in case the multiple chromosome sets are from different species), and fixes
heterozygosity; leading to genetic variation between offspring and parents; resulting into hybrid
vigour/good phenotypic variations such as increased resistance to diseases; that promote survival
of the offspring when natural selection operates and eliminates the parents hence causing
evolution.
(v)
Meiosis
END
NEVER FEAR TO WALK THE JOURNEY ALONE!!!!
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