6.1.2 Patterns of inheritance
Specification summary
(a) (i) the contribution of both environmental and genetic factors to phenotypic variation
(a) (ii) how sexual reproduction can lead to genetic variation within a species
(b) (i) genetic diagrams to show patterns of inheritance
(b) (ii) the use of phenotypic ratios to identify linkage (autosomal and sex linkage) and epistasis
(c) using the chi-squared (χ2) test to determine the
significance of the difference between observed and expected results
(d) the genetic basis of continuous and discontinuous variation
(e) the factors that can affect the evolution of a species
(f) the use of the Hardy–Weinberg principle to calculate allele frequencies in populations
(g) the role of isolating mechanisms in the evolution of new species
(h) (i) the principles of artificial selection and its uses
(h)(ii) the ethical considerations surrounding the use of artificial selection.
1
(a) (i) the contribution of both environmental and genetic factors to phenotypic
variation
Keywords: interspecific, intraspecific, variation
Lesson objectives:
Recap inter and intraspecific, continuous and
discontinuous variation.
Define keywords for genetics
Explain how genes and the environment affect
different organisms
Link to 4.2.2
Types of variation
What is the difference between intra
and interspecific variation
(year 12 4.2.2)
Interspecific variation
4.2.2 Types of
variation
The differences between different species
Intraspecific variation
4.2.2 Types of
variation
The differences between individuals of the same
species
Easy way to remember…
4.2.2 Types of
variation
Interspecific – Different species
Intraspecific – Same species
Line up in height order
Ext. Why are you all different heights?
4.2.2 Types of
variation
4.2.2 Types of
variation
Get into groups of similar eye colour
Ext. Could you get into a line of eye colour and
give a reason for your answer
Continuous variation
4.2.2 Types of
variation
Quantitative - Any feature that can be measured.
Controlled by both genes and the environment
e.g. height, length of leaves, length of stalk, number of
flagellum
Tongue rolling
4.2.2 Types of
variation
Discontinuous variation
4.2.2 Types of
variation
Qualitative - Any feature that can’t be measured.
Controlled by genes
e.g. blood group, eye colour, shape of bacteria
Comparing continuous and
discontinuous variation
4.2.2 Types of
variation
There are now over 7 billion people in the
world, what makes us different?
Keywords you should have used…
DNA
Genes
Alleles
Proteins
Amino acids
Base sequence
Environment
Keyword definitions
Allele- form of a gene
Genotype –the alleles an organism has
Phenotype – the characteristics displayed by an
organism
Allele- form of a gene
E.g.
Genotype- the genetic
make-up – the alleles
an organism has
Phenotype- the
observable
characteristics
Spec link 2.1.6
How does sexual reproduction lead to
variation within genotypes
• Crossing over
• Independent assortment
• Random fertilisation
Skin colour
Continuous (controlled by many genes)
POLYGENIC
Colour of violet flowers
Discontinuous (controlled by one gene)
MONOGENIC
Environment
Phenotypic variation can be affected by the
environment:
Diet
Climate
Lifestyle
Light
Nutrients
Etiolation
Plants grow abnormally long and spindly – due
to not enough light
Chlorosis
Plants do not produce enough chlorophyll.
Due to a lack of magnesium ( Iron can also cause
this)
Genes and the environment
Height- you might have the genes to be 6’2 but
whether or not you reach that height depends
on your diet.
Genes and the environment
Melanin production is partially controlled by genes
but also the amount of sunlight that a person is
exposed to.
Debate
Obesity – environment or genes?
Obesity
Environment: Availability of food
Humans stop eating when dopamine levels
reach a certain level
Genes : people with one particular allele have
30% fewer dopamine receptors - overeat
Twin studies
Identical
Non - identical
Complete Qs 20.1 in your booklet
2
(a) (ii) how sexual reproduction can lead to genetic variation within a species
Keywords: gamete, haploid, diploid
Lesson objectives:
State 3 ways how meiosis creates genetic
variation
Describe the stages of meiosis
Spec link 2.1.6
Can you talk about this image for more
than 60 seconds?
Use stopwatches
to time each other
Spec link 2.1.6
Words I expect you to have used…
Egg
Sperm
Fertilisation
Gamete
Haploid
Chromosomes
Genetic
Genes
• How many chromosomes are in a
diploid cell?
• How many are in a haploid cell?
Spec link 2.1.6
Creating genetic variation
1. Crossing over
Chromatids twist around
each other and swap.
Contain the same genes
but different alleles.
Spec link 2.1.6
Creating genetic variation
2. Independent segregation of chromosomes
Chromosomes align on the metaphase plate randomly
during metaphase. One from each pair passes into a
daughter cell.
There are
8.5million
combinations.
Spec link 2.1.6
Creating genetic variation
3. Random fertilisation
• Any sperm can fertilise any egg.
• Each individual is unlike any to have existed before or
any that will exist again.
Spec link 2.1.6
The process of Meiosis
Which are haploid?
Which are diploid?
Spec link 2.1.6
Interphase DNA duplicates during
S phase
Spec link 2.1.6
Meiosis I (first division)
Each chromosome is made up of 2 sister
chromatids
Genetic recombination takes place
At random -Chromosomes cross over
and swap blocks of genes
Chiasma
Homologous chromosomes segregate
into 2 nuclei.
Nuclear envelope forming
Prophase 1, metaphase 1, anaphase 1, telophase 1
Spec link 2.1.6
Meiosis II (Second division)
Daughter chromosomes separate
Independent assortment:
4 haploid gametes formed
Prophase 2, metaphase 2, anaphase 2, telophase 2
Spec link 2.1.6
Gametes
(how many chromosomes at each stage?)
Summary
1. Why is it essential that gametes are haploid?
2. Describe 3 ways that genetic variation is
achieved
Complete Qs 20.2 in your booklet
3
(b) (i) genetic diagrams to show patterns of inheritance
Keywords: monogenetic, dihybrid
Lesson objectives:
Define keywords: Gene, Dominant, Recessive,
Homozygous, Heterozygous
Use a punnett square to find a phenotypic ratio
Draw genetic diagrams for monogenetic inheritance
including co-dominance
Keywords you should know from GCSE
What do they mean?
Gene
Dominant
Recessive
Homozygous
Heterozygous
Locus
5 mins to write your
own definitions for
these
Gene – sequence of nucleotide bases
that codes for a protein (polypeptides)
Dominant – allele that
always expresses if
present
Recessive – allele that
does not express (unless
they are both recessive)
Homozygous – the two
alleles are the same
Heterozygous – The two
alleles are different
heterozygous
(different)
homozygous
(Same)
Ext. Which other combination is homozygous?
Locus – where on the chromosome the
alleles are
b B
B B
b b
Monogenetic inheritance
The inheritance of characteristic controlled by a
single gene
e.g. Wing length in fruit flies
NN
nn
Punnett squares – remember GCSE?
Crossing homozygous parents
Crossing heterozygous parents
Phenotypic ratio
3:1
Normal : Vestigial
This is the expected ratio.
In practice it is likely to be slightly
different.
Monogenetic inheritance of
codominant alleles
Codominance – both alleles are expressed and neither one is recessive.
Normal haemoglobin
Sickle-cell trait
Sickle-cell anaemia
Phenotypic
ratio
1
:
2
:
1
How does this occur?
Co-dominant alleles
Not the usual one dominant, one recessive.
Both alleles are dominant
Eg. Snapdragons
CR CR = red
CW CW = white
CR CW = pink
a) Draw a genetic cross diagram for crossing a red snap
dragon (CR CR )with a white snap dragon (CWCW) what
colour are the offspring? ( this is called the F1
generation)
b) Draw a genetic cross diagram when two of the
offspring are crossed, what colour are the offspring?
(F2 generation)
CR
CR
CR
CW
CRCR
CRCW
CW
CRCW CRCW
CR
CW
CRCW
CW CRCW CWCW
CRCW
100% Pink flowers
25% Red
50% Pink
25% White
Practice question 1
Practice question 2
Dihybrid inheritance
The inheritance of two characteristics controlled
by different genes.
The classic example uses peas…
• Which are dominant?
• What are the results if you cross two heterozygous
parents?
Work out the alleles that the gametes
would have
Ext. Can you predict the outcome of the offspring?
Work out the possible offspring
Ext. What are the phenotypes?
RrYy x RrYy (F1 cross)
Phenotypes
Phenotypes
Ext. What is the phenotypic ratio?
Dihybrid cross practice question
4
(b) (i) genetic diagrams to show patterns of inheritance
Keywords: allele, immunoglobin
Lesson objectives:
Multiple alleles
Practice questions
What about blood?!
Multiple alleles
1. What are the 4 blood groups?
A, B, AB, O
Fun fact!
2. How many alleles are there?
I stands for
Immunoglobulin
A
B
O
3–I I I
3. What are the allele combinations of each
blood group?
A = IAIA or IAIO
B = IBIB or IBIO
AB = IAIB
O = IOIO
Need to know
IA and IB are co-dominant
Io is recessive to both.
Q. Draw a genetic cross for a child produced by some one who is
blood group AB with someone who is blood group O.
Write the probability of the child having each blood group.
IA
IB
IO
IA IO
IBIO
IO
IA IO
IBIO
50% group A
50% group B
Ext. A man claims that he is not the father of a child, he is
blood group O. The child has blood group AB, the mother
is blood group A. Could he be the father?
Complete Qs 20.3 in your booklet
5
(b) (ii) the use of phenotypic ratios to identify linkage (autosomal and sex
linkage) and epistasis
Keywords: phenotypic, autosomal, epistasis
Lesson objectives:
Sex-linkage
Linkage of autosomal genes
epistasis
Sex Linkage
Draw a genetic cross diagram to
represent the inheritance of sex
Sex linkage
Any gene that is carried on the X or Y chromosome is said to be
‘sex linked’.
Recessive conditions on the sex chromosomes are more likely to
appear in men as there is no dominant allele on the X
Sex linked genetic disorders
• Haemophilia
• Colour blindness
• Duchenne muscular dystrophy
Colour blindness example
3:1 offspring without colour blindness : offspring with colour blindness
Or
2:1:1 females without: males without: males with
What if you had a female carrier and a
male with colour blindness?
Phenotypic ratio:
1:1
With colour blindness : without colour blindness
Linkage of autosomal genes
Autosome – chromosome that is not a sex
chromosome
Genes on the same chromosome are ‘linked’
Crossing over during meiosis
(prophase I)
The closer the genes the less likely it is that they will
be separated.
Independent assortment during
meiosis I
Genes on the same autosome are said to be
‘linked’ as they stay together during independent
assortment.
Autosomal linkage and phenotypic
ratios
• If two genes are autosomally linked, you won’t
get the phenotypic ratio you expect.
• For dihybrid you would usually expect
(9:3:3:1)
• If two genes are inherited together it is more
like a monohybrid cross (3:1)
Practice question
In corn plants, the allele for glossy leaves (G)is dominant to the gene for normal leaves
(g) and the gene for branching of ears (B) is dominant to the gene for no branching
(b). A cross is carried out between a plant that is heterozygous for glossy leaves and
branching of ears (GgBb) and a plant that is homozygous recessive (ggbb).
a) Use a genetic diagram to work out the expected phenotypic ratio in the offspring
b) The results of the cross are shown below. What is the observed phenotypic ratio
in the offspring?
Number of
offspring
c)
Glossy leaves, lots of branching (GgBb)
126
Glossy leaves, no branching (Ggbb)
81
Normal leaves, lots of branching (ggBb)
74
Normal leaves, no branching (ggbb)
133
Suggest why the observed ratio is different from the expected ratio.
Answers
Fast recall questions
1.
What is the probability of having a female child?
50%
2.
3.
Some characteristics are sex- linked, what does this mean?
Allele for the characteristic is on a sex
chromosome (X or Y)
Why are X linked disorders more common in males than females?
Males only have one X chromosome and express the characteristic
on that chromosome whether it is dominant or recessive
4.
What is an autosome?
A chromosome that is not a sex chromosome
5.
Why are genes on the same autosome said to be linked?
Stay together during independent assortment of chromosomes
during meiosis I (unless crossing over splints them up first)
Epistasis
Epistasis – when one gene masks or supresses
the expression of another
Classic example –Widow’s peak
Widow’s peak is a V shaped hair growth.
In humans gene 1 controls Widow’s peak, gene 2
controls baldness.
If you have the gene for baldness then it doesn’t matter
if you have the gene for Widow’s peak as you have no
hair!
The baldness gene ‘masks’ the Widow’s peak gene
Recessive epistatic alleles example –
flower colour
Gene 1 codes for a yellow pigment (Y) and gene 2 codes for an
enzyme that turns the yellow pigment orange (R)
If you don’t have the dominant Y it doesn’t matter if you have
the R allele as the flower will be colourless. Gene 1 is epistatic to
gene 2 as it can mask the expression of gene 2.
Crossing homozygous parents will give a 9:3:4 (orange: yellow:
white) phenotypic ratio in the F2 generation
Work out the gamete alleles
Possible offspring
Dominant epistatic alleles
If the epistatic allele is dominant, then having
one copy of it will mask the expression of the
other gene.
Crossing homozygous parents will result in a
12:3:1 phenotypic ratio in the F2 generation
Squash colour
The no-colour white allele (W) is dominant over the colour allele (w)
The yellow allele (Y) is dominant over the green allele (y)
If the plant has at least one W it will be white
Complete the cross.
Possible offspring
6
Summary
Complete the cards in the circus
Complete the dingbats for genetics keyterms
One fourth of the offspring will be homozygous
dominant (SS), one half will be heterozygous (Ss),
and one fourth will be homozygous recessive (ss)
Mendel first proposed that alleles
segregate from one another during
the formation of gametes.
All of the F1 plants were true hybrids with a
phenotype of Ss. The recessive trait reappears
in the F2 generation
Only 25% of F2 plants will have the
recessive phenotype.
Among the F2 plants of a Yy x Yy cross, 25%
will be yy with the recessive, green-seeded
phenotype.
The F1 plants are all Tt hybrids. The recessive
trait (tt) reappears in the F2 generation in
about 25% of the plants.
A cross with the homozygous recessive (yy) is
a test cross. If the parent of unknown
genotype is heterozygous (Yy), half of the
offspring will have the recessive trait. The
unknown genotype could also be determined
by a cross with a known heterozygote (Yy)
The heterozygous offspring (Tt) would be of
intermediate height.
Tt x Tt = TT, Tt, Tt and tt
The features of crosses involving incomplete dominance
are intermediate phenotype of heterozygous individuals,
and parental phenotypes reappear in F2 when
heterozygotes are crossed.
But if the man was type O rather than type B,
offspring of type B and type AB would not be
possible
The predicted segregation pattern in the F2
generation is 1/4 normal (homozygous), 1/2 Manx
phenotype (heterozygous), an 1/4 embryonic lethal
(homozygous for the Manx allele)
A cross between individuals that are of
genotype IAIBx ii can yield offspring that are
either IAi or IBi. Their blood type will be A or B
GENETICS KEYWORDS DINGBATS
SAY WHAT YOU SEE
i
7
(c) using the chi-squared (χ2) test to determine the
significance of the difference between observed and expected results
Keywords: chi squared, observed, expected
Lesson objectives:
objective
Chi-squared test
Why do we use it?
It measures the size of the difference between the observed and the expected
results
It helps us to determine whether the differences are significant or not.
It is used to test the null hypothesis
Null hypothesis – no significant difference between what we expect and
observe (differences are due to chance)
Worked example
Phenotype
Number of plants with trait
Yellow and round
169
Green and round
54
Yellow and wrinkled
51
Green and wrinkled
14
Assuming a 9:3:3:1 ratio, with 288 individuals, what would you expect for each category?
Step 1. Complete the table
O-E
(O-E)2
(O-E)2
E
162
7
49
0.30
54
54
0
0
0.00
Yellow,
wrinkled
51
54
-3
9
0.17
Green,
wrinkled
14
18
-4
16
0.88
Category
Observed
(O)
Expected (E)
yellow,
round
169
Green,
round
Χ2 =
1.35
Step 2. Degrees of freedom
4 categories
4-1 = 3
Phenotype
Number of plants with trait
Yellow and round
169
Green and round
54
Yellow and wrinkled
51
Green and wrinkled
14
Step 3.
Look up the value of Χ2 in a distribution table
No.
classes
Degrees
of
freedom
2
1
0.00
0.10
0.45
1.32
2.71
3.84
5.41
6.64
3
2
0.02
0.58
1.39
2.77
4.61
5.99
7.82
9.21
4
3
0.12
1.21
2.37
4.11
6.25
7.82
9.84
11.34
5
4
0.30
1.92
3.36
5.39
7.78
9.49
11.67
13.28
6
5
0.55
2.67
4.35
6.63
9.24
11.07
13.39
15.09
0.99
0.75
0.50
0.25
0.10
0.05
0.02
0.01
Probability that
deviation is due to
chance alone
Χ2
Accept null hypothesis. Any difference is due to
chance and not significant
(probability due to chance is greater than 5%)
Reject null hypothesis, difference is
significant and not due to chance
(probability due to chance is less
than 5%)
95% certain that result is not
due to chance (critical value
the cut off point!)
Step 4 – reject or accept null hypothesis
Critical value for 3 degrees of freedom = 7.82
Our value, 1.35, is smaller so difference between observed and
expected is due to chance and not significant
= accept null hypothesis
Complete Qs 20.4 in your booklet
8
(e) the factors that can affect the evolution of a species
Keywords: stabilising, directional
Lesson objectives:
Recall examples of continuous and
discontinuous variation
Describe stabilising selection and directional
selection
Stand in order of
height
Stand in order of shoe size
Get into groups of eye colour
Get into groups of those who can roll
their tongue and those who can’t
Get into groups of attached ear lobes
and free earlobes
No two people are the same – Why?
Continuous
variation
Discontinuous
variation
Can you give examples for each?
What is selection?
Reproductive success and allele
frequency
Most populations have a relatively
stable size.
All organisms produce more
offspring than can be supported
by the supply of food/light/space
There is competition between
members of the same species
Reproductive success and allele
frequency
Within any population there is a
gene pool with a wide variety of
alleles
Some individuals have
combinations of alleles that make
them better at surviving
Therefore reproduce and pass
their alleles on
Types of selection
Directional selection – individuals favoured in
one direction
Stabilising selection – average individuals
favoured
Ext. Can you think of any examples of these?
Directional selection
‘Individuals favoured in one direction’
What will happen if it gets colder?
If the temperature falls, the individuals with longer fur length are
at an advantage as they have better insulation against the cold.
There is a selection pressure favouring the animals with longer fur
so these animals are more likely to survive and thus reproduce.
Over several generations, the average fur length increases as more
young have inherited the genes for long fur.
When the mean fur length has reached the
most advantageous length, the selection pressure ceases.
Stabilising selection
In years when it is hot,
short fur is favoured.
In years when it is cold,
long fur is favoured.
When temperature remains constant, individuals at the extremes will
never be at an advantage. The mean fur length will be favoured.
How will this change the shape of the graph?
The mean will remain the same but there will be fewer
individuals at the extremes
9
(e) the factors that can affect the evolution of a species
Keywords: drift, bottleneck, founder
Lesson objectives:
Describe the following:
• Genetic bottleneck
• Founder effect
• Genetic drift
Genetic Bottlenecks
Genetic bottleneck summary
Original population
Large numbers of the population die
Reduced population – some alleles are lost from the
original population
Reproduction
New population- genetic diversity is greatly reduced.
The Elephant seal
The Founder Effect
Original population
Founder
population
New population
1. Draw your own founder effect diagrams
2. Link to genetic diversity
3. Think of reasons why the founder effect might occur
Original population
Founder
population
New population
Genetic drift
Genetic drift says that characteristics are passed
on by chance rather than due to factors that
affect the individuals ability to survive and
reproduce.
Fast recall questions
1. Define the term gene pool
The complete range of alleles in a population
2. Define the term allele frequency
How often an allele appears in a population
3. How are allele frequency and evolution related?
Evolution is the change in the frequency of an allele in a population over
time
4. Explain why variation is needed for evolution to take place
Individuals vary, some are better adapted to selection pressures than
others. Survive, reproduce and pass on alleles
5. Explain why the founder effect can lead to an increased
incidence of genetic disease.
Allele frequency is higher than in original population. If one of these
alleles represents a genetic disorder this would mean that it has an
increased incidence in the population.
Complete Qs 20.5 in your booklet
10
(f) the use of the Hardy–Weinberg principle to calculate allele frequencies in
populations
Keywords: H-W, allele frequency, phenotype,
genotype
Lesson objectives:
Describe the purpose of the H-W principle
Complete calculation questions
Key terms
Gene pool: All alleles of all genes of all individuals in a
population at any one time
Allelic frequency: The number of times an allele occurs
within a gene pool
Hardy- Weinberg principle
It predicts the proportion of dominant and recessive alleles in a
population. This stays the same provided that:
• No mutations arise
• The population is isolated (no alleles in or out)
• There is no selection ( all alleles are equally likely to be passed
on)
• The population is large
• Mating within the population is random
If a gene has two alleles, a dominant (A) and recessive (a)
Let A = p
Let a = q
p + q = 1.0 (100%)
There are 4 possible arrangements of the 2 alleles:
AA + Aa + aA + aa = 1.0
p2 + 2 pq + q2 = 1.0
Equations
2
p
2
q
+ 2pq + = 1.0
Use when given information about
phenotypes/genotypes
p + q = 1.0
Use when given information about
allele frequency
Using the calculation
1 in 25000 people display a recessive characteristic. Calculate the
frequency of the dominant allele. ( use a and A)
Recessive must have aa
q2 = 1/25000 or 0.00004
q = √0.00004 or 0.00063
p + q = 1.0 ( to work out p 1.0 – q)
1.0 – 0.00063 = 0.9937 (this is the frequency of allele A)
Ext. How many people are carriers?
Ext. How many people are carriers?
From HWP we know that the frequency of
heterozygotes is ‘2pq’
2 x 0.9937 x 0.00063 = 0.0125
125/10000 are heterozygous
This is 313 in 25000
Question 1
If 98 out of 200 individuals in a population
express the recessive phenotype, what percent
of the population would you predict would be
heterozygotes?
Answer 1
• 98/200 = (q2)
• 0.49 = q2
• 0.7 = q
• p+q=1
• p = 1 – 0.7
• p = 0.3
• 2pq = 2(0.3)(0.7) = 0.42 = 42% heterozygotes
Question 2
Your original population of 200 in Q1 was hit by a
tidal wave and 100 organisms were wiped out,
leaving 36 homozygous recessive out of the 100
survivors. If we assume that all individuals were
equally likely to be wiped out, how did the tidal
wave affect the predicted frequencies of the alleles
in the population?
Calculate the predicted heterozygous population
and homozygous dominant
Answer 2
• 36/100 = q2
• 0.6 = q
• p+q=1
• p = 0.4
• Heterozygous = 2 (0.4)(0.6) = 0.48 = 48%
• Homozygous dominant = (0.4)(0.4) = 0.16 = 16%
Question 3
Lets say that brown fur coloring is dominant to
grey fur colour in mice. If you have 168 brown
mice in a population of 200 mice........
What is the predicted frequency of
– Homozygous dominants
– Heterozygotes
– Homozygous recessives
Answer 3
•
•
•
•
•
•
200 mice in total
168 = brown = p2 + 2pq
32/200 = grey fur = q2
0.16 = q2
0.4 = q
p = 0.6 (p + q = 1)
• p2 = 0.36 = 36%
• 2pq = 0.48 = 48%
• q2 = 0.16 = 16%
Starter – define the keywords
Gene pool
Total number of alleles in a particular
population at a specific time
Allele
The form of a gene
Phenotype
Genotype
The characteristics of an organism
The genetic composition of an organism – the
alleles it posseses
11
(g) the role of isolating mechanisms in the evolution of new species
Keywords: geographic, allopatric, sympatric
Lesson objectives:
Describe the process of allopatric speciation
Compare allopatric and sympatric speciation
What is a species?
A group of organisms that can interbreed to
produce fertile offspring
Speciation
• The development of a new species
• Occurs when populations of the same species become
reproductively isolated
• Changes in allele frequency lead to a change in phenotype
• Can no longer interbreed
Meet the Wibbleys
Oh no a mountain!
Geographic
Isolation
Mutation
Mutation
Separate gene pools, no interbreeding between the 2
new populations
Years pass…
Natural
selection
Hair is better for staying
warm
Blue is better for
camouflage in this
environment
• Variation due to mutation
• Different biotic/abiotic factors/ selection factors
• Differential reproductive success / (selected) organisms survive
and reproduce
• Leads to change in allele frequency
More years pass…
Mountain barrier is removed
New species have been created and cannot
breed together – called speciation
More years pass…
Mountain barrier is removed
New species have been created and cannot
breed together – called speciation
The Wibbley song!
http://www.youtube.com/watch?v=WDPsZPKS
EFg
Describe Allopatric speciation
Keywords to use:
Geographical isolation, variation, mutation,
selection, characteristic, allele frequency
Reproductive Isolation
Changes in alleles and phenotype of two populations
prevent them from successfully breeding together.
Seasonal changes – different flowering/mating seasons
Mechanical changes – changes in genitalia prevent
successful mating
Behavioural changes – different courtship rituals
developed.
Sympatric Speciation
Do not have to reproductively isolated to become reproductively
isolated
Random mutations could lead to reproductive isolation.
This is rare as it is difficult for sections or a population to be
reproductively isolated.
Remember key terms
Allopatric = Away from each other
Sympatric = Same place
AQA Jan 2011 8c
[5 marks]
Mark scheme
1. Geographical isolation;
2. Separate gene pools / no interbreeding (between populations);
3. Variation due to mutation;
4. Different environmental/abiotic/biotic conditions / selection
pressures;
5. Selection for different/advantageous,
features/characteristics/mutation/ /allele;
6. Differential reproductive success / (selected) organisms survive and
reproduce;
7. Leads to change in allele frequency;
8. Occurs over a long period of time;
Complete Qs 20.6 in your booklet
12
(h) (i) the principles of artificial selection and its uses
Keywords: artificial, inbreeding, gene pool
Lesson objectives:
Describe the process of artificial selection
Link to examples
Consider ethical issues
Selective breeding (artificial selection)
Who started it?
Imagine you are a caveman or cavewoman
You found one of these:
Which will grow up into one of these:
This might end well for you and your
family..
Or less well:
So you keep the friendly ones and deal
with the ones that are dangerous
You choose to breed together the friendly ones
and over time you create a new species- the
dog.
Some of these animals look pretty much like a
wolf still-
Some not so much:
Some not at all:
In fact by choosing the parents we
have created dogs for many different
purposes:
Pomsky
Bullshit
Jackshit
Cockapoo
We have also selectively bred many
other species:
From this
To this
This
To this
And it has been the same for plants
species as well
Wild bananas are full of seeds, modern
bananas are sterile
In fact we can really mess about with
plants
And animals…
Artificial selection
Humans select individuals in a population to
breed together to get desirable traits
Which characteristics are desirable for
Chickens?
Which characteristics are desirable for
Chickens?
• Have little fat
and lots of
muscle.
• Large muscular
legs.
• Grow fast.
• Produce large
numbers of
eggs.
• Produce eggs of
a similar size.
• Produce eggs
that are an even
colour.
Dairy cattle
How are modern dairy cattle artificially selected?
Female with high milk yield
+
Male whose mother has a high milk yield
Breed them together
Select offspring with highest milk yield and breed
together
Repeat!
Other characteristics to consider…
• Milk quality
• Lactation period (how long cow produces milk
for)
• Large udders (milking process is easier)
• Resistance to mastasis (inflammation)
• Calm temperament
Wheat example
Wheat
Large ear plant bred with large ear plant
( high yielding)
Offspring selected with large ears and
bread together
Process repeated over generations to
produce very large ears = very high yield
Other characteristics we like wheat to
have:
• Tolerance to cold
• Short stalks (sturdier and more
energy goes into the ears!)
• Uniform stalk height = easier harvest
Problems with selective breeding
• Can cause health problems. Certain traits may be
exaggerated
• Reduces genetic diversity / reduced gene pool
– more susceptible to genetic disease
– Potentially useful alleles for the future are lost
Pugs– nasal passage too short so
difficulty breathing
Chinese Sher-Pei – prone to skin
rashes – the folds are ideal to harbour
bacteria
Common problems caused by artificial
selection
Breed of dog
Conditions to which the breed is susceptible
Boxer
Caner and heart disease
German Shepherd
Heart disease, cancer, lack of digestive enzymes, skin
infection
Cocker Spaniel
Inflammation of ear, glaucoma,
Bulldog/ Pekingese
Breathing, hip and joint problems
Dalmatian
Deafness, heart disease skin infections
(h)(ii) the ethical considerations surrounding the use of artificial selection.
Domesticated animals more docile
Less able to defend themselves, easy prey
Livestock animals more lean
Less fat so in colder weather need to be housed
Inbreeding
More susceptible to disease
Colour of coats
Loss of camouflage
Fast recall questions
1. Define artificial selection
Humans select individuals with desirable traits to breed together
2. Outline an example of selective breeding in plants
Bread wheat – selectively bred to have large yield/ large ears/ high tolerance
to cold/ short stalks/ uniform stalk height
3. Describe two potential problems associated with selective breeding
Reduce gene pool of a species, could lead to problems in the future with
resistance to new strains of pathogen, can also lead to health issues that are
unforeseen.
13
Revision
Year 12 revision
‘genetic biodiversity’
4.2.1 Biodiversity
Genetic biodiversity – the variation of
alleles within a species
The importance of genetic diversity
Individuals of the same species have the same
genes but they may have different versions of
the genes ( alleles).
The importance of genetic diversity
If genetic diversity is low, then the species may
be more susceptible to changes in the
environment. The whole population could then
be wiped out by a single event or disease.
Elephant Seal hunted for their blubber to
use as oil in 1900’s until ~100 left.
Now ~100,000 but have very low genetic
diversity
More Genetic diversity key terms
Polymorphism: when a gene has more than one
allele. eg. Hair colour, eye colour and blood
type.
Monomorphism: When a gene has only one
allele. Most genes are ‘monomorphic’ this
ensures that the basic structure of a species is
similar
Locus – where on the chromosome the
alleles are
b B
B B
b b
Calculating genetic diversity
Producing gametes
Brown
Blue
Blue
B b
B
Blue
b b
b
b
b
Drawing a genetic cross
B
b
b
Bb
Bb
50% brown eyes
50% blue eyes
b
bb
bb
Q. Draw genetic diagrams for the following crosses and for
each state the probability of getting free earlobes and
attached earlobes
a) EE x ee
b) Ee x Ee
c) Ee x ee
E
E
E
e
E
e
Ee
Ee
E
EE
Ee
e
Ee
ee
e Ee
Ee
e Ee
ee
e Ee
ee
e
100% free earlobes
( all heterozygous)
75% free earlobes
25% attached
50% free earlobes
50% attached
Ext. Try describing each outcome using the keywords, recessive, dominant,
heterozygous, homozygous
Pedigree chart
Complete Qs on page 120 in textbook
Answers
1. Because the ancestors from whom they are
descended (edward VII and Victoria) did not have or
carry alleles for haemophilia
2. a) only appears in males
b) Parents without the disease have children who do (
give example)
3. a) XHXH b) XhY c) XHXh
4. Anastasia could have either XHXH or XHXh. Waldemar’s
gentoype must be XhY.
5. Sons could have XhY or XHY
Daughters must inherit Xh from father so possible
genotypes XHXh or XhXh
Question 4
• If 81% of a population is homozygous
recessive for a given trait. Calculate
– Frequency of homozygous dominant
– Frequency of heterozygotes
– Frequency of dominant and recessive alleles
Answer 4
• q2 = 0.81
• q = 0.9
• p = 0.1
• p2 = 0.01
• 2pq = 0.18
Question 5
• If 51% of the population carries at least one
copy of the recessive allele
– what is the predicted frequency of the population
expressing the dominant phenotype
Answer 5
•
•
•
•
51% = 2pq + q2
49% = 0.49 = p2
0.7 = p
0.3 = q
• p2 + 2pq =
• 0.49 + 0.42 = 0.91 have dominant phenotype
Question 6
• Albinism is a rare genetically inherited trait that is only
expressed in the phenotype of homozygous recessive
individuals (aa). The most characteristic symptom is a
marked deficiency in the skin and hair pigment
melanin. This condition can occur among any human
group as well as among other animal species. The
average human frequency of albinism in North America
is only about 1 in 20,000.
• calculate the frequency of the dominant allele in North
America
• the frequency of people expressing the normal
phenotype in
Answer 6
• q2 = 1/20,000
• q = 0.0071
• p = 0.9929
• dominant phenotype =
• p2 + 2pq = 0.9859 + 0.1409
Question 7
• 1 in 1700 US Caucasian new borns have cystic
fibrosis.
• calculate the frequency of the recessive cystic
fibrosis allele and the dominant allele in the
population
• calculate the frequency of non cystic fibrosis
sufferers in the population
Answer 7
• q2 = 1/1700
• q = 0.0243
• p = 0.09757
• p2 + 2pq
• (0.09757)(0.09757) + 2(0.09757)(0.0243)
• 0.9567
Question 8
• If 9% of an African population is born with a
severe form of sickle-cell anemia (ss), what
percentage of the population will be more
resistant to malaria because they are
heterozygous(Ss) for the sickle-cell gene?
Answer 8
•
•
•
•
q2 = 9%
q2 = 0.09
q = 0.3
p = 0.7
• 2pq = 2(0.3)(0.7) = 0.42 = 42%
Question 9
• The allele y occurs with a frequency of 0.8 in a
population of clams. Give the frequency of
• genotypes YY, Yy, and yy. Show your work!
Answer 9
• The allele y (recessive) has a frequency q =
0.8.
• p + q = 1, then p = 1 – 0.8 = 0.2
•
•
•
•
genotype:
YY genotype frequency = p2 = 0.04
Yy genotype frequency = 2pq = 0.32
yy genotype frequency = q2 = 0.64.
Question 10
• In the year 2374, humans finally developed the technology
necessary for time travels. You are a scientist interested in the
population genetics of extinct animals. Taking advantage of this
technological advance, you decide to go to the past 8 million years
to conduct a field work in Venezuela to study a population of
Phoberomys pattersoni*, the world’s largest extinct rodent
weighing approximately 700 kg (1500 lb) and looking vaguely like a
giant guinea pig.
• The coat color of this rodent varies between tan (dominant) and
brown (recessive). Assume the population is in Hardy-Weinberg
equilibrium. You observed 336 tan Phoberomys and 64 brown
Phoberomys during your study.
– What is the frequency of the homozygous recessive genotype
– What is the allelic frequency of the dominant (tan) allele in the
population?
– Of the animals you observed, how many were heterozygous?
Answer 10
• There are 336 + 64 = 400 animals in the population.
• 64 are homozygous recessive (brown)
• Frequency of homozygous recessive = q2 = 64/400 = 0.16
• Since q2 = 0.16, take the square root to get q = 0.4
• p + q = 1 (formula for allele frequencies)
• Frequency of the dominant allele p = 0.6
• Since q2 = 0.16, take the square root to get q = 0.4
• Remember that p + q = 1 (formula for allele frequencies)
• Frequency of the dominant allele p = 0.6
Question 12
• You make another trip to Venezuela and this
time you observe 650 animals.
– How many of the 650 animals would you expect
to be tan, assuming the population is still in
Hardy-Weinberg equilibrium?
– How many of these tan animals are homozygous
for the dominant allele?
– How many of these 650 animals would you expect
to be brown, assuming the population is still in
Hardy-Weinberg equilibrium?
Answer 12
• If the population is still in H-W equilibrium, then the allele
frequencies would be the same: p = 0.6, q = 0.4
• The tan phenotype = p2 + 2pq
• (0.6)2 + (2)*(0.6)*(0.4) = 0.84
• 0.84 * 650 = 546 tan
• p2 = (0.6)2 = 0.36,
• (0.36)*(650) = 234
• Brown animals are homozygous recessive
• Frequency of brown is q2 = (0.4)2 = 0.16
• (0.16)*(650) = 104