Port Said University
Faculty of Science
Zoology Department
Prepared by
Prof. Dr. Ali Hussein Abu Almaaty
Level: Second (Computing and Bioinformatics)
2024/2025
Code: BnfBi203
Introduction
**********************
Genetics is the study of genes, genetic variation, and heredity in living organisms. It
is generally considered a field of biology, but intersects frequently with many other
life sciences and is strongly linked with the study of information systems.
The discoverer of genetics is Gregor Mendel, a late 19th-century scientist and
Augustinian friar. Mendel studied "trait inheritance", patterns in the way traits are
handed down from parents to offspring. He observed that organisms (pea plants)
inherit traits by way of discrete "units of inheritance". This term, still used today, is a
somewhat ambiguous definition of what is referred to as a gene.
Trait inheritance and molecular inheritance mechanisms of genes are still primary
principles of genetics in the 21st century, but modern genetics has expanded beyond
inheritance to studying the function and behavior of genes. Gene structure and
function, variation, and distribution are studied within the context of the cell, the
organism (e.g. dominance), and within the context of a population. Genetics has
given rise to a number of subfields, including epigenetics and population genetics.
Organisms studied within the broad field span the domains of life (archaea, bacteria,
and eukarya).
Genetic processes work in combination with an organism's environment and
experiences to influence development and behavior, often referred to as nature versus
nurture. The intracellular or extracellular environment of a cell or organism may
switch gene transcription on or off. A classic example is two seeds of genetically
identical corn, one placed in a temperate climate and one in an arid climate. While the
average height of the two corn stalks may be genetically determined to be equal, the
one in the arid climate only grows to half the height of the one in the temperate
climate due to lack of water and nutrients in its environment.
2
Content
*********************
Subject
Page
1-Introduction………………/…..………………………………….2
2- Mendelian Genetics……………….…….....…………………….4
3- Monohybrid cross……………………………..…………….…...6
4- Dihybrid cross…...……………………………………...………..15
5- Family pedigree……….………………………………………….20
6- Incomplete Dominance……………………….…..…………..….26
7- Nonmendelian Genetics…………………………..………...…....27
8- Population Genetics……………………..…………………….....35
9- Linkage and crossing over…………..………………………......46
10- Cell division………………………...……………………….......68
11- Chromosomes………………………………..……….…..…......96
12- Mutations……………….…..…………………………….…....110
13- Chromosomal Aberration…………………….…..,……….....114
14- Chromosomal Disorder…………...…………………..……....127
15- Sex Determination……………………………………………..135
16- Nucleic Acids …………………………………………………..145
3
Mendelelian Genetics
*************************
**Gregor Mendel (1822-1884)
- Responsible for the Laws governing Inheritance of Traits.
- Austrian monk studied the inheritance of traits in pea plants Developed
the laws of inheritance.
- Mendel's work was not recognized until the turn of the 20th century.
- Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea
plants.
- He found that the plants' offspring retained traits of the parents.
- Called the “Father of Genetics"
**Mendel‟s Pea Plant Experiments
Why peas, Pisum sativum?
 Can be grown in a small area
 Plants have sexual processes very similar to animals.
 A pea plant has gametes and both male and female reproductive organs
 Produce lots of offspring
 Produce pure plants when allowed to self-pollinate several generations
 Can be artificially cross-pollinated
 He found 7 lines of peas that differed from each other for 7 distinct traits.
4
Seven Traits of Pea
5
How Mendel Began
Mendel produced pure strains by allowing the plants to self-pollinate for
several generations.
Mendel‟s Experimental Methods
- Mendel hand-pollinated flowers using paint brush.
- He could snip the stamens to prevent self-pollination.
- Covered each flower with a cloth bag.
- He traced traits through the several generations.
**Types of Genetic Crosses
1- Monohybrid cross - cross involving a single trait e.g. flower color.
2- Dihybrid cross - cross involving two traits e.g. flower color & plant height.
**Monohybrid Cross
• We are first going to look at what happens when plants with different
traits are crossed, then go through Mendel's explanation.
• Purple flowers vs. white flowers. The original parental lines are truebreeding, or pure-breeding. All offspring within the lines gave the same
flower color for an arbitrary number of generations.
First Cross
• True-breeding purple x true-breeding white. All offspring are purple.
The parent lines are the P generation; the offspring are the F1 (first filial)
generation.
6
• All the F1's are purple regardless of which parent (father or mother) was
purple and which was white.
• Note: no blending occurs. The purple F1 plants look exactly like the
purple parentals.
• We say that purple is dominant because it appears in the F1 hybrid.
White is recessive because it does not appear in the F1 hybrid.
Selfing the F1
• Self-pollinate the F1 plants to get the F2 (second filial) generation.
• The F2 appear in a ratio of 3/4 purple to 1/4 white.
• Note: white has re-appeared in the F2.
a.
Parental cross: PP x pp
Gamete from parent
with white flowers
p
Gamete from
parent with
purple flowers
All offspring
have purple
flowers
P
Pp
Mendel’s parental cross between true-breeding pea
plants with purple flowers and white flowers, producing
an F1 generation consisting of all purple-flowered plants.
Fig. 12-5a, p. 239
7
b.
F1 x F1 cross: Pp x Pp
Gametes from
one Pp F1 plant
P
Gametes from
another Pp
F1 plant
P
p
PP
Pp
Pp
pp
p
Mendel’s cross between F1 plants with purple flowers,
producing an F2 generation consisting of 3/4 purpleflowered and 1/4 white-flowered plants.
Fig. 12-5b, p. 239
Punnett Square
- The Punnett square diagram is an easy way to show the results of any cross.
- Used to help solve genetics problems.
8
**Explanation:
• Pea plants, like humans and most other higher organisms, are diploid:
there are 2 copies of each gene. One copy came from the father and the
other copy came from the mother.
• A gene can have many different versions, called alleles. In this case, the
flower color gene has a purple allele (symbolized P) and a white allele
(symbolized p).
• The true breeding lines are homozygous for the gene being examined:
both copies of the gene are the same allele. Thus in the initial cross, one
parent was PP and the other was pp.
• Hybrid lines are heterozygous: the two copies of the gene being examined
are different. A Pp plant is a heterozygote.
• When diploid organisms reproduce, they make gametes (sperm and eggs)
that are haploid: they have only 1 copy of each gene. Which copy goes
into the gamete is a random process.
• The male and female gametes combine at random to form zygotes, the
first diploid cells of the next generation
• In the case we are discussing, the PP plant produces P gametes and the pp
plant produces p gametes.
These gametes combine to form Pp F1
individuals.
• Since P (purple) is dominant to p (white), the Pp plants are purple.
• Thus there are 2 forms of purple plant: the PP homozygotes and the Pp
heterozygotes.
These plants have the same phenotype (physical
appearance) but different genotypes (genetic constitution).
Much of
genetics is an attempt to determine the relationship between phenotypes
and genotypes.
9
• When the F1 (Pp) plant makes gametes, each gamete gets either a P allele
or a p allele. This happens randomly, so 1/2 of the gametes are P and 1/2
are p.
• The gametes combine at random. So:
•
1/2 x 1/2 = 1/4 of the zygotes come from a P egg meeting a P sperm, giving
a PP F2 plant.
•
1/2 x 1/2 = 1/4 of the zygotes come from a p egg meeting a p sperm = pp
F2 plant.
•
1/2 x 1/2 = 1/4 are P egg meeting a p sperm, and 1/2 x 1/2 = 1/4 are p egg
meeting a P sperm. Both of these give a Pp F2 plant, so 1/2 of the F2 are
Pp.
• In summary, 1/4 of the F2 genotypes are PP, 1/2 are Pp, and 1/4 are pp.
• Since P (purple) is dominant, both PP and Pp plants are purple. Thus, 3/4
of the phenotypes are purple, and 1/4 is white.
• Mendel found that this rule worked for all 7 of his traits, to within what he
considered reasonable accuracy.
Mendel‟s first Law (Law of segregation
• The two members of a gene pair segregate randomly and equally into the
gametes, which then combine at random to form the next generation. Or
during the formation of gametes (eggs or sperm), the two alleles
responsible for a trait separate from each other.
• Alleles for a trait are then "recombined" at fertilization, producing the
genotype for the traits of the offspring.
01
Fig. 14-4
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
00
**Genetic Terminology
 Trait - any characteristic that can be passed from parent to offspring
 Heredity - passing of traits from parent to offspring
 Genetics - study of heredity
 Pollination: transfer of pollen (male gametes) from a male reproductive
organ to a female reproductive organ in a plant
 Fertilization: the male gamete unites with the female gamete to form a
zygote (a fertilized cell)
 Alleles - versions of gene have two forms of a gene (dominant & recessive)
 Dominant allele - stronger of two genes expressed in the hybrid;
represented by a capital letter (R)
 Recessive allele - gene that shows up less often in a cross; represented by a
lowercase letter (r)
 Genotype - gene combination for a trait (e.g. RR, Rr, rr)

Phenotype - the physical feature resulting from a genotype (e.g. red,
white)
 Homozygous genotype - gene combination involving 2 dominant or 2
recessive genes (e.g. RR or rr); also called pure.

Heterozygous genotype - gene combination of one dominant & one
recessive allele (e.g. Rr); also called hybrid
Dominance –
The ability of one allele to express its phenotype at the expense of an
alternate allele; the major form of interaction between alleles; generally the
dominant allele will make a gene product that the recessive can not; therefore
the dominant allele will express itself whenever it is present
02
The rule of dominance
 Dominant trait: “stronger” trait that shows up when the dominant allele is
present. Represented by a capital letter; B is for brown eyes
 Recessive trait: “weaker” trait that shows up only when the dominant allele
is not present. Represented by a lowercase letter; b is for blue eyes
Test cross –
The cross of any individual to a homozygous recessive parent; used to
determine if the individual is homozygous dominant or heterozygous
When you are trying to find out the genotype of an organism, you simply
cross it with a known pure recessive mate. If 100% of the F1 generation show
the dominant phenotype, the unknown organism was purebred for the dominant
trait. If 50% of the F1 generation was dominant and 50% was recessive, then
the unknown organism was a hybrid.
03
Back cross –
• A backcross involves mating the F1 hybrid to one of the homozygous
parental types. There are 2 possible backcrosses in the system we are
examining.
• Pp x PP. Back crossing to the dominant parent. The Pp plant will
produce 1/2 P gametes and 1/2 p gametes. The PP plant will produce only
P gametes. The offspring will thus be 1/2 PP and 1/2 Pp. Both of these
types are purple, so the result of a backcross to the dominant parent is all
offspring with the dominant type.
• Pp x pp. back crossing to the recessive parent. Again, the Pp parent
produces 1/2 P gametes and 1/2 p gametes, and the pp parent produces
only p gametes. The offspring will be 1/2 Pp (purple) and 1/2 pp (white).
04
Mendel's Dihybrid crosses
Law of independent assortment
(Second law)
*********************************************
• "Each pair of alleles segregates into gametes independently or
Genes for different traits are inherited independently from each
other."
or "For unlinked genes, the alleles from each gene
segregate into the gametes independently of one another."
 Mendel‟s experiments that followed the inheritance of flower color or
other characters focused on only a single character via monohybrid
crosses.
 He conduced other experiments in which he followed the inheritance of
two different characters, a dihybrid cross.
 In one dihybrid cross experiment, Mendel studied the inheritance of seed
color and seed shape.
 The allele for yellow seeds (Y) is dominant to the allele for green
seeds (y).
 The allele for round seeds (R) is dominant to the allele for wrinkled
seeds (r).
 Mendel crossed true-breeding plants that had yellow, round seeds (YYRR)
with true-breeding plants that has green, wrinkled seeds (yyrr).
 The resulting F1 dihybrid progeny were heterozygous for both traits
(RrYy) and had round yellow seeds, the dominant phenotypes.
 From the F1 generation, Mendel could not tell if the two characters were
inherited independently or not, so he allowed the F1 progeny to selfpollinate.
05
Mendel considered two alternate hypothesis:
Hypothesis 1: If the two characters segregate together, the F1 hybrids can only
produce the same two classes of gametes
(RY and ry) that they received from the parents and the F2 progeny will show a
3:1 phenotypic ratio.
Hypothesis 2: If the two characters segregate independently, the F1 hybrids will
produce four classes of gametes (RY, Ry, rY, ry)
And the F2 progeny will show a 9:3:3:1 ratio
Experiment: Mendel performed a dihybrid cross by allowing self-pollination of
the F1 plants (RrYy X RrYy)
Results: Mendel categorized the F2 progeny and determined a ratio of
315:108:101:32 which approximates 9:3:3:1.
Conclusion: The experimental results supported the hypothesis that each allele
pair segregates independently during gamete formation
 Mendel repeated the dihybrid cross experiment for other pairs of
characters and always observed a 9:3:3:1 phenotypic ration in the F2
generation.
 Each character appeared to be inherited independently.
 The independent assortment of each pair of alleles during gamete
formation is now called Mendel‟s law of independent assortment.
 One other aspect that you can notice in the dihybrid cross experiment is
that if you follow just one character, you will observe a 3:1 F2 ratio for
each, just as if this were a monohybrid cross.
 One possibility is that the two characters are transmitted from parents to
offspring as a package.
 The Y and R alleles and y and r alleles stay together.
 If this were the case, the F1 offspring would produce
yellow, round seeds.
06
 The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a
monohybrid cross.
 This was not consistent with Mendel‟s results.
 An alternative hypothesis is that the two pairs of alleles segregate
independently of each other.
 The presence of one specific allele for one trait has no impact on the
presence of a specific allele for the second trait.
 In our example, the F1 offspring would still produce yellow, round seeds.
 However, when the F1‟s produced gametes, genes would be packaged into
gametes with all possible allelic combinations.
 Four classes of gametes (YR, Yr, yR, and yr) would be produced in
equal amounts.
07
 When sperm with four classes of alleles and ova with four classes of
alleles
combined,
there
would
be
16
equally
probable
ways in which the alleles can combine in the F2 generation.
 These combinations produce four distinct phenotypes in a
9:3:3:1 ratio.
 This was consistent with Mendel‟s results.
 Mendel repeated the dihybrid cross experiment for other pairs of
characters and always observed a 9:3:3:1 phenotypic ration in the F2
generation.
 Each character appeared to be inherited independently.
 The independent assortment of each pair of alleles during gamete
formation is now called Mendel‟s law of independent assortment.
 One other aspect that you can notice in the dihybrid cross experiment is
that if you follow just one character, you will observe a 3:1 F2 ratio for
each, just as if this were a monohybrid cross.
08
09
Family Pedigree
***********************
Tracking Family History
Goals of Pedigrees
 A chart that shows multiple family generations and relationships to track
the inheritance of genetic traits.
 Can be used to determine genotypes of family members.
 Can be used to help predict probability of future generations expressing
certain traits.
 Important tool for genetic counselors
 Determine the mode of inheritance:
dominant, recessive, partial
dominance, sex-linked, autosomal, mitochondrial, maternal effect.
21
Basic Symbols
Y-Linked Inheritance
• We will now look at how various kinds of traits are inherited from a
pedigree point of view.
• Traits on the Y chromosome are only found in males, never in females.
• The father‟s traits are passed to all sons.
• Dominance is irrelevant: there is only 1 copy of each Y-linked gene
(hemizygous).
Mitochondrial Genes
• Mitochondria are only inherited from the mother.
• If a female has a mitochondrial
trait, all of her offspring inherit it.
• If a male has a mitochondrial trait,
none of his offspring inherit it.
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• Note that only 1 allele is present in each individual, so dominance is not an
issue.
• Outsider Rules
• In any pedigree there are people whose parents are unknown. These
people are called “outsiders”, and we need to make some assumptions
about their genotypes.
• Sometimes the assumptions are proved wrong when the outsiders have
children. Also, a given problem might specify the genotype of an outsider.
• Outsider rule for dominant pedigrees: affected outsiders are assumed to
be heterozygotes.
• Outsider rule for recessive pedigrees: unaffected (normal) outsiders are
assumed to be homozygotes.
• Both of these rules are derived from the observation that mutant alleles
are rare.
Maternal Effect Genes
• The maternal effect rule: “Mother‟s genotype determines offspring‟s
phenotype.”
• Assume that the trait is
recessive, in a complete
dominance situation.
• Also
assume
all
“outsiders” (people with
unknown parents) are
homozygous
for
the
allele they are expressing
: the dominant allele if they are unaffected, and the recessive allele if they
are affected
22
Sex-Influenced Trait
• Assume that the trait is dominant in males but recessive in females.
• Assume all outsiders are homozygotes.
• Thus:
– DD is always affected
– dd is always normal
– Dd is affected in males, but
normal in females
Sex-Limited Trait
• There are several possibilities for dominance, but for this problem assume
the trait is dominant but only expressed in males.
• Affected outsider males are heterozygous; unaffected males are
homozygous normal
• Assume that outsider females
are homozygous normal.
Sex-Linked Dominant
• Mothers pass their X‟s to both sons and daughters
• Fathers pass their X to daughters only.
• Normal outsider rule for dominant pedigrees for females, but for sexlinked traits remember that males are
hemizygous and express whichever gene
is on their X.
• XD = dominant mutant allele
23
• Xd = recessive normal allele
Sex-Linked Recessive
• males get their X from their mother
• fathers pass their X to daughters only
• females express it only if they get a copy from both parents.
• expressed in males if present
• recessive in females
• Outsider rule for recessives (only affects females in sex-linked situations):
normal outsiders are assumed to be homozygous.
Autosomal Dominant
• Assume affected outsiders are assumed to be heterozygotes.
• All unaffected individuals are homozygous for the normal recessive allele.
24
Autosomal Recessive
• All affected are homozygotes.
• Unaffected outsiders are
assumed to be homozygous
normal
• Consanguineous mating's
are often (but not always)
involved.
25
Incomplete Dominance
********************************
• incomplete dominance: one allele is not dominant over another
•
Occurs when hybrids have an appearance between the phenotypes of the
parental varieties.
▫ The hetereozygote is intermediate in phenotype
▫ Example
 The color between red and white is Pink
 phenotypic ratio of F2 are 1 red:2 pink:1 white
• Ex. Snapdragons
CWCW x CRCR
White x Red
F1
CWCR (Pink)
26
Genetic (Genes) Interaction
Non Mendelian Genetics
***********************************************
Gene interactions occur when two or more different genes influence the
outcome of a single trait.
**A Cross Involving a Two-Gene Interaction can still produce a 9:3:3:1 ratio.
Inheritance of comb morphology in chicken
 First example of gene interaction
 William
Bateson
and
Reginald
Punnett in 1906
 Four different comb morphologies
The crosses of Bateson and Punnett
 F2 generation consisted of chickens with four types of combs
 9 walnut : 3 rose : 3 pea : 1 single
 Bateson and Punnett reasoned that comb morphology is determined by
two different genes
 R (rose comb) is dominant to r
 P (pea comb) is dominant to p
 R and P are codominant (walnut comb)
 rrpp produces single comb
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Complementary Genes: (9 : 7)
Enzyme C and enzyme P cooperate to make a product,
therefore they complement one another.
**Cross between two pea plants different in colour of flowers.
• When at least one dominant gene at two loci must both be
present for the development of a characteristic.
• eg purple flowers on sweet peas require the presence of C and P.
The genotypes for purple flowers are CCPP, CCPp, CcPP or
CcPp
(also written as C-P-). Any other genotype produces white
flowers.
• There are only two possible phenotypes.
A Cross Producing a 9:7 ratio
9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp
purple
28
white
Duplicate genes (15:1)
 Enzyme 1 and enzyme 2 are redundant
 They both make product C, therefore they duplicate each other
**Cross between two pea plants different in shape of seed capsules
x
TTVV
Triangular
ttvv
Ovate
F1 generation
TtVv
All triangular
F1 (TtVv) x F1 (TtVv)
15:1 ratio results
TV
Tv
tV
tv
TV
Tv
tV
tv
TTVV
TTVv
TtVV
TtVv
TTVv
TTvv
TtVv
Ttvv
TtVV
TtVv
ttVV
ttVv
TtVv
Ttvv
ttVv
ttvv
(b) The crosses of Shull
Supplementary Genes
• When an allele at one locus is necessary for the expression of alleles at
another locus
• Mice homozygous for recessive allele c cannot produce melanin and are
albino.
• C- guinea pig are coloured.
• C second locus influences amount of pigment in hairs. B gives black, b
gives brown
29
• C-B- = black
• C-bb = brown
ccB- = albino
ccbb = albino
• The C/c locus is said to be at a higher level of importance than the B/b
locus since it „permits‟ the expression of the B and b alleles.
• There are three possible phenotypes
Cross between two Guinea pig different in coat colour
*There are three types of Guinea pig
Black (C- B-); Albino (ccB- or ccbb) and brown (C-bb)
*When occur cross between albino ccBB with brown CCbb, F1 is black CcBb
*When Two of F1 are mated, the offspring appear in the F2 in the ratio 9 black:
3 brown: 4 albinos
31
Epistasis
Epistatis occurs when one gene (epistatic) alters the expression of another
gene (hypostatic) or the phenotypic expression of one gene at one locus is
affected by another gene at a second locus.. The genes are independent of each
other.
Difference between Dominance and Epistasis:
Dominance involves intra-genetic or inter-allelic gene suppression, or the
masking effect which one allele has upon the expression of another allele at the
same locus.
Epistasis involves inter-geneic suppression or the masking effect which
one gene locus has upon the expression of another.
Dominant epistasis: (12:3:1)
Colour the feather in fowls
Cross between two fowls different in colour of feather
Allele I (inhibitor gene) prevent the expression of colour allele B
(hypostatic gene), the alleles of hypostatic gene locus (BB, Bb, or bb) express
only when two recessive alleles (ii) occur on epistatic locus.
White feathers
Brown feathers
IIBB
X
F1
iibb
IiBb
White feathers
When F1 mating with F1
IiBb
X
IiBb
30
F2
G
IB
Ib
iB
ib
IB
IIBB
IIBb
IiBB
IiBb
White
White
White
White
IIBb
IIbb
IiBb
Iibb
White
White
White
White
IiBB
IiBb
iiBB
iiBb
White
White
Black
Black
IiBb
Iibb
iiBb
iibb
White
White
Black
Brown
Ib
iB
ib
12 white : 3 black :1 brown
32
**Duplicate Genes with cumulative effect: (9:6:1)
9 means A-B6 means A-bb or aaB1 means aabb
Example:
In the summer squash (Cucurbita pepo) spherical fruit is recessive to disc.
Spherical races from different geographical region were crossed. The F1's were
disc and F2's segregate 35 disc, 25 spherical and 4 elongate. Explain these
results.
Answer
****************
AAbb (spherical) X aaBB (spherical)
F1 Aa Bb (disc)
Aa Bb (disc) X Aa Bb (disc)
33
G
AB
Ab
aB
ab
AB
AABB
Ab
AABb
aB
AaBB
ab
AaBb
disc
disc
disc
disc
AABb
AAbb
AaBb
Aabb
disc
spherical
disc
spherical
AaBB
AaBb
aaBB
aaBb
disc
disc
spherical
spherical
AaBb
Aabb
aaBb
aabb
disc
spherical
spherical
elongate
( 35)
(25)
(4)
6 spherical:
1 elongate
9 disc:
This is Duplicate Genes with cumulative effect: (9:6:1)
34
Population genetics
*****************************
The study of genes and alleles in entire populations
• Population = A group of organisms in one area that regularly mate
with each other
• If mating is random and both alleles of a gene are equally beneficial,
the frequency of each allele will stay the same from generation to
generation
√ Allele frequency = The percent of a gene in the population
that is one particular allele
Example allele frequency calculation:
A population is made up of 10 BB individuals, 10 Bb individuals, and
40 bb individuals
• The allele frequency of B is 25% (30 B alleles/120 total alleles)
• The allele frequency of b is 75% (90 b alleles/120 total alleles)
Hardy-Weinberg equilibrium
An equation that relates the genotypes in a population to the allele
frequencies
p2
% Homozygous
+
2pq
q2 =
+
% Heterozygous
dominant
% Homozygous
recessive
• p = The allele frequency of the dominant allele
• q = The allele frequency of the recessive allele
•p+q=1
35
1
**The chi-square test
– A goodness-of-fit test
– Works for count data if you know the expected
counts
– Null hypothesis is that the observed data do not
differ from the predicted data
χ2 = Σ d2/e
o = observed value,
d=o-e
e = expected value
The more the data deviates from the expected, the higher the chi-square
will be. A large enough chi-square value means that the observed data is a poor
fit to the expected data, leading you to reject the null.
dF = n-1 (f=degree of freedom and n= number of classes or traits)
36
Lethal Genes
*************************
Lethal allele (lethal gene)
The genes (A mutant form of a gene that eventually results in the death of
an organism if expressed in the phenotype) that cause the death of the organism
if find it in a "pure" either dominant or recessive because it stops a vitally
important activities which disrupt growth, which leads to the death of the
individual sooner or later.
Most lethal genes are recessive; for example, sickle-cell anaemia (see
polymorphism) results from a recessive lethal gene that causes the production of
abnormal and inefficient haemoglobin.
Recessive Lethal Genes (3 : 0)
Sickle cell anemia, blood cells (3:0)
The production of hemoglobin in humans is controlled by dominant gene
S and alternative recessive s prevents formation of hemoglobin naturally so it is
deadly.
Therefore, individuals clean SS (sound) because all the red blood cells
have hemoglobin disc. And individuals Ss hybrid (carrier of the disease), some
blood cells have their hemoglobin sickle, therefore, does not seem to have
symptoms of the disease only when lack of oxygen so is anemia, blood cells,
sickle (the case of the rule of non-full) which is characterized by models genetic
hybrid Boutrz I look clear when lack of oxygen only. Pure individual's ss (sick)
die before reaching puberty, because all the red blood cells have sickle
hemoglobin.
37
Dominant Lethal Genes (2: 1)
The fur color in mice (2:1)
Mice strains, namely: (yellow dominant gene Y deadly - gray recessive gene y is
not fatal.
Yy: yellow hybrid mice live
yy : gray pure mice live
YY: yellow pure mice dead
38
Multiple alleles
************************
When there are three or more alleles for a single gene as in ABO blood
groups.
The color of the fur rabbits (3 alternatives of gene) C. Ch. Ca
**There are three breeds of rabbits are:
1. Coloured: body as a whole has a specific colour (black or brown) With
dominant Gene C.
2. Himalayan: White body but the parties color (black) and Ch recessive gene for
colored cushions on the albino.
3. Albino: the entire body as well as Parties to be white and Ca recessive gene for
the Himalaya and coloured, so there are three phenotypes with 6 genotype:
Coloured - pure CC - hybrid or CCh Cca
Himalayan - pure or hybrid ChCh ChCa
Albino - always pure CaCa
Example: 1 - a series of alleles in the color of fur in rabbits
P1
Albino CaCa X CC colored agouty
CCa
Albino agouty
P2
CCa X CCa
CaCa: CCa: CC: CCa
Albino 1: Colorful 3
Himalaya X colored
P1
Chch X
CC
Cch
P2
Cch X Cch
ChCh 1: CCh 2: CC 1
Himalaya 1: Colorful 3
39
Codominance
The situation where the phenotypes of both alleles are exhibited in the
heterozygote. Hence two alleles are expressed in heterozygous individuals.
Example: blood type example (Multiple alleles and codominance)
There are three alleles of ABO blood type gene
IA
or A promotes blood type A
IB
or B promotes blood type B
i
or O promotes blood type O
1.
type A
= IAIA or IAi
2.
type B
= IBIB or IBi
3.
type AB
= IAIB
4.
type O
= ii
Codominance Problem
Example:
IB
B
I
homozygous male Type B (IBIB) X heterozygous female Type A (IAi)
IA
i
IAIB
IBi
IAIB
IBi
A B
1/2 = I I
B
1/2 = I i
Another Codominance Problem
• Example: male Type O (ii) x
female type AB (IAIB)
41
IA
IB
i
IAi
IBi
i
A
B
I i
A
1/2 = I i
B
1/2 = I i
I i
Question:
If a boy has a blood type O and his sister has blood type AB, what are the
genotypes and phenotypes of their parents?
boy - type O (ii) and girl - type AB (IAIB)
Answer:
IA
i
IB IAIB
i
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
40
copyright cmassengale
64
Sex-linked Traits
*************************************
A Traits (genes) located on a sex chromosomes. Sex linked traits express
themselves more in one sex than the other-often more in males than females
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on X chromosome
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
copyright cmassengale
66
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x XrXr
Remember: the Y chromosome in males does not carry traits.
RR = red eyed
Xr
Xr
Rr = red eyed
rr = white eyed
XY = male
XR
XX = female
Y
42
copyright cmassengale
67
Xr
Xr
XR
XR Xr XR Xr
Y
Xr Y
Xr Y
50% red eyed
female
50% white eyed
male
copyright cmassengale
Sex Influences traits
Sex-limited inheritance - only one sex can express phenotype
Sex-influenced inheritance - sex determines how phenotype is seen.
Autosomal genes involved and hormones involved
Example Pattern Baldness
Female
Male
bb
Bald
Bald
b+b
Not Bald
Bald
b+b+
Not Bald
Not Bald
43
68
44
POLYGENIC INHERITANCE
This type of inheritance involves .....“Many genes” influencing the
inheritance of one trait or characteristic
PLEIOTROPHIC INHERITANCE
This is simply the opposite of polygenic One gene influences the inheritance
of many characteristics...
Rh Factor
• Determines whether someone has positive or negative blood
• A protein antigen that is on the surface of blood cells and if that
antigen is present, the individual is positive
– A+; B+; O+; AB+
• If the antigen is not present, then the individual is negative
– A-; B-; O-; AB• If an RH-negative mother is exposed to blood from an Rh-positive
fetus, the mother‟s blood will produce antibodies that will attack the
blood of the fetus--potentially killing the unborn child.
• This is why, blood types should be determined before having children
• If, the male and female are negative, and positive, the mother must
receive medication to prevent her immune system from attacking the
child.
45
Linkage and Crossing over
********************************
When genes
are located on separate chromosomes, they
sort
independently of each other during meiosis. But what happens when genes are
located on the same chromosome?
The alleles for these genes would not tend to be sorted into gametes
independently since they would stay together during meiosis. The only way such
alleles can sort independently is if crossing over during meiosis separates them.
Linkage:
1- Tendency of genes of same chromosome to remain together.
2- Linked loci (genes) tend to be transmitted as a unit. Not Independently
assorted. Segregate together.
3- Because they are a group of genes linked together, chromosomes are
functionally linkage groups.
Linked Genes
F1
F2
Notice how linked genes can be written
46
Factors which effect on linkage
1- Age
2-Temperature
3- X-Rays
Crossing over (Genetic Recombination)
1- The exchange of genes between homologous chromosomes.
2- Non-sister chromatids of homologous chromosomes exchange DNA
segments
3- Occur in Eukaryotes but limited in Prokaryotes.
4- Crossing Over ensures a combination of the maternal and paternal genes
we inherited
5- Genes that are far from each other on a chromosome are more likely to be
separated by crossing-over than are genes that are close to each other.
6- Causes loci that are far apart on the same chromosome to sometimes
independently assort
7- Known as incomplete linkage
8- Crossing over (meiotic recombination)
9- Occurs during prophase I of meiosis at the bivalent stage
47
Steps of Crossing-over
1- Synapsis of homologous chromosomes –Zygotene
2- Tetrad formation
3- Crossing over-Pachytene
4- Disjunction
Factors which effect on crossing over
1- Temrperature
2- X-Rays
3- Chemicals
4- Age
5- Interference
6- Sex
Significance of Crossing-over
1- Provide genetic variation during meiosis and produces new combinations of
traits.
2- Forms raw material for evolution.
3- Establishes concept of linear arrangement of genes.
4- Helps to determine loci of genes in the chromosomes.
Example on linkage and crossing over
In the fruit fly (Drosophila): gene of Wing length L is dominant gene
linked with dominant gene of gray colour body G on the same chromosome, as
well as gene of wing reduced l is recessive gene linked with recessive gene g of
black colour body on the same chromosome. Explain the following crosse.
1- GLGL X glgl.
2- GLgl X glgl with crossing over occur.
48
The first
P1
GLGL
G
GL
X glgl
X
gl
F1
GLgl
Wing tall and gray body
P2
GLgl
X GLgl
F2
G
GL
gl
GL
Gl
GLGL
GLgl
Wing tall and gray body
Wing tall and gray body
GLgl
glgl
Wing tall and gray body
Wing reduced and black body
3 Wing tall and gray body : 1 Wing reduced and black body
The second
GLGL X glgl with crossing over occur.
P1
GLgl
X glgl
crossing over occur.
G
F1
GL Gl gL
GLgl
1
Glgl
: 1
gl X
gl
gLgl
1
:
:
glgl
1
Wing tall and gray body : Wing reduced and gray body: Wing tall and black body: Wing reduced and
black body
49
The allele for red flower colour (R) in a certain plant is incompletely
dominant with the allele for white flowers (R‟). Thus a plant with the
genotype RR‟ has pink flowers. Tall (D) is dominant to dwarf (d). What
would be the expected phenotypic ratio from a cross of RR‟dd plants with
R‟R‟Dd plants?
A.
9:3:3:1
B.
50 % pink 50 % white, and all tall
C.
1:1:1:1, in which 50 % are tall, 50 % dwarf, 50 % pink and 50 %
white
D.
3:1
51
Genetic problems:
1-In humans, brown eyes (B) are dominant over blue (b)*. A browneyed man marries a blue-eyed woman and they have three children,
two of whom are brown-eyed and one of whom is blue-eyed. Draw the
Punnett square that illustrates this marriage. What is the man‟s
genotype? What are the genotypes of the children?
P
Man
Bb
X
Wamen
bb
G
B b
X
b b
F1
Bb
Bb
bb
bb
2-Assume right-handedness (R) dominates over left-handedness (r) in
humans, and that brown eyes (B) are dominant over blue (b). A righthanded, blue-eyed man marries a right-handed, brown-eyed woman.
One of their two children is right-handed/blue-eyed, while the other is
left-handed/brown-eyed. The man marries again, and this time the
woman is right-handed and brown-eyed. They have 10 children, all
right-handed and brown-eyed. What are the genotypes of the husband
and two wives?
Realize that the father exhibited the dominant right-handedness trait
and the recessive blue-eyed trait. Thus his genotype is best given as
R_bb.
The first wife was right-handed and brown-eyed (dominant): R_B_.
The second wife was also right-handed and brown-eyed: R_B_
The first marriage yielded one right-handed, blue-eyed child (R_bb)
and one left-handed, brown-eyed child (rrBb).
The only way to get a left-handed child is if both parents offered a lefthanded allele, r. Therefore both parents must have been heterozygous
with respect to handedness (i.e., father = Rr; and first wife = Rr).
50
The only way to get a blue-eyed child is if both parents offered a blueeyed allele, b. Therefore the mother (first wife) must be heterozygous
(Bb) with respect to eye color. The father's eye color genotype was bb.
Therefore the genotypes must have been as follows:
Father: Rrbb
First Wife: RrBb
The second marriage yields 10 right-handed and brown-eyed children.
If she were heterozygous for either character, chances are some of the
offspring would have exhibited the recessive trait for the character,
because occasionally a recessive allele from the father would be
matched with a recessive allele from the second wife in the zygote.
However, none of the offspring exhibited the recessive trait for either
character. Therefore, the second wife must be homozygous dominant
for both characters (RRBB).
2-Yellow guinea pigs crossed with white ones always produce cream
colored offspring. Two cream colored guinea pigs when crossed
produced yellow, cream and white offspring in the ratio of l yellow: 2
cream: l white. How are these colors inherited?
This problem appears to be an example of incomplete dominance.
First define alleles:
Iy = yellow allele
Iw = white alllele
---------------------------------------------------------------Genotypes and their respective phenotypes:
Iy Iy (homozygous yellow alleles) = yellow
Iw Iw (homozygous white alleles) = white
52
Iy Iw (heterozygous) = cream
---------------------------------------------------------------Parental (P) Cross
Yellow x White
Iy Iy x Iw Iw
---------------------------------------------------------------F1 Generation
All (100%) Cream: Iy Iw (100% heterozygous)
---------------------------------------------------------------F2 Generation
Punnett Square Illustrating F1 Generation Cross:
F2 Generation Genotype/Phenotype Ratio:
Iy Iy : Iy Iw : Iw Iw
yellow : cream : white
1:2:1
53
3- In humans the blood groups are produced by various combinations
of three alleles IA, IB, and i. Blood type A is caused by
either IA IA or IA i; type B by IB IB or IB i; type AB by IA IB; and type O
by i i. Suppose a child is of blood type A and the mother is of type 0.
What type or types may the father belong to?
Since the mother can only provide alleles for O type blood (i), the
father must provide the allele for blood type A (IA). Three genotypes
can provide the IA allele: IA IA (blood type A), IA i (blood type A),
or IA IB (blood type AB). So the father must be either blood type A or
blood type AB. The child (with blood type A) must be
heterozygous, IA i (remember the O allele, i, is recessive to both the A
and the B alleles).
4-Suppose a father of blood type B and a mother of blood type O have
a child of type O. What are the chances that their next child will be
blood type O? Type B? Type A? Type AB?
Once again, because type O blood results from the homozygous
recessive genotype (i i ), the only way to produce a type O child is if
both parents provide an O allele (i ).
Since the father has blood type B, he must be heterozygous (IB i ).
Because the mother has blood type O, she must be homozygous for the
O allele (i i ).
Construct a Punnett square diagram to determine the possible
offspring of these two parents:
54
Based upon these results, we can see that these parents may produce
offspring with the following blood type phenotypes: B and O, each
with a 50% chance of occurring in their next child.
It would be impossible to produce children with either type A or type
AB blood. In such a situation, the mother could be in a lot of
trouble!
4- How many different kinds of gametes could the following
individuals produce?
1.
2.
3.
4.
5.
aaBb
CCDdee
AABbCcDD
MmNnOoPpQq
UUVVWWXXYYZz
1. aaBb
=2
2.
CCDdee
=2
3.
AABbCcDD
=4
4.
MmNnOoPpQq
= 32
5.
UUVVWWXXYYZz = 2
55
•
5-Sixteen percent of the human population is known to be able to
wiggle their ears.
• This trait is determined to be a recessive gene.
• These are a population genetics question.
• Use the following equation: 1 = p2 + 2pq + q2
• What of the population is homozygous dominant for this trait?
• q2 = 16% or .16:
• then use :
•
•
•
•
q2 = .16
q = .4
1=p+q
1 = p + .4
1- .4 = p
p = .6
Now use p2 for answer: .62 = .36 or 36%
What of the population is heterozygous for this trait?
We know that q = .4 and p = .6
Now use 2pq for answer: 2(.6)(.4) = .48 or 48%
56
Vocabulary:
1. *Trait - a heritable feature such as flower color (purple color
trait, white color trait etc.)
2. *True Breeding - plants that – when self-pollinated – always
produce the same phenotypic traits. (ie homozygous dominant
or homozygous recessive_
3. *hybridization: The mating or crossing of two true-breeding
varieties (true breeding parents- P generation - produce all
heterozygous F1 offspring
4. *law of segregation: Mendel‟s first law – stating that each allele
in a pair (diploid) separates into a different gamete (haploid)
during gamete formation
5. *Allele: alternate versions of a gene that produce different
phenotypes
6. *genotype: The genetic makeup or set of alleles ( ie. Aa, AA, aa)
of an organism
7. * phenotype: The physical traits which are determined by its
genotype. (Remember, many phenotypes are microscopic as in
the shape of a hemoglobin molecule in sickle cell)
8. * dominant allele: An allele that is fully expressed in the
phenotype of a heterozygote
9. * recessive allele: An allele whose phenotype is not observed in
the heterozygote
10. *homozygous: Having two identical alleles for a given gene
11. * heterozygous: Having two different alleles for a given gene
12. *monohybrid cross: The cross between two heterozygotes (F1
generation) for a single trait. Ie. Bb x Bb
13. *dihybrid cross: The cross between two heterozygous (F1
generation) for two traits Ie. YyRr x YyRr
14. *carrier: An individual who is heterozygous with one normal
allele and one potentially harmful recessive allele. The
individual is phenotypically normal but can pass on the
harmful allele.
15. *test cross: Breeding of an organism of an unknown
genotype (heterozygous or homozygous dominant) with a
homozygous recessive individual to determine the unknown
genotype. The ratio of phenotypes in the offspring determines
the unknown genotype
57
16. *law of independent assortment: Mendel‟s second law, that
each pair of alleles separates independently during gamete
formation. This law only when genes for two traits are located
on different chromosomes.
17. *complete dominance: The situation when the phenotypes of
the heterozygote and the dominant homozygote are
indistinguishable.
18. *codominance: The situation where the phenotypes of both
alleles are exhibited in the heterozygote
19. *incomplete dominance: The situation in which the phenotype
of heterozygotes in intermediate between the phenotypes of
individuals homozygous for either allele.
20. *multiple alleles: When there are three or more alleles for a
single gene as in ABO blood groups.
21. *polygenic inheritance: An additive effect of two or more
genes on the a single phenotypic trait
22. *pedigree analysis: predicting the genotypes of individuals in
a pedigree chart based on the phenotypes of the offspring.
23. *Sex-linked Traits: A gene located on a sex chromosome.
Sex linked traits express themselves more in one sex than the
other-often more in males than females
58
4.3 Theoretical genetics
4.3.1
Define genotype, phenotype, dominant allele,
recessive allele, codominant alleles, locus,
homozygous, heterozygous, carrier and test
cross.
4.3.2
Determine the genotypes and phenotypes of the
offspring of a monohybrid cross using a Punnett
grid.
4.3.3
State that some genes have more than two alleles
(multiple alleles).
4.3.4
Describe ABO blood groups as an example of
codominance and multiple alleles.
4.3.5
Explain how the sex chromosomes control gender
by referring to the inheritance of X and Y
chromosomes in humans.
4.3.6
State that some genes are present on the X
chromosome and absent from the shorter Y
chromosome in humans.
4.3.7
Define sex linkage.
4.3.8
Describe the inheritance of colour blindness and
hemophilia as examples of sex linkage.
4.3.9
State that a human female can be homozygous or
heterozygous with respect to sex-linked genes.
4.3.10
Explain that female carriers are heterozygous for
X-linked recessive alleles.
4.3.11
Predict the genotypic and phenotypic ratios of
offspring of monohybrid crosses involving any of
the above patterns of inheritance.
4.3.12
Deduce the genotypes and phenotypes of
individuals in pedigree charts.
59
10.2 Dihybrid crosses and gene linkage
10.2.1
Calculate and predict the genotypic and
phenotypic ratio of offspring of dihybrid crosses
involving unlinked autosomal genes.
10.2.2
Distinguish between autosomes and sex
chromosomes.
10.2.3
Explain how crossing over between non-sister
chromatids of a homologous pair in prophase I
can result in an exchange of alleles.
10.2.4
Define linkage group.
10.2.5
Explain an example of a cross between two linked
genes.
10.2.6
Identify which of the offspring are recombinants
in a dihybrid cross involving linked genes.
10.3 Polygenic inheritance
10.3.1
Define polygenic inheritance.
10.3.2
Explain that polygenic inheritance can contribute
to continuous variation using two examples, one
of which must be human skin colour.
*Explain the relationship between Mendel‟s law of segregation
and meiosis.
 law of segregation states that one half of the alleles enter one
gamete
and the other half enter the other gamete;
 meiosis reduces the chromosome number by half / diploid to
haploid;
 homologues carrying alleles separate (in anaphase I);
 end result is four cells, half with one allele/homologue and the
other half
with the other allele;
61
1. * In garden peas, the pairs of alleles coding for seed shape and
seed colour are unlinked. The allele for smooth seeds (S) is
dominant over the allele for wrinkled seeds (s). The allele for
yellow seeds (Y) is dominant over the allele for green seeds
(y).
If a plant of genotype Ssyy is crossed with a plant of genotype
ssYy, which offspring are recombinants?
A. SsYy and Ssyy
B. SsYy and ssYy
C. SsYy and ssyy
D. Ssyy and ssYy
2. * In peas the allele for round seed (R) is dominant over the
allele for wrinkled seed (r). The allele for yellow seed (Y) is
dominant over the allele for green seed (y).
If two pea plants with the genotypes YyRr and Yyrr are
crossed together, what ratio of phenotypes is expected in the
offspring?
A. 9 round yellow : 3 round green : 3 wrinkled yellow : 1
wrinkled green
B. 3 round yellow : 3 round green : 1 wrinkled yellow : 1
wrinkled green
C. 3 round yellow : 1 round green : 3 wrinkled yellow : 1
wrinkled green
D. 1 round yellow : 1 round green : 1 wrinkled yellow : 1
wrinkled green
3. A parent organism of unknown genotype is mated in a test
cross. Half of the offspring have the same phenotype as the
parent. What can be concluded from this result?
A. The parent is heterozygous for the trait.
60
B. The trait being inherited is polygenic.
C. The parent is homozygous dominant for the trait.
D. The parent is homozygous recessive for the trait
4. The allele for red flower colour (R) in a certain plant is
incompletely dominant with the allele for white flowers (R‟).
Thus a plant with the genotype RR‟ has pink flowers. Tall (D)
is dominant to dwarf (d). What would be the expected
phenotypic ratio from a cross of RR‟dd plants with R‟R‟Dd
plants?
A. 9:3:3:1
B. 50 % pink 50 % white, and all tall
C. 1:1:1:1, in which 50 % are tall, 50 % dwarf, 50 % pink
and 50 % white
D. 3:1
5.* A polygenic character is controlled by two genes each with
two alleles. How many different possible genotypes are there
for this character?
A. 2
B. 4
C. 9
D. 16
6.* A woman who is a carrier of hemophilia marries a man who is
not affected. What are the possible genotypes of their
children?
A. XHXh, XHXH, XHY, XhY
B. XHXh, XHXH, XHYh, XHYH
62
C. XHXh, XhXh, XHYh, XhYh
D. XHXh, XhXh, XHY, XhY
7. A cross is performed between two organisms with the
genotypes AaBb and aabb.
What genotypes in the offspring are the result of
recombination?
A. Aabb, AaBb
B. AaBb, aabb
C. aabb, Aabb
D. Aabb, aaBb
Answer Key: 1. c 2 c 3 a 4. c 5. c 6. a 7. d
In Zea mays, the allele for coloured seed (C) is dominant over the
allele for colourless seed (c). The allele for starchy endosperm
(W) is dominant over the allele for waxy endosperm (w). Pure
breeding plants with coloured seeds and starchy endosperm
were crossed with pure breeding plants with colourless seeds
and waxy endosperm.
(a) State the genotype and the phenotype of the F1
individuals produced as a result of this cross.
 genotype
CcWw
 phenotype
all are coloured starchy
(2)
63
(b) The F1 plants were crossed with plants that had the genotype c c w
w. Calculate the expected ratio of phenotypes in the F2 generation,
assuming that there is independent assortment.
 gametes are C W, C w, c W, c w and c w;
 F2 genotypes are CcWw, Ccww, ccWw and
ccww;
 1 coloured starchy: 1 coloured waxy: 1 colourless
starchy: 1 colourless waxy;
64
Pedigree Analysis for a Sex-Linked Trait
The diagram below shows the pedigree of a family with red green
colour-blindness, a sex-linked condition.
A male and female with normal color vision each have a
father who is color blind. They are planning to have
children. Predict, showing your working, the possible
phenotypes and genotypes of male and female children.
Key
male
female
affected male
affected female
?
 parent genotypes (XCY and XCXc) / same genotypes using
alternative symbols /
four offspring genotypes (XCXc, XCXC, XCY,XcY);
 Punnett square showing cross / other acceptable working;
 all/100 % daughters normal colour vision (phenotype);
 half/50 % the sons normal and half/50 % are colour blind
(phenotype); 3 max
Pedigree Analysis with Multiple Alleles
The following diagram represents a two generation pedigree
showing the blood groups of the individuals. The female has
been married to two different individuals.
O
A
AB
Key
1st generation
1
2
Male
3
Female
B
O
A
AB
2nd generation
1
2
3
65
4
(a) Deduce with a reason the probable father of 2nd
generation-1.
 1st generation–3 / father 3;
father 1 can only donate an O allele / B allele cannot
come from O parent;
(b) If 2nd generation-3 marries a man with blood group AB,
predict the possible genotypes of the children.
 IA IA/ AA;
IA IB / AB;
IA IO and IB IO / AO and BO / IA i and IB i;
Pedigree analysis of a Sex-linked Trait
Key
1st generation
1
normal male
2
normal female
male with condition
2nd generation
1
2
1
2
3
4
5
3
4
female with condition
3rd generation
(a) Define the term sex-linkage.
A gene / trait / allele carried on a sex chromosome
(b) Deduce, with a reason, whether the allele producing the
condition is dominant or recessive.
 recessive;
 evidence from the pedigree such as the 2nd
generation-2 and 3 do not have the condition but
have one child who does
(c) (i) Determine all the possible genotypes of the individual
66
(2nd generation-1) using appropriate symbols.
 Xa Y (where a = condition);
(ii) Determine all the possible genotypes of the
individual (3rd generation-4) using
appropriate symbols.
 XAXa or XAXA where A = normal, a = condition (must
have both);
67
68
Cell Cycle
**********************************************************
The sex cells or gametes are the only link between the parents and
offspring. So, the pattern of inheritance must pass through these germ cells and
be governed by the chromosomes during cell division. To maintain its continuity,
a cell has to divide in such a way that the two daughter cells produced from it are
similar to the parent cell and also to each other in every respect This is true for
mitosis only which takes place in somatic cells, but in case of meiosis occurring in
germ cells, the daughter cells differ from the parent cell, but still they have
common features which are essential for their continuity. The division of cells is
a cyclic event divided into four stages, namely G1, S, G2 and M (cell division).
The first three phases are combined to form interphase.
G1 phase: It is called as the first gap phase since no DNA synthesis takes place
during this stage. It may be also called as first growth phase, as it involves the
synthesis of RNA, proteins and membranes which lead to the formation of
nuclear membrane and cytoplasmic membrane of daughter cells. G1 phase
roughly occupies 30 to 50 percent of the total time of the cell cycle. Terminally
differentiated cells (such as neurons and striated muscle cells) that no longer
divide are arrested at the G1 phase; such type of G1 phase is called as G0 phase.
S phase: It is called as the synthetic phase, since replication of DNA and
synthesis of histone proteins takes place in it. At the end of S phase, each
chromosome has two DNA molecules. S phase occupies 35 to 45 percent of total
time of the cell cycle.
G2 phase: It is called as the second gap phase or second growth phase. In this
phase synthesis of RNA and proteins occurs required for the cell growth. It
occupies 10 to 20 percent of the total time of the cell cycle. After G phase the cell
enters the M phase, either mitosis or meiosis.
69
The cell cycle.
Cell Cycle
interphase
G1
S
Mitosis
telophase
anaphase
metaphase
prophase
G2
71
Regulation of Cell Cycle
************************
70
72
Regulation of cell cycle in mammalian cells
*****************************
73
Regulation of cell cycle in yeast cells
*****************************
74
Cell division
*******************************
MITOSIS
(Introduction)
Mitosis divides genetic information during cell division. Mitosis is
the process by which a cell separates its duplicated genome into two
identical halves. It is followed immediately by cytokinesis which
divides the cytoplasm and cell membrane, resulting in two identical
daughter cells. Mitosis and cytokinesis together is defined as the
mitotic (M) phase of the cell cycle, the division of the mother cell (2n)
into two daughter cells (2n), each the genetic equivalent of the parent
cell. Mitosis occurs exclusively in eukaryotic cells. In multicellular
organisms, the somatic cells undergo mitosis, while germ cells divide
by a related process called meiosis. Prokaryotic cells, which lack a
nucleus, divide by a process called binary fission.
The process of mitosis is complex and highly regulated. The
sequence of events is divided into five phases called as prophase,
prometaphase, metaphase, telophase and anaphase. During the process
of mitosis the pairs of chromosomes condense and attach to fibers that
pull the sister chromatids to opposite sides of the cell. The cell then
divides in cytokinesis, to produce two identical daughter cells. Mitosis
is for replacement and repairing the damage cells and growth.
The primary result of mitosis is the division of the parent cell's
genome into two daughter cells. The genome is composed of a number
of chromosomes, complexes of tightly-coiled DNA that contain genetic
information vital for proper cell function. Because each resultant
75
daughter cell should be genetically identical to the parent cell, the
parent cell must make a copy of each chromosome before mitosis. This
occurs during the middle of interphase, the period that precedes the
mitotic phase in the cell cycle where preparation for mitosis occurs.
Each chromosome now contains two identical copies of itself,
called sister chromatids, attached together in a specialized region of
the chromosome known as the centromere.
In eukaryotes, the nuclear envelope that separates the DNA from
the cytoplasm degrades, and its fluid spills out into the cytoplasm. The
chromosomes align themselves in a line spanning the cell.
Microtubules, essentially miniature strings, splay out from opposite
ends of the cell and shorten, pulling apart the sister chromatids of each
chromosome. As a matter of convention, each sister chromatid is now
considered a chromosome, so they are renamed to sister chromosomes.
As the cell elongates, corresponding sister chromosomes are pulled
toward opposite ends. A new nuclear envelope forms around the
separated sister chromosomes.
As mitosis completes cytokinesis starts. In animal cells, the cell
pinches inward, separating the two developing nuclei. In plant cells,
the daughter cells will construct a new dividing cell wall between each
other. Eventually, the mother cell will be split in half, giving rise to two
daughter cells, each with an equivalent and complete copy of the
original genome.
Prokaryotic cells undergo a process similar to mitosis called
binary fission. However, prokaryotes cannot be properly said to
76
undergo mitosis because they lack a nucleus and only have a single
chromosome with no centromere.
Interphase.
The mitotic phase is a relatively short action-packed period of the
cell cycle. It alternates with the much longer prophase, where the cell
prepares itself for division. Interphase is divided into three phases, G1
(first gap), S (synthesis), and G2 (second gap). During all three phases,
the cell grows by producing proteins and cytoplasmic organelles.
However, chromosomes are replicated only during the S phase. Thus,
a cell grows (G1), continues to grow as it duplicates its chromosomes
(S), grows more and prepares for mitosis (G2), and divides (M).
Mitosis is a continual and dynamic process. For purposes of
description, however, mitosis is broken down into five subphases:
prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase.
Normally, the genetic material in the
nucleus is in a loosely bundled coil called
chromatin. At the onset of prophase,
chromatin condenses together into a highly
ordered structure called a chromosome.
Since the genetic material has already been
duplicated earlier in S phase, the replicated
chromosomes have two sister chromatids, bound together at the
centromere by the protein cohesion.
77
Just outside the nucleus are two centrioles. Each centriole, which was
replicated earlier independent of mitosis, acts as a coordinating center
for the cell's microtubules. The two centrioles push themselves to
opposite ends of the cell forming spindle fibres or microtubules. The
network of microtubules is the beginning of the mitotic spindle. Later,
during prophase, the nucleolus gradually disintegrates, and nuclear
envelope disappears marking the end of prophase.
Prometaphase.
The nuclear envelope dissolves
and
microtubules
(spindle
fibres)
invade the nuclear space. This is
called open mitosis, and it occurs in
most multicellular organisms. Some
protists, such as algae, undergo a
variation called closed mitosis where
the microtubules are able to penetrate
an intact nuclear envelope.
Each chromosome forms two
kinetochores at the centromere, one
attached
at
kinetochore
each
chromatid.
is
point
the
A
where
microtubules attach themselves to the
chromosome.
Although
the
kinetochore is not fully understood, it
78
is known that it contains some form of molecular motor. When a
microtubule connects with the kinetochore, the motor activates, using
energy from ATP to "crawl" up the tube toward the originating
centrosome. This motor activity, coupled with polymerization and
depolymerisation of microtubules, provides the pulling force necessary
to later separate the chromosome's two chromatids. Thus, sister
chromatids become attached by their kinetochores to opposite poles.
Metaphase.
As microtubules find and attach
to kinetochores in prometaphase, the
centromeres
convene
or
of
the
chromosomes
aligned
along
the
metaphase plate or equatorial plane,
an imaginary line that is equidistant
from the two centrosome poles. This
even
alignment
is
due
to
the
counterbalance of the pulling powers
generated
by
the
opposing
kinetochores.
Because proper chromosome
separation requires that every kinetochore be attached to a bundle of
microtubules
(spindle
fibers),
it
is
thought
that
unattached
kinetochores generate a signal to prevent premature progression to
anaphase without all chromosomes being aligned. The signal creates
the mitotic spindle checkpoint
79
Anaphase.
In this phase, first removal
the proteins that bind sister
chromatids
cohesion,
together
allowing
them
are
to
separate. These sister chromatids
turned sister chromosomes are
pulled
apart
by
shortening
kinetochore
microtubules
toward
the
and
respective
centrosomes to which they are
attached.
nonkinetochore
elongate,
Next,
the
microtubules
pushing
the
centrosomes (and the set of
chromosomes (chnromatids) to
which they are attached) apart to
opposite ends of the cell. These
two stages are sometimes called
early and late anaphase. At the
end of anaphase, the cell has succeeded in separating identical copies
of the genetic material into two distinct populations.
81
Telophase.
Telophase is a reversal of prophase
and prometaphase events. It "cleans up"
the after-effects of mitosis. At telophase,
the nonkinetochore microtubules continue
to lengthen, elongating the cell even more.
Corresponding sister chromosomes attach
at opposite ends of the cell. A new nuclear
envelope, using fragments of the parent
cell's nuclear membrane, forms around
each set of separated sister chromosomes.
Both sets of chromosomes, now surrounded
by new nuclei, unfold back into chromatin.
Spindle fibres disappear.
Cytokinesis.
Cytokinesis is the division of the cytoplasm into
nearly equal
halves. It begins in telophase.
Cytokinesis in cells without a cell wall (In animal cells)
-The cell membrane begins to pinch inward, caused by a ring of actin
and myosin filaments constricts the -plasma membrane at the equator
-The groove formed by the pinching inward is called the cleavage
furrow.
80
-When the two sides of the furrow meet, the cell splits into two
daughter cells.
Cytokinesis in cells with a cell wall (in plant cells)
-Vesicles from the Golgi bodies fuse in the center of the cell, forming a
structure called the cell plate.
-The cell plate is built outward as more vesicles are added.
-When the cell plate reaches the cell membrane, the cell has been
divided into two daughter cells.
-The cell plate is used as a frame to build a cell wall for each daughter
cell.
Cytokinesis: Animal Cell
Cytokinesis: Plant Cells
 Cleavage furrow
 Cell Plate Formation
Figure 9.9
Page 159
Figure 9.8
Page 158
82
Endomitosis
Endomitosis is a variant of mitosis without nuclear or cellular
division, resulting in cells with many copies of the same chromosome
occupying a single nucleus.
INTERPHASE
PROPHASE
Figure 8.6
METAPHASE
ANAPHASE
Spindle
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Metaphase
plate
Daughter
chromosomes
Figure 8.6 (continued)
Mitosis
83
Nuclear
envelope
forming
Nucleolus
forming
Binary Fission
84
MEIOSIS
(Introduction)
The word "meiosis" comes from the meaning "to make smaller,"
since it results in a reduction in chromosome number. Meiosis is
essential for sexual reproduction and therefore occurs in many
eukaryotes. Meiosis does not occur in prokaryotes, which reproduce
by asexual cell division processes.
During meiosis, the genome of a diploid (2n) germ cell, which is
composed of long segments of DNA called chromosomes, undergoes
DNA replication followed by two rounds of division, resulting in four
haploid cells (n) called gametes. Each gamete contains one complete set
of chromosomes, or half of the genetic content of the original cell.
These resultant haploid cells can fuse with other haploid cells of the
opposite gender or mating type during fertilization to create a new
diploid cell, or zygote. Thus, the division mechanism of meiosis is a
reciprocal process to the joining of two genomes that occurs at
fertilization. Because the chromosomes of each parent undergo genetic
recombination during meiosis, each gamete, and thus each zygote, will
have a unique genetic blueprint encoded in its DNA. In other words,
meiosis is the process that produces genetic variation.
Biochemically, meiosis uses some of the same mechanisms
employed during mitosis to accomplish the redistribution of
chromosomes. There are several features unique to meiosis, most
importantly the pairing and recombination between homologous
chromosomes, which enable them to separate from each other.
85
Interphase is divided into three phases:
-Gap 1 (G1) phase: Characterized by increase in cell size due to
accelerated manufacture of organelles, proteins, and other cellular
matter.
-Synthesis (S) phase: The genetic material is replicated: each of its
chromosomes duplicates. The cell can thereby be said to be currently
tetraploid.
-Gap 2 (G2) phase: The cell continues to grow.
Interphase is immediately followed by meiosis I and meiosis II.
Meiosis I consists of segregating the homologous chromosomes from
each other, then dividing the tetraploid cell into two diploid cells each
containing one of the segregates. Meiosis II consists of decoupling each
chromosome's sister strands (chromatids), segregating the DNA into
two sets of strands (each set containing one of each homologue), and
dividing both diploid cells to produce four haploid cells. Meiosis I and
II are both divided into prophase, metaphase, anaphase, and telophase
subphases, similar in purpose to their analogous subphases in the
mitotic cell cycle. Therefore, meiosis includes the interphase (G1, S,
G2), meiosis I and meiosis II.
86
MEIOSIS I
Prophase I
Prophase I is the largest stage of meiosis, divided into following
sub-stages1. Leptotene.
During this stage the individual chromosomes begin to
condense into long strands within the nucleus and appear as thin
thread with the beaded appearance called as chromomeres. The
chromosomes at this stage take up a specific orientation inside the
nucleus; the ends of chromosomes converge towards one side of the
nucleus, that side where the centrosome lies (bouquet stage). The
centriole duplicates and moves to opposite poles; on reaching there,
they divide again so that each pole possesses two centrioles.
2. Zygotene.
During this stage the chromosomes approximately line up with
each other; pairing of homologous chromosomes takes place by a
process known as synapsis. The pairing of homologous chromosomes is
facilitated by proteinaceous framework called as synaptonemal
complex. Synapsis provide the ground for the genetic recombination
during the later stages. The combined homologous chromosomes are
said to be bivalent. They may also be referred to as a tetrad, a
reference to the four sister chromatids.
87
3. Pachytene.
During this stage pairing of homologous chromosomes which is
at its extreme and the homologous pairs appear as bivalent with tetrad
(four sister chromatids). At this stage genetic recombination called
crossing over takes place at molecular level by the formation of
recombination nodules called as chaisma. The synaptonemal complex
stabilizes the pairing during the exchange of homologus non-sister
chromatid segments.
4. Diplotene.
At this stage bivalent becomes more apparent because at this
stage the homologus chromosomes pair start separating by the process
called as desynapsis, and the synaptonemal complex degrades. They
are held together by virtue of recombination nodules, at the sites of
previous crossing over, the chiasmata. Homologous chromosomes fall
apart and begin to repel each other.
5. Diakinesis.
At this stage bivalent becomes more condense and appear as
four discrete or distinct chromatids. Sites of crossing over entangle
together, effectively overlapping, making chiasma clearly visible. In
general, every chromosome will have crossed over at least once. This
means that the pairs of homologous chromosomes are so tightly
packed they exchange genetic material. The chaisma moves from the
centromere of the chromosome towards the end of chromosome and
ultimately disappears. This movement of chaismata is called as
88
terminalization. The tetrad becomes angular or oval in appearance
and finally they migrate towards the equatorial plate. Lastly the
nucleoli disappears and the nuclear membrane disintegrates into
vesicles.
Prometaphase.
This stage is marked by the disintegration of nuclear envelope,
movement of centrioles at opposite poles and the microtubules get
arranged in a form of spindle in between the two centrioles. The
chromosomes become tightly coiled and are arranged on the equatorial
plate.
89
Metaphase I
As kinetochore microtubules from both centrioles attach to
their respective kinetochores, the homologous chromosomes align
equidistant above and below an imaginary equatorial plane, due to
continuous counter-balancing forces exerted by the two kinetochores
of the bivalent. Because of independent assortment, the orientation of
the bivalent along the plane is random. Maternal or paternal
homologues may point to either pole.
Anaphase I
Kinetochore microtubules shorten, severing the recombination
nodules and pulling homologous chromosomes apart. Since each
chromosome only has one kinetochore, whole chromosomes are pulled
toward opposing poles, forming two haploid sets. Each chromosome
still contains a pair of sister chromatids. The cell elongates in
preparation for division down the middle. Also, homologous pairs line
up together at the metaphase plate.
Telophase I
The first meiotic division effectively ends when the centromeres
arrive at the poles. Each daughter cell now has half the number of
chromosomes but each chromosome consists of a pair of chromatids.
The microtubules that make up the spindle network disappear, and a
new nuclear membrane surrounds each haploid set. The chromosomes
uncoil back into chromatin. Cytokinesis, the pinching of the cell
91
membrane in animal cells or the formation of the cell wall in plant
cells, occurs, completing the creation of two daughter cells.
Cells enter a period of rest known as interkinesis or interphase II.
No DNA replication occurs during this stage. Note that many plants
skip telophase I and interphase II, going immediately into prophase II.
MEIOSIS II
Prophase II.
In this prophase we see the disappearance of the nucleoli and
the nuclear envelope again as well as the shortening and thickening of
the chromatids. Each centrioles divides into two, thus two pairs of
centrioles are present at each pole. Each pair move to the Polar
Regions and are arranged by spindle fibres. The new equatorial plane
is rotated by 90 degrees when compared to meiosis I.
Metaphase II.
The chromosomes get arranged on the equatorial plate in the way
same as in mitotic metaphase, attached with the microtubule of
spindle, by the centromere.
Anaphase II.
The centromeres are cleaved, allowing the kinetochores to pull
the sister chromatids apart. The sister chromatids by convention are
now called sister chromosomes, and they are pulled toward opposing
poles.
90
Telophase II.
It is similar to telophase I, marked by uncoiling, lengthening,
and disappearance of the chromosomes occur as the disappearance of
the microtubules. Nuclear envelopes reform; cleavage or cell wall
formation eventually produces a total of four daughter cells, each with
a haploid set of chromosomes.
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Microtubules
attached to
Spindle kinetochore
Sites of crossing over
Chromatin
Sister
chromatids
Tetrad
ANAPHASE I
Metaphase
plate
Centromere
(with kinetochore)
Sister chromatids
remain attached
Homologous
chromosomes separate
Figure 8.14, part 1
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
92
Figure 8.14, part 2
Haploid
daughter cells
forming
Mitosis
93
Terms to know

anaphase - The third stage of mitosis during which all of the
sister chromatid pairs break simultaneously and are tugged
toward opposite ends of the cell by the spindle fibers.

cell cycle - A description of the general stages of life of a
eukaryotic cell. It is divided into mitosis and interphase.

cell plate - A structure made of flattened vesicles which is built
from the center toward the cell membrane during cytokinesis in
cells which have a cell wall.

centrioles - Two structures which, during mitosis, move to
opposite ends of the cell and direct the action of the spindle
fibers.

centromere - A region at which a pair of sister chromatids are
attached to one another.

chromatin - The organization of a eukaroytic cell's DNA when it
is not dividing. Chromatin is simply a large, dense mass of DNA.

chromosome - A term which refers to each half of the sister
chromatids after they split during mitosis.

cleavage furrow - The deep groove formed when the cell
membrane pinches inward during cytokinesis in cells without a
cell wall.

cytokinesis - The division of the cytoplasm after mitosis, resulting
in an approximately equal distribution of organelles in each of
the daughter cells.
94

DNA - Deoxyribonucleic acid. DNA is a long molecule composed
of deoxyribose, phosphate groups, and nitrogenous bases which
indirectly dictates the production of proteins in a cell.

interphase - The stage of the cell cycle between each cell division.
Interphase is divided into three phases: the G1 phase, the S
phase, and the G2 phase.

metaphase - The second stage of mitosis during which the spindle
fibers attach to the kinetochore of each sister chromatid
structure and pull them to the center of the cell.

mitosis - The process by which a cell's DNA is copied and then
distributed so that each daughter cell receives an identical copy
of the original DNA.

prophase - The first stage of mitosis, during which the nuclear
membrane and nucleolus disappear, the chromatin condenses
into sister chromatid structures, the centrioles begin to move
apart, and the spindle fibers begint to form.

sister chromatids - The individual copies of portions of the DNA
molecule formed after the chromatin condenses.

spindle fibers - The structures which, during mitosis, direct the
movement of the chromosomes for proper cell division.

telophase - The final stage of mitosis during which the
spindle fibers break apart, new membranes begin to form
around each set of chromosomes, the nuclear membrane and
nucleoli reappear, and the chromosomes begin to disperse
back into chromatin.
95
Chromosomes
************************
Chromosomes are the rod-shaped, filamentous bodies present in the
nucleus, which become visible during cell division. They are the carriers of the
gene or unit of heredity.
Chromosomes are not visible in active nucleus due to their high water
content, but are clearly seen during cell division.
Chromosomes were first described by Strausberger in 1875. The
term“Chromosome”, however was first used by Waldeyer in 1888. They were
given the name chromosome (Chromo = colour; Soma = body) due to their
marked affinity for basic dyes. Their number can be counted easily only during
mitotic metaphase. Chromosomes are composed of thin chromatin threads
called Chromatin fibers.
These fibers undergo folding, coiling and supercoiling during prophase so
that the chromosomes become progressively thicker and smaller. Therefore,
chromosomes become readily observable under light microscope. At the end of
cell division, on the other hand, the fibers uncoil and extend as fine chromatin
threads, which are not visible at light microscope
96
Morphology of chromosomes:
It has been established that each prophase chromosome is composed of two
longitudinal halves called chromatids. The chromatids are made up of
chromomers which connected by non-staining intervals called interchromomers
and the division of the chromosome is brought about by the doubling of the
chromomeres.
Types of chromosomes according to position of centromere:
Chromosomes may differ in the position of the Centromere, the place on the
chromosome where spindle fibers are attached during cell division.
1- Metacentric chromosome or v-shaped chromosomes in which the two
arms are equal or nearly equal.
2- Submetacentric chromosome or hook-shaped chromosomes, in which the
two arms are unequal.
3- Acrocentric or subtelocentric chromosome or rod-like chromosomes, each
being formed of a straight rod (two, arms, one of which is very small).
4- Telocentric or rod-like chromosome, in which the centromere lies at the
proximal end.
97
Centromeres and Telomeres
 Centromeres and telomeres are two essential features of all eukaryotic
chromosomes.
 Each provide a unique function i.e., absolutely necessary for the stability
of the chromosome.
 Centromeres are required for the segregation of the chromosomes during
meiosis and mitosis.
 Teleomeres provide terminal stability to the chromosome and ensure its
survival
Centromere (Primary constriction)
The region where two sister chromatids of a chromosome appear to be
joined or “held together” during mitotic metaphase is called Centromere. When
chromosomes are stained they typically show a dark-stained region that is the
centromere.
Also termed as Primary constriction. During mitosis, the centromere that
is shared by the sister chromatids must divide so that the chromatids can
migrate to opposite poles of the cell.
On the other hand, during the first meiotic division the centromere of
sister chromatids must remain intact. Whereas during meiosis II they must act
as they do during mitosis. Therefore the centromere is an important component
of chromosome structure and segregation.
As a result, centromeres are the first parts of chromosomes to be seen
moving towards the opposite poles during anaphase. The remaining regions of
chromosomes lag behind and appear as if they were being pulled by the
centromere.
Kinetochore
Within the centromere region, most species have several locations where
spindle fibers attach, and these sites consist of DNA as well as protein.The actual
98
location where the attachment occurs is called the kinetochore which composed
of both DNA and protein.
Telomere
The two ends of a chromosome or specialized sequences of DNA are
present at two ends of chromosomes known as telomeres. It required for the
replication and stability of the chromosome. When telomeres are damaged or
removed due to chromosome breakage, the damaged chromosome ends can
readily fuse or unite with broken ends of other chromosome. Thus it is generally
accepted that structural integrity and individuality chromosomes is maintained
due to telomeres. Also the telomere prevents analysis of chromosome ends by the
analyzed enzymes.
Secondary constriction:
Chromosomes may possess secondary constrictions on one or both their
arms; the position of which is also constant. The secondary constriction is
distinguished from primary constriction by the absence of marked angular
deviation of the segments of the chromosome on either side.
Nucleolar zone:
There is another constriction on some chromosomes; this is not easily
distinguished from the secondary constriction, called the nucleolar zone or
nucleolar organizer of nucleolus. It is closely related to the formation of the
nucleolus.
Satellite:
Small segment of the chromosome distal to the nucleolar zone. It is a
rounded or elongated prominence separated from the body of the chromosome
by delicate chromatic thread.
99
Number of chromosomes
 Normally, all the individuals of a species have the same number of
chromosomes.
 Closely related species usually have similar chromosome numbers.
 Presence of whole sets of chromosomes is called euploidy.
 It includes haploids, diploids, triploids, tetraploids etc.
 Gametes normally contain only one set of chromosome – this number is
called Haploid.
 Somatic cells usually contain two sets of chromosome - 2n: Diploid, 3n –
triploid and 4n – tetraploid.
 The condition in which the chromosomes sets are present in a multiples of
“n” is Polyploidy.
 When a change in the chromosome number does not involve entire sets of
chromosomes, but only a few of the chromosomes - is Aneuploidy.
Monosomics (2n-1), Trisomics (2n+1), Nullisomics (2n-2) and Tetrasomics
(2n+2)
Chromosome Size
 In contrast to other cell organelles, the size of chromosomes shows a
remarkable variation depending upon the stages of cell division.
 Interphase: chromosome are longest & thinnest
 Prophase: there is a progressive decrease in their length accompanied with
an increase in thickness
 Anaphase: chromosomes are smallest.
 Metaphase: Chromosomes are the most easily observed and studied
during metaphase when they are very thick, quite short and well spread in
the cell.
 Therefore, chromosomes measurements are generally taken during
mitotic metaphase
011
The size of the chromosomes in mitotic phase of animal and plants sp
generally varies between 0.5 µ and 32 µ in length, and between 0.2 µ and 3.0 µ in
diameter.
The longest metaphase chromosomes found in Trillium - 32 µ.
The giant chromosomes found in diptera and they may be as long as 300 µ and
up to 10 µ in diameter.
In general, plants have longer chromosomes than animal and species
having lower chromosome numbers have long chromosomes than those having
higher chromosome numbers.
Among plants, dicots in general, have a higher number of chromosomes
than monocots. Chromosomes are longer in monocot than dicots.
Chemical structure of chromosomes (Chromatin)
The complexes between eukaryotic DNA and proteins are called
Chromatin, which typically contains about twice as much protein as DNA. The
major proteins of chromatin are the histones – small proteins containing a high
proportion of basic aminoacids (arginine and lysine) that facilitate binding
negatively charged DNA molecule. There are 5 major types of histones: H1,
H2A, H2B, H3, and H4 – which are very similar among different sp of
eukaryotes. The histones are extremely abundant proteins in eukaryotic cells.
Their mass is approximately equal to that of the cell‟s DNA
The DNA double helix is bound to proteins called histones. The histones
have positively charged (basic) amino acids to bind the negatively charged
(acidic) DNA.
In addition, chromatin contains an approximately equal mass of a wide
variety of non-histone chromosomal proteins. There are more than a thousand
different types of these proteins, which are involved in a range of activities,
including DNA replication and gene expression.
The DNA of prokaryotes is similarly associated with proteins, some of
which presumably function as histones do, packing the DNA within the bacterial
010
cell. Histones, however are unique feature of eukaryotic cells and are responsible
for distinct structural organization of eukaryotic chromatin.
The basic structural unit of chromatin, the nucleosome, was described by
Roger Kornberg in 1974.
According to some authors the prophase chromosome consists of a protein
fibres called chromonema, in certain regions of which DNA accumulates to
produce small beads- like structure called chromomeres.
Euchromatin and Heterochromatin
Chromosomes may be identified by regions that stain in a particular
manner when treated with various chemicals. Several different chemical
techniques are used to identify certain chromosomal regions by staining then so
that they form chromosomal bands. For example, darker bands are generally
found near the centromeres or on the ends (telomeres) of the chromosome, while
other regions do not stain as strongly.
The positions of the dark-staining are heterochromatic region or
heterochromatin and light staining are euchromatic region or euchromatin.
Heterochromatin is classified into two groups: (i) Constitutive and (ii)
Facultative.
 Constitutive
heterochromatin
remains
permanently
in
the
heterochromatic stage, i.e., it does not revert to the euchromatic stage.
 In contrast, facultative heterochromatin consists of euchromatin that
takes on the staining and compactness characteristics of heterochromatin
during some phase of development.
Satellite DNAs
When the DNA of a prokaryote, such as E.coli, is isolated, fragmented and
centrifuged to equilibrium in a Cesium chloride (CsCl) density gradient, the
DNA usually forms a single band in the gradient.
012
On the other hand, CsCl density-gradient analysis of DNA from
eukaryotes usually reveals the presence of one large band of DNA (usually called
“Mainband” DNA) and one to several small bands. These small bands are
referred to as “Satellite DNAs”. For e.g., in Drosophila virilis, contain three
distinct satellite DNAs.
Karyotype:
The karyotype is general morphology of the somatic chromosome.
Generally, karyotypes represent by arranging the chromosomes in the
descending order according to length and size of them. It is given to the group of
constant or characteristics (number, form, size, and others) which should be
taken into consideration when identifying a particular chromosomal set.
Idiotype:
The karyotype of a species may be represented diagrammatically,
showing all the morphological features of the chromosome; such a diagram is
known as Idiotype or ideogram.
Homologous Chromosomes
 Chromosomes that pair up during meiosis
 Contain the same genes
 May have different alleles of these genes
 One came from each parent
 Each chromosome contains one long DNA molecule. A gene is a short
region of the molecule. Each chromosome can have > 1,000 genes
013
Homologous
Chromosomes
Types of chromosomes:
1- Sex chromosomes
Chromosomes or group of chromosomes in eukaryotes in which the sexes
are represented differently. Typically designated X and Y (sometimes Typically
designated X and Y (sometimes W and Z)
2- Autosomal chromosomes: All of the other chromosomes.
Human contain 44 autosomal +XX in female or 44+XY in male
014
Prokaryotic and Eukaryotic Chromosomes
Eukaryotic chromosome
Prokaryotic chromosome
015
Prokaryotic and Eukaryotic Chromosomes
************************************************
Not only the genomes of eukaryotes chromosomes are more
complex than prokaryotes, but the DNA of eukaryotic cell is also
organized differently from that of prokaryotic cells.
The
genomes
of
prokaryotes
are
contained
in
single
chromosomes, which are usually circular DNA molecules.
In contrast, the genomes of eukaryotes are composed of
multiple chromosomes, each containing a linear molecular of DNA.
Although the numbers and sizes of chromosomes vary
considerably between different species, their basic structure is the
same in all eukaryotes.
The DNA of chromosomes of eukaryotic cell is tightly bound to
small basic proteins (histones) that package the DNA in an orderly
way in the cell nucleus. This task is substantial (necessary), given the
DNA content of most eukaryotes
Prokaryotic chromosome
The prokaryotes usually have only one chromosome, and it
bears little morphological resemblance to eukaryotic chromosomes.
Among prokaryotes there is considerable variation in genome
length bearing genes. The genome length is smallest in RNA viruses.
In this case, the organism is provided with only a few genes in its
chromosome.
016
The number of gene may be as high as 150 in some larger
bacteriophage genome.
Bacterial chromosome is single, circular DNA molecule located
in the nucleolus region of cell.
In E.coli, about 3000 to 4000 genes are organized into its one
circular chromosome. The chromosome exists as a highly folded and
coiled structure dispersed throughout the cell. The folded nature of
chromosome is due to the incorporation of RNA with DNA.
017
Specialized chromosomes
Giant chromosomes
Found in certain tissues e.g., salivary glands of larvae, gut epithelium,
Malphigian tubules and some fat bodies, of some Diptera (Drosophila)
These chromosomes are very long and thick (upto 200 times their size
during mitotic metaphase in the case of Drosophila).
The total length of
D.melanogater giant chromosomes is about 2,000µ.
They are first discovered by Balbiani in 1881 in dipteran salivary glands
and thus also known as salivary gland chromosomes.
Giant chromosomes are made up of several dark staining regions called
“bands”. It can be separated by relatively light or non-staining “interband”
regions
All the available evidence clearly shows that each giant chromosome is
composed of numerous strands, each strand representing one chromatid.
Therefore, these chromosomes are also known as “Polytene chromosome”, and
the condition is referred to as “Polytene”
The numerous strands of these chromosomes are produced due to
repeated replication of the paired chromosomes without any nuclear or cell
division. So that the number of strands (chromatids) in a chromosome doubles
after every round of DNA replication
018
Lampbrush Chromosome
It was given this name because it is similar in appearance to the
brushes. First observed by Flemming in 1882. The name lampbrush
was given by Ruckert in 1892.
These are found in oocytic nuclei of vertebrates (sharks,
amphibians, reptiles and birds) as well as in invertebrates (sepia, and
several species of insects). Also found in plants – but most experiments
in oocytes.
Lampbrush chromosomes are up to 800 µm long; thus they
provide very favorable material for cytological studies.
Each lampbrush chromosome contains a central axial region,
where the two chromatids are highly condensed. Each chromosome
has several chromomeres distributed over its length. From each
chromomere, a pair of loops emerges in the opposite directions
vertical to the main chromosomal axis.
019
Mutations
*********************
Mutation
Damage or change in genetic material of organism or any
change in the DNA sequence of an organism that affect genetic
information.
Mutations are the source of the altered versions of genes that
provide the raw material for evolution.
- Mutations May occur in somatic cells (aren‟t passed to
offspring).
- Mutations May occur in gametes (eggs & sperm) and be passed
to offspring (inherited).
- Some of mutations are harmful and some are beneficial.
Causes of mutation:
1- Spontaneous mutation: Almost all mutations are natural
(inherited).
2- Induced mutation (acquired) by the following:
a. Radiation e.g. UV.
b. Toxic Chemicals (e.g. Cigarette Smoke and pesticides, etc)
c. High Temperatures
001
Types of mutations
***********************
1- Gene mutations:
Changes in the nucleotide sequence of DNA of one gene.
Types of Gene Mutations:
1- Point Mutations:
Change of a single nucleotide. Includes the deletion, insertion,
or substitution of ONE nucleotide in a gene.
Substitution is equal (Transition) through substitution purine
by purine (A and G) but it is unequal (Transversion) through
substitution purine with Pyrimidine (T and C).
Example:
Substitution - when a base is replaced with
different base.
CGG CCC AAT to CGG CGC AAT
Guanine for Cytosine
Insertion - when a base is added
CGG CCC AAT to CGG CGC CAA T
Guanine is added
Deletion - the loss of a base
CGG CCC AAT to CGG CCA A T
loss of Cytosine
000
a
**Substitutions of point mutations can be classified according to
their effects on the protein (or mRNA) produced by the gene that is
mutated:
• Silent: change in 3rd position of codon usually but same amino
acid, so change here has no effect.
• Missense (Faux sens): typically a single nucleotide change, causes
change in amino acid and noticeable effect.
• Nonsense: change amino acid codon to STOP codon.
• Sense: is opposite of nonsense mutations. Here, a stop codon is
converted into an amino acid codon.
2-Frame shift Mutations:
Inserting or deleting one or more nucleotides. Changes the
“reading frame”. A frame shift mutation results from a base deletion
or insertion. Each of these changes the triplets (codons) that follow the
mutation. Frame shift mutations have greater effects than a point
mutation because they involve more triplets
CGG CCC AAT to CGG CGC CAA T
- The Frame shift changes the mRNA produced.
- mRNA from DNA as expected……..
002
GGG CCC TTT AAA
CCC GGG AAA UUU
- Mutated DNA GGC GCC CTT TAA A
CCG CGG GAA AUU U
All the triplets are changed; this in turn changes the amino
acids of the protein.
3- Reversion mutation:
This is a mutation which through it the phenotype changes from
mutant shape to the natural shape.
2- Chromosomal Mutations (Chromosomal aberrations):
Affecting whole or a part of a chromosome by change in
structure or number of chromosomes.
003
Chromosomal Aberrations
(Variations)
************************************
The somatic (2n) and gametic (n) chromosome numbers of a
species ordinarily remain constant. This is due to the extremely
precise mitotic and meiotic cell division.
Somatic cells of a diploid species contain two copies of each
chromosome, which are called homologous chromosome. Their
gametes therefore contain only one copy of each chromosome, which is
they contain one chromosome complement or genome.
Each chromosome of a genome contains a definite numbers and
kinds of genes, which are arranged in a definite sequence.
Sometime due to mutation or spontaneous (without any known
causal factors), variation in chromosomal number or structure do
arise in nature.
Chromosomal aberrations may be grouped into two broad classes:
1. Structural
2. Numerical
004
Structural Chromosomal Aberrations
(Variation in chromosome structure)
****************************************************
Chromosome structure variations result from chromosome
breakage. Broken chromosomes tend to re-join; if there is more than
one break, rejoining occurs at random and not necessarily with the
correct ends. The result is structural changes in the chromosomes.
Chromosome breakage is caused by X-rays, various chemicals,
and can also occur spontaneously.
- There are four common types of structural aberrations:
1. Deletion or Deficiency
2. Duplication or Repeat
3. Inversion, and
4. Translocation.
- Consider a normal chromosome with genes in alphabetical order: a b
cdefghi
1. Deletion: part of the chromosome has been removed: a b c g h i
2. Duplication: part of the chromosome is duplicated:
abcdefdefghi
3. Inversion: part of the chromosome has been re-inserted in reverse
order: a b c f e d g h i
005
4. Translocation:
parts of two non-homologous chromosomes are
joined:
If one normal chromosome is a b c d e f g h i and the other
chromosome is u v w x y z, then a translocation between them would
be
a b c d e f x y z and u v w g h i.
Ring chromosome: the ends of the chromosome are joined together to
make a ring.
006
Deletion or deficiency
Loss of a chromosome segment is known as deletion or
deficiency. It can be terminal deletion or interstitial (intercalary)
deletion.
A single break near the end of the chromosome would be
expected to result in terminal deficiency. If two breaks occur, a section
may be deleted and an intercalary deficiency created.
Terminal deficiencies might seem less complicated. But
majority of deficiencies detected are intercalary type within the
chromosome.
Deletion was the first structural aberration detected by Bridges
in 1917 from his genetic studies on X chromosome of Drosophila.
007
Duplication:
The presence of an additional chromosome segment, as
compared to that normally present in a nucleus is known as
Duplication (Doubling of a segment of a chromosome). In a diploid
organism, presence of a chromosome segment in more than two copies
per nucleus is called duplication.
Types of Duplication:
There are four types of duplication:
1. Tandem duplication:
The extra chromosome segment may be located immediately
after the normal segment in precisely the same orientation forms the
tandem. (Tandem may be terminal).
2. Reverse tandem duplication:
When the gene sequence in the extra segment of a tandem in
the reverse order i.e, inverted, it is known as reverse tandem
duplication
3. Displaced duplication:
In some cases, the extra segment may be located in the same
chromosome but away from the normal segment – termed as displaced
duplication
4. Translocation duplication:
The additional chromosome segment is located in a nonhomologous chromosome is translocation duplication.
008
009
Inversion:
When a segment of chromosome is oriented in the reverse
direction, such segment said to be inverted and the phenomenon is
termed as inversion.
The existence of inversion was first detected by Strutevant and
Plunkett in 1926.
Inversion occurs when parts of chromosomes become detached,
turn through 1800 and are reinserted in such a way that the genes are
in reversed order.
 Inversion may be classified into two types:
 Pericentric - include the centromere
 Paracentric - do not include the centromere
021
Translocation
Integration of a chromosome segment into a non homologous
chromosome is known as translocation or a piece of a chromosome
winds up attached to another chromosome.
Types of translocation:
1- Nonreciprocal translocation: it divided into
a- (Nonreciprocal interchromaosomal): it divided into:
i- Simple translocation
In this case, terminal segment of a chromosome is integrated at
one end of a non-homologous region. Simple translocations are rather
rare.
ii-Shift translocation
In shift, an intercalary segment of a chromosome is integrated
within a non-homologous chromosome.
b-Nonreciprocal intrachromosomal
Piece of one chromosome breaks off, attaches to same
chromosome. But on other position from chromosome.
2-Reciprocal translocation
It is produced when two non-homologous chromosomes
exchange
segments
–
i.e.,
segments
Translocation of this type is most common.
020
reciprocally
transferred.
Numerical Chromosomal Aberrations
(Variation in chromosome number)
**************************************************
Organism with one complete set of chromosomes is said to be
euploid (applies to haploid and diploid organisms).
Euploidy – the correct number of sets of chromosomes in an organism.
( n for Gametes and 2n for somatic cells).
**But there are some variation in chromosome number as following:
1- Monoploidy – only one set of chromosomes in somatic cells of
organism. Male wasps, ants, and bees are monoploids because they
develop from unfertilized eggs
2- Polyploidy – having three or more sets of chromosomes
– Usually occurs due to a breakdown of the spindle
– Usually occurs in plants (self-fertilization)
– In humans, triploidy is usually lethal.
– Polyploidy divided into:
a- Autopolyploidy
– All sets of chromosomes originate in the same species/defect
in meiosis.
– Example – a diploid gamete fuses with a haploid gamete to
produce a triploid zygote (plants give large size fruits with
seedless fruits)
b-Allopolyploidy
022
– Sets of chromosomes originate from different species
though usually related
– Because of differences between chromosomes, the hybrid,
no crossing over occurs and no viable gametes produced
making hybrids sterile
– Such as marriage between donkey and horse to give the
mule.
3- Aneuploidy
Variation in the number of individual chromosomes (but not
the total number of sets of chromosomes). The variation due to nondisjunction of chromosomes during the meiosis division. This
variations such as:
- Nullisomy - loss of one homologous chromosome pair; 2N – 2
- Monosomy – loss of a single chromosome; 2N – 1
- Trisomy – one extra chromosome; 2N + 1
- Tetrasomy – one extra chromosome pair; 2N + 2
023
Non-Disjunction
Generally during gametogenesis the homologous chromosomes
of each pair separate out (disjunction) and are equally distributed in
the daughter cells.
But sometime there is an unequal distribution of chromosomes
in the daughter cells. The failure of separation of homologous
chromosome is called non-disjunction. This can occur either during
mitosis or meiosis or embryogenesis.
Mitotic non-disjunction:
The failure of separation of homologous chromosomes during
mitosis is called mitotic non-disjunction. It occurs after fertilization.
May happen during first or second cleavage. Here, one blastomere will
receive 45 chromosomes, while other will receive 47.
Meiotic non-disjunction:
The failure of separation of homologous chromosomes during
meiosis
is
called
mitotic
non-disjunction.
It
occurs
during
gametogensis. Here, one type contains 22 chromosomes, while other
will be 24.
024
025
Chromosomal Disorders (Syndromes)
*******************************************
Disorders of Chromosome structure:
1-Cri-du-chat (Cat cry syndrome):
The name of the syndrome came from a catlike mewing cry
from small weak infants with the disorder. Other characteristics are
small head, broad face and mental retardation.
Cri-du-chat patients die in infancy or early childhood. The
chromosome deficiency is in the short arm of chromosome 5.
2- Myelocytic leukemia
Another human disorder that is associated with a chromosome
abnormality
is
chronic
myelocytic
leukemia.
A
deletion
of
chromosome 22 was described by P.C.Nowell and Hungerford and
was called “Philadelphia” (Ph‟) chromosome.
Disorders of Chromosome number:
1- Down Syndrome (Trisomy 21) (47, +21)
The best known and most common chromosome related
syndrome. Formerly known as “Mongolism”.
In 1866, when a physician named John Langdon Down
published an essay in England in which he described a set of children
with common features who were distinct from other children with
mental retardation he referred to as “Mongoloids.”
026
One child (male or female) in every 800-1000 births has Down
syndrome.
Reason of Down syndrome
Down syndrome results if the extra chromosome with pair of
homologous chromosomes number 21 (three copies) due to nondisjunction during meiosis in pair of somatic homologous chromosome
number 21 hence produce egg or sperm with 24 chromosomes.
For
example
when
egg
(23+X)
chromosomes
unite
with sperm (22+X or 22+Y) to give zygote with 47 chromosomes
(45+XX female or 45+XY male ) (trisomy 21 or Down syndrome).
Symptoms of Down syndrome
Short in stature, broad short skulls, flat nostrils, open small
mouth, large protruding tongue, folded ears, short broad hands, short
neck, short fingers, Space between first and second toe, transverse
palmer crease, skin folds on eye and mental retardation.
Male, Trisomy 21 (Down’
(Down’s)
2n = 47 28
027
Female Down’
Down’s Syndrome
2n = 47 28
2- Edward's syndrome, (Trisomy 18) 47, +18
Is the second most common trisomy after Down's syndrome.
Edward's
syndrome
occurs
when
three
copies
(trisomy)
of
chromosome 18 occur.
Trisomy 18 is therefore caused by a genetic abnormality such as
trisomy 21 but in chromosome 18.
Trisomy 18 was discovered by John Hilton Edwards, a British
geneticist, in 1960. His discovery of it led to the association of his name
with the syndrome. One child (male or female) in every (8000-11000)
births has Trisomy 18.
028
Symptoms of Edward's syndrome
Multiple
congenital
malformations
of
many
organs,
malformed ears, small mouth and nose with general elfin appearance,
short neck, large protruding heels, prominent sternum. 90% die in
the first 6 months. Very rare.
029
3- Patau syndrome, ( Trisomy 13) 47, +13
Patau syndrome occurs when three copies (trisomy) of
chromosome 13 occur. Trisomy 13 is therefore caused by a genetic
abnormality such as trisomy 21 and in trisomy 18 but in chromosome
13. One child (male or female) in every (15000 - 20000) births has
Trisomy 13.
Symptoms of Patau syndrome
Mental retardation, deafness, cleft lip, defects in small eyes,
extra fingers or toes, and large protruding heels, and malformations of
brain, nervous system, and heart.
031
4- Klinefelter Syndrome, male with (44+XXY).
- Called the XXY syndrome
- 1 : 500-1000 male births
Reason of Klinefelter Syndrome
This syndrome due to non-disjunction during meiosis in pair of
sex chromosomes, hence produce egg with (22 +XX) chromosomes.
When this egg unites with sperm 23 chromosomes (22+Y) give male
with 47 chromosomes (44+XXY).
Symptoms of Klinefelter syndrome
Long stature, incomplete sexual development, rudimentary
small testes and prostate, long limbs, large hands and feet, little body
hairs, some breast tissue development. Most common cause of male
infertility.
Klinefelter’
Klinefelter’s Syndrome
2n = 47
30
030
5- Turner Syndrome, female with (44+X0)
-Called the XO syndrome,
-1 in 2,500- 3000 female births
-99% of affected fetuses die in uterus
Reason of Turner Syndrome
Due to non-disjunction during meiosis in pair of sex
chromosomes, hence produce egg with 22 chromosomes. When this
egg unites with sperm 23 chromosomes (22+X) give female with 45
chromosomes (44+X).
Symptoms of Turner syndrome
Short stature, broad chest, webbing of skin in neck
region, cardiovascular abnormalities, incomplete sexual
development
(infertile),
and
impaired
hearing.
retardation.
2n = 45
30
032
Mental
Barr Bodies
 1940‟s two Canadian scientists noticed a dark staining mass in
the nuclei of cat brain cells
 Found these dark staining spots in female but not males
 This held for cats and humans
 They thought the spot was a tightly condensed X chromosome
 Barr bodies represent the inactive X chromosome and are
normally found only in female somatic cells.
 A woman with the chromosome constitution 47, XXX should
have 2 Barr bodies in each cell.
 XXY individuals are male, but have a Barr body.
 XO individuals are female but have no Barr bodies.
Number of Barr Bodies = (number of chromosome X -1)
Female with 46 chromosomes (44+XX) has one barr body.
033
Sex Determination
**********************************
All the sexually reproducing organisms produce two different
types of gametes. In case of monoecious, both the type of gametes are
produced by the same individual while in dioecious the male and
female gametes are produced by different individuals. In dioecious
forms there is a specific problem of finding out the factors which are
responsible for determining whether a particular individual would be
a male or female. According to the work of modern geneticists, sex is
determined and controlled by many factors like genetic mechanism,
hormonal mechanism and environmental mechanism but the most
significant is the genetic mechanism of sex determination as far as
humans are concerned.
Of the 23 homologus pairs of human chromosomes, 22 pairs are
similar in appearance in men and women and are called as autosomes.
The 23rd pair is different in males and females and is called as sex
chromosomes or allosomes. Males have two different types of sex
chromosomes, namely XY, and so produces two different types of
gametes, are referred to as heterogametic sex. The females, on the
other hand have same sex chromosomes, and produces similar type of
gametes, are known as homogametic sex.
034
1. Autosomes
2. Sex chromosomes
3. Heterogametic sex (2 types of gametes)
4. Homogametic sex (1 type of gamete)
5. Males are not always heterogametic sex - females are
heterogametic in birds, moths, fish and chickens
XX/XY – male heterogametic sex
ZZ/ZW – female heterogametic sex
035
**Sexual Differentiation
In multicellular organisms, it is important to distinguish between:
1-primary sexual differentiation
involves only the gonads where gametes are produced
2-secondary sexual differentiation
involves the overall appearance of the organism
3-Unisexual, Dioecious, Gonochoric – containing only male or female
reproductive organs
4-Bisexual, Monoecious, Hermaphroditic – both male and female
reproductive organs
Sexual Differentiation in Humans XX-XY Type
• Early embryonic gonads can become testes or ovaries
– Y chromosome induces formation of testes
• Testosterone (T) from testes induces formation of male
sex organs
• In absence of T, female sex organs develop
1. All human embryos undergo a hermaphroditic period
2. 5th week of gestation, gonadal primordia arise
3. Primordail germ cells become cortex/inner medulla
4. Cortex – develop into ovary
5. Medulla – develop into testis
036
6. 7th week of gestation, if XY chromosomes are present, medulla
develops into testis. If no Y present, cortex forms ovarian tissue
7. Initiation of testis stimulates production of two hormones that is
needed for male sex differentiation
8. Males produce spermatocytes at puberty
9. Females arrest eggs in meiosis by 25th week
X and Y chromosomes
• The X is BIG (5-6% of genome) with lots of genes (mostly
encoding somatic function) markers, and disease-associated
mutations.
• The Y is small (though variable in length)…but it does have some
genes.
Y chromosome in Male Development
The Y chromosome Determines Maleness in Humans. Y has
many parts
SRY:
- Sex determining region Y (encodes for TDF)
- Y chromosome contains:
1. Females X Y – missing SRY region on the Y chromosome.
2. Males X X – attached SRY region to the X chromosome.
037
Pseudoautosomal Regions (PARs)
- Regions on Y chromosome that share homology with regions on the
X chromosome
- Synapse and recombine with it during meiosis
-Presence of such a pairing region is critical to segregation of the X
and Y chromosomes during male gametogenesis
-These regions synapse/located on ends of Y chromosome
- All the rest of Y is called NRY (nonrecombining regions)
Testis-determining factor (TDF)
A protein encoded by a gene in the SRY that triggers testes
formation.
Male-specific region of the Y (MSY)
The MSY consists of three regions:
• X-transposed region
• X-degenerative region
• ampliconic region
038
039
Sexual Differentiation in insects XX-XO Type.
This mechanism operates in certain insects such as cockroaches,
grasshoppers and bugs. depends on random distribution of the X
chromosome into half of the male gametes.
The female has two homomorphic sex chromosomes XX and is
homogametic. The male has one sex chromosome only. It produces
two
types
of sperms-
gynosperms
with
X-chromosome
and
androsperms with no X-chromosome. Fertilization of an egg by Xbearing sperm yields female offspring and by no X-chromosome
sperm yields male offspring. The presence of an unpaired Xchromosome determines the male sex.
Parents:
Gametes:
F1:
Female (XX)
X
Male (XO)
X
XX (female)
041
O
XO (male)
040
Sexual Differentiation in birds ZW-ZZ Type
This mechanism operates in certain insects (butterflies and
moths) and certain vertebrates (fishes, reptiles and birds). The male has
two homomorphic sex chromosomes (ZZ) and is homogametic, while
the female has two heteromorphic sex chromosomes (ZW) and is
heterogametic. There are thus two types of eggs- with Z and W sex
chromosomes, while males have only one type of sperm, i.e., each with
Z sex chromosomes. Fertilization of an egg containing Z chromosome
by a sperm having Z chromosome produces a zygote with ZZ
chromosomes, developing into a male. Fertilization of an egg
containing W chromosome by a sperm with Z chromosome yields a
zygote with ZW chromosomes, developing into a female.
Parents:
Male (ZZ)
Gametes:
Z
F1:
Female (ZW)
Z
ZZ (male)
042
W
ZW (female)
043
Nucleic Acids
********************
• Nucleic acid that composes chromosomes and carries genetic
information.
DNA (DEOXYRIBONUCLEIC ACID)
DNA Structure:
**Structure discovered by Watson & Crick in 1953.
DNA as coiled double stranded molecule known as double helix. Make up
chromosomes in the nucleus. Subunits of DNA called nucleotides. Nucleotides
contain a phosphate, a Deoxyribose sugar, and one nitrogen base (A,T,C, or G)
Sides made of pentose (5-sided) sugars attached to phosphate groups by
phosphodiester bonds. Pentose sugar called Deoxyribose. Steps or rungs of DNA
made of 4 nitrogen-containing bases held together by weak hydrogen bonds.
Nitrogen bases are Purines (double carbon-nitrogen rings) include
adenine (A) and guanine (G). Pyrimidines (single carbon-nitrogen rings) include
thymine (T) and cytosine (C). Base pairing means a purine bonds to a
pyrimidine (Example: A --- T and C --- G). Two H bonds for A-T and three
H bonds for G-C. Free nucleotides also exist in nucleus. Each turn in the helix is
34Å long and involves 10 successive nucleotide pairs. One strand of DNA the
nucleotides construction is in direction from 5' to 3' and other strand is from 3'
to 5'.
**Sugar + Base = Nucleoside
044
DNA Replication:
Process by which DNA makes a copy of itself. Occurs during S phase of
interphase before cell division. Extremely rapid and accurate (only 1 in a billion
are incorrectly paired). Requires many enzymes & ATP (energy). Begins at
special sites along DNA called origins of replication where 2 strands open &
separate making a replication fork. Each strand from the parent molecule serve
as a template for new srrand.
Nucleotides added & new strand forms at replication forks. DNA helicase
(unwind DNA) (enzyme) uncoils & breaks the weak hydrogen bonds between
complementary bases (strands separate). DNA polymerase adds new nucleotides
to the exposed bases.
DNA polymerase proofreads the new DNA checking for errors &
repairing them; called excision repair, DNA polymerases can move only in 3‟ to
5‟ direction, but in 5' to 3' of DNA the nucleotides in new strand add in shape
pieces called Okazaki fragments. DNA ligase helps join segments together,
stitches Okazaki fragments. Helicase recoils the two, new identical DNA
molecules.
RESULTS OF REPLICATION
• Two molecules of DNA those are identical.
• Each is half old (strand from parent) and half new (strand synthesized by
DNA polymerase).
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GENETIC CODE
The “language” that translates the sequence of nitrogen bases in DNA
(mRNA) into the amino acids of a protein.
• Codon = three nucleotides on DNA or mRNA
• One codon specifies one amino acid
• Some codons are redundant (code for the same amino acid)
• The genetic code is universal to all organisms
• One start codon (AUG=methionine), and three stop codons (UAA, UAG
or UGA)
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RNA (RIBONUCLEIC ACID)
**Nucleic acid involved in the synthesis of proteins
RNA STRUCTURE
Composed of nucleotides, but differs from DNA in four ways.
1. Single strand of nucleotides instead of double stranded.
2. Has uracil instead of thymine.
3. Contains ribose instead of deoxyribose.
4. Found in nucleus & cytoplasm
Transcription: Synthesis of Messenger RNA (mRNA)
**Steps in Transcription
 DNA helicase (enzyme) uncoils the DNA molecule
 RNA polymerase (enzyme) binds to a region of DNA called the promoter
which has the start codon TAC to code for the amino acid methionine
 Promoters mark the beginning of a DNA chain in prokaryotes, but mark
the beginning of 1 to several related genes in eukaryotes
 The 2 DNA strands separate, but only one will serve as the template & be
copied
 Free nucleotides are joined to the template by RNA polymerase in the 5‟
to 3‟ direction to form the mRNA strand
 mRNA sequence is built until the enzyme reaches an area on DNA called
the termination signal
 RNA polymerase breaks loose from DNA and the newly made mRNA
 Eukaryotic mRNA is modified (unneeded sections snipped out by enzymes
& rejoined) before leaving the nucleus through nuclear pores, but in
prokaryotic transcription RNA isn‟t. No cap, no poly(A) tail and no
introns.
 All 3 types of RNA called transcripts are produced by this method
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RNA processing
** mRNA transcripts are modified before use as a template for translation:
1- Addition of capping nucleotide at the 5‟ end, 7-methylguanosine at terminus
Protects mRNA from nucleases.
2- Addition of polyadenylated tail to 3‟ end.
3-Splicing occurs removing internal sequences, introns are sequences removed
And exons are sequences remaining.
Important for:
- moving transcript out of nucleus to cytoplasm.
- regulating when translation occurs.
Notice:
** An intron (intervening sequence) is a
segment of DNA which is transcribed into
mRNA but not actually used when a protein is expressed.
**An exon (expressed sequence) in the part of the DNA gene which is expressed.
** Each gene usually contains a number of introns and exons.
- Introns are excised from mRNA after transcription.
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**Types of RNA
1- mRNA




Messenger RNA
Single, uncoiled, straight strand of nucleic acid
Found in the nucleus & cytoplasm
Copies DNA‟s instructions & carries them to the ribosomes where
proteins can be made
 mRNA‟s base sequence is translated into the amino acid sequence of a
protein
 Three consecutive bases on mRNA called a codon (e.g. UAA, CGC, AGU)
 Reusable
2- tRNA
 Transfer RNA
 Single stranded molecule containing 80 nucleotides in
the shape of a cloverleaf
 Carries amino acids in the cytoplasm to ribosomes for
protein assembly
 Three bases on tRNA that are complementary to a
codon on mRNA are called anticodons (e.g. codonUUA; anticodon- AAU)
 Amino Acid attachment site across from anticodon
site on tRNA
 Enters a ribosome & reads mRNA codons and links
together correct sequence of amino acids to make a
protein
 Reusable
3- rRNA
 Ribosomal RNA
 Globular shape
 Helps make up the structure of the
ribosomes
 rRNA & protein make up the large &
small subunits of ribosomes
 Ribosomes are the site of translation
(making polypeptides)
 Aids in moving ribosomes along the
mRNA strand as amino acids are
linked together to make a proteins
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Protein Synthesis (Translation)
**Steps in Transcription
• mRNA brings the copied DNA code from the nucleus to the cytoplasm
• First the mRNA attaches itself to a ribosome (to the small subunit) called
initiation.
• Six bases of the mRNA are exposed.
• A complementary tRNA molecule with its attached amino acid
(methionine) base pairs via its anticodon UAC with the AUG on the
mRNA in the first position P.
• Another tRNA base pairs with the other three mRNA bases in the
ribosome at position A.
• The enzyme peptidyl transferase forms a peptide bond between the two
amino acids.
• The first tRNA (without its amino acid) leaves the ribosome.
• The ribosome moves along the mRNA to the next codon (three bases).
• Two amino acids at a time are linked together by peptide bonds to make
polypeptide -chains (protein subunits); called elongation
• The second tRNA molecule moves into position P.
• Another tRNA molecule pairs with the mRNA in position A bringing its
amino acid.
• Ribosomes) move along the mRNA strand until they reach a stop codon
(UAA, UGA, or UAG); A growing polypeptide is formed called
termination
• A growing polypeptide is formed in this way until a stop codon is
reached.
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Large
subunit
P
Site
A
Site
m RN A
AUG C U ACUUCG
Small subunit
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