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GENETICS
Genetics is the branch of biology that deals with the study of Heredity and Variations. Heredity
is the transfer of parental characters to the off springs. Variation is the differences between the
parents and off springs and also between the off springs of a set of parents.
Variations are of two types
1.Somatic variations: These are variations that affect only the somatic cells or body cells. These
are not heritable. These are acquired by the effect of environmental factors, use and disuse of
organs or by conscious effort.
2.Germinal variations: These are variations that affect the reproductive cells. These are
heritable. These may be Continuous or Discontinuous. Continuous variations very small indistinct
variations. These are also called Fluctuating variations. They develop during gamete formation
and will not contribute evolutionary changes. Discontinuous variations develop as sudden
changes due to mutations. They are also called Sports or saltations. They are the most
important factors that contribute raw materials for evolutionary changes.
Branches of Genetics
1.Classical genetics or Transmission genetics: deals with the principles of Mendelian
Inheritance
2.Molecular genetics: deals with the molecular structure of genes
3.Population or biometrical genetics: deals with the behavior and effect of genes in
Population
Common Terms in Genetics
1.Phenotype
2.Genotype
3.Genome
4.Haploid
5.Diploid
6.Genes
6.Allele
External or visible characters of an organism
Genetic makeup of an organism
Haploid set of chromosome in a diploid cell
Organism with half set of chromosomes. Gametes are haploids
Organism with full set (2 genomes) of chromosomes
Physical and chemical basis of heredity. Reside in chromosomes
Alternate forms of a gene located in the same loci of homologous chromosomes
Also called Allelomorphs
7.Locus
Position of gene in the chromosome
8.Homozygous An organism with identical genes of a character .AA or aa
9.Heterozygous An organism with non-identical genes for a character. Aa
10.Dominant
Character that is expressing.
11.Recessive Character that is masked in the presence of a dominant gene.
12.Back cross A cross between F1 hybrid with any one of the parents.
13.Test cross A cross between F1 hybrid with the recessive parent.
Scientists associated with Genetics
1.Mendel
2.Hugo Devries
3.Punnet
4.Carl Correns
5.Erich Von Tschermak
6.Correns
7.Karl Land Steiner
8.Von Decastello, Sturli
Father of genetics
Discovered mutation
Introduced Checker board or punnet squre for Mendelian cross
Rediscovered Mendelism
Rediscovered Mendelism
Discovered Incomplete dominance
Discovered blood groups
Discovered AB blood group
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9.Bernstein
10.Bateson and Punnet
11.Landsteiner and Weiner
12.Davenport
13.Robert Brown
14.Hammerling
15.Sutton and Bovery
16.Waldeyer
17.Balbiani
18.Watson and Crick
19.T.H.Morgan
20.T.H.Morgan
21.Bridges
22.Blakeslee
23.Altenberg
24.Muller
25.Auerbach
26.Beadle and Tatum
27.Griffith
28.Avery, MacLeod, Mc Carthy
29.Harshey and Chase
30.Fraenkel conrat
31.Friedrich Miescher
32.Mary Rosalind
33.Meselson and Stahl
34.Archibald Garrod
35.Beadle and Tatum
36.Jacob and Monod
37.Holley
38.Philip Sharp, Richard Robert
39.Briggs and Thomas
40.Francis Galton
41.Alec Jeffreys
Multiple alleles in blood groups
Discovered Complementary action of genes in Lathyrus odoratus
Discovered Rh blood group
Discovered Polygenic inheritance
Discovered nucleus
Demonstrated the role of nucleus to control characters.
proposed the chromosome theory of inheritance
Coined the term Chromosome
Discovered Giant chromosomes in Drosophila
Proposed the double helical model of DNA
Discovered Linkage
Produced artificial mutation in Drosophila for the first time
Sex linked inheritance
Found out trisomic plants for the first time in Datura
Discovered the mutagenic property of U.V rays
Discovered mutagenic property of X rays.
Discovered mutagenic property of chemicals
Conducted experiments in Neurospora
Discovered bacterial transformation
Proved DNA as the genetic material
Conducted experiments in viral multiplication
Discovered RNA also act as genetic material
Isolated nucleic acid from nucleus
Lady behind the DNA.Helped Watson to make the DNA model
Discovered semi conservative method f DNA replication
Discovered Inborn Errors of Metabolism
Proposed One Gene One Enzyme hypothesis
Proposed Operon concept
Proposed the Cloverleaf model of t-RNA
Discovered Split Genes
Nuclear transplantation experiment
Founder of Eugenics
Introduced DNA Finger printing
Mendels’s work on Heredity
Greogor Johan Mendel, an Austrian Monk conducted a number of experiments in garden pea
Pisum sativum and formulated the principles and laws of inheritance of characters. Because of
his contributions in genetics he was called as the Father of Genetics.
1.Mendel’s
pea plant
Mendel selected pea plant as his experimental material because of the
following reasons
A. Pea plants have large number of observable characters and most of them are true breeds.
B.Pea plants are self-pollinating and the keel petal of the flower prevents contamination
with foreign pollens.
C.they have short life span
D.they produce large number of seeds.
E.they are easy to cultivate.
Mendel’s Experiment
Mendel performed the breeding experiment in three stages
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1.Selection of Parents: Mendel selected 14 pure or true breeds with contrasting characters as
parents
2.Hybridisation for F1: Mendel artificially cross-pollinated the F1 plants by dusting the pollens
collected from one flower on the stigma of an emasculated flower.
Bagging the pollinated flower prevented contamination.
3.Self-breeding F1.
Mendel allowed the F1 hybrids to self pollinate and raised F2 and F3
Mendel’s findings
After conducting experiments in garden pea Mendel concluded the results as follows:
1.Hybrids of F1 generation resembled one of their parents. The result of the reciprocal crosses
were also same.
2.In the F2 generation both the parental characters appeared.
3.Both the parental characters appeared in the ratio 3:1 in the F2.This is called Monohybrid ratio.
In the F2 generation of a cross between yellow and green seeds, Mendel obtained 6022 yellow
and 2001 green seeds. This was approximately in the ratio 3:1.
4.In the F2 Mendel found that all the recessive plants were true breeding forms.
5.Mendel observed that one of the parental characters that was not appeared in F1 reappeared
in F2.
6.He also found that the inheritance was particulate in nature and the characters are transmitted
by factors and random mixing of characters during fertilization leads to the appearance of
parental characters.
MONOHYBRID CROSSMendel selected true breeding pea plants for the monohybrid cross. He fixed one plant as
male and the other as female .The anthers from the female plant were removed (emasculation)
and bagged the flowers to prevent contamination with other pollens. Then he dusted the pollens
collected from the male plant on the stigma of the female plant. Pollinated flowers were bagged
again and raised the F1 generation from the seeds obtained from the cross pollinated flower. He
also conducted reciprocal crosses by interchanging the parents. The F1 plants were then allowed
to self-pollinate to raise the F2 generation. Similarly F3 and F4 generations were produced.
Results of Monohybrid cross
When true breeding tall and dwarf plants were cross-pollinated only tall plants appeared in the F1
generation. When F1 plants were self-pollinated both tall and dwarf plants appeared in the ratio
3:1.The reciprocal cross also gave the same result. Mendel found that the tall nature of plant
appeared in the F1 generation was due to the dominant nature of the factor controlling height of
the plant. In the presence of Dominant factor the recessive trait dwarf ness was masked. Thus
Mendel concluded that the dominant factor only will appear in some generations and the
recessive character will appear only the plant becomes homozygous.
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MONOHYBRID CROSS
Tall
X
Dwarf
Parents
TT
X
tt
Genotype
T
T
t
t
Tall
F1 generation
Tall
Tall
Tt
Tt
T
t
P gametes
T
Selfing
t
F1 gametes
Male
T
t
TT
Tt
Tt
Tt
T
Female
t
Monohybrid cross
Phenotypic ratio: 3 Tall: 1 Dwarf
Genotypic ratio 1 TT : 2 T t : 1 tt
3:1
1:2:1
DIHYBRID CROSS
After the monohybrid cross, Mendel conducted dihybrid crosses in pea plant taking two pairs of
contrasting characters. He selected two characters namely pod colour and seed shape. Yellow
colour of pod is dominant over green colour and round shape of seed is dominant to wrinkled
seed. Mendel crossed a pure breed Round Yellow plant with a double recessive Wrinkled Green
plant. In the F1 generation he got only Round Yellow plants as in the monohybrid cross. But when
he self pollinated the F1 plants, four varieties of plant namely Round Yellow, Round Green,
Wrinkled Yellow and Wrinkled Green appeared in the ratio 9:3:3:1.This ratio is called dihybrid
ratio. After analyzing the ratio, Mendel found that the factors responsible for colour of pod and
shape of seed separated independently and segregated into different gametes of the F1 plants.
The random fusion of gametes produced all the four possible varieties in the F2 generation.
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Mendel’s dihybrid cross
Round Yellow
Rr Yy
X
Rr Yy
RY
Ry
RY
rY
Ry
ry
Wrinkled Green
rr yy
P1
Round Yellow
F1
F1 Gametes
rY
ry
RY
RRYY
RRYy
RrYy
RrYy
Ry
RRYy
Rryy
RrYy
Rryy
rY
RrYY
rRyY
rrYy
rrYy
ry
RrYy
Rryy
rrYy
rryy
9 Round Yellow :3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled green
MEDELIAN PRINCIPLES AND LAWS
After the experiments, Mendel formulated some principles and laws to explain the behavior of
factors during inheritance. These are called Mendelian laws and principles. This became the
basis of genetics.
Principle of dominance: This principle says that during inheritance some characters will appear
while others remain hidden or masked. The character that is appearing is called as dominant
character and the one masked as recessive character. Eg. Tall plant is dominant over drawf.
Principle of Combination: This principle says that different characters in an organism are
produced by the combination of factors present in the gametes. The random fusion of gametes
result in the appearance of new characters.
Eg. Round green, Wrinkled Yellow.
Law of segregation: Mendel formulated his first law of inheritance after his monohybrid cross.
This law is also called law of purity of gamates. According to the law of segregation, the different
factors present in an organism separate and segregate into different gametes at the time of
gamete formation. This law can be explained by the monohybrid cross.
Law of Independent Assortment: Mendel formulated his second law after conducting di hybrid
cross in pea plants. According to this law, when two or more pairs of contrasting characters are
present, the separation or segregation of characters in any one pair is independent to the
separation of characters in the other pair. This law can be explained using the dihybrid cross.
VARIATIONS OF MENDELISM
Normally in most of the organisms the hereditary characters are inherited according to the
Mendelian pattern. But there are exceptions in which the inheritance shows variations from
Mendelism. A few examples of such variations are:
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INCOMPLETE DOMINANCE
Generally in Mendelian pattern of inheritance, the dominant trait is appear in the F1 generation
and the recessive trait does not appear in F1.This is due to the presence of dominant gene that
masks the effect of the recessive gene. But in some organisms the character in the F1 generation
is a mixture of the effect of both dominant and recessive genes. The dominant gene is expressing
partially allowing the recessive gene also to express. This phenomenon is called incomplete or
partial dominance.
A very good example of incomplete dominance is the flower colour in the common 4–O clock
plant Mirabilis jalapa. In this plant the Red flower is dominant over White flower. When a red
flowered plant is crossed with white flowered one, all the F1 plants produced Pink flowers. In the
F2 generation Red, Pink and White flowered plants will appeared in the ratio 1: 2: 1.That is, the
character in the F1 will be a mixture of both dominant and recessive genes. Here the genotypic
and phenotypic ratios will be the same.
Red
x
White
RR
Parents
rr
R
R
r
r
P – gametes
F1 – Pink
Rr
R
r
RR
Rr
Rr
rr
R
r
1 Red : 2 Pink : 1White
Genotypic ratio 1:2:1
Phenotypic ratio 1:2:1
Other example 1.Sickle cell anemia
Sickle cell trait Hb A / Hb S genotype has partial normal
hemoglobin and partial sickle hemoglobin
CO DOMINANCE
In this type of gene expression both the dominant genes of character will express together if they
come together and produce a mixture of dominant characters. So neither allele is dominant to the
other. The inheritance of AB blood group provides an example for codominance. If a person
inherits both I A and I B genes, the blood group will be AB.Both antigen A and B will be present
on the surface of RBC.
A – Group
X
IA/IA
B – Group
IB/ IB
IA/IB
AB Blood group
Eg. 2. Coat colour inheritance in Cattle
Red coat colour ( I R ) and White ( I W ) have no dominant or recessive relations. In a
cross between Red and White cattles the F1 will be Roan with red and white patches on the
body. In the F2 Red, Roan and White will appear in the ratio 1:2:1
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MULTIPLE ALLELISM
Generally a character is controlled by two alleles, either dominant, recessive or in the
heterozygous conditions. But some characters are controlled by more than two alleles. This
phenomenon is called as multiple allelism and the alleles as multiple alleles. They are different
from multiple genes since they are the alternate forms of the same gene located in the same loci
of the homologous chromosomes. Even though a number of alleles are present in the group, only
two alleles will be present in an organism.
Multiple allelism is exemplified by the blood group inheritance in man. Three alleles are present in
man to control blood group inheritance. I A and I B genes are dominant and i gene recessive. The
I A and I B genes originated from i gene by two dominant or reverse mutations. Thus all the three
alleles control the same character namely blood group inheritance.
Other examples
1.Rh inheritance in man
Rh factor present on the RBC of man is controlled by 8 alleles.
They are treated as multiple alleles by Weiner but Fischer considered them as pseudo genes.
2.Coat colour inheritance in Rabbit: There are four coat colours in rabbit namely Agouti,
Chinchilla, Himalayan and Albino. These coat colours are controlled by alleles I C , I ch , I h and
I a.
POLYGENIC INHERITANCE OR MULTIPLE GENE INHERITANCE
The quantitative traits of man are controlled by a group of genes called polygenes. They are
separate genes forming a group to control a character. Each gene in the group can contributes
some amount of character and all the contributions of genes are added together to produce the
character. Therefore the effect is called Additive effect or cumulative effect. Davenport in
1913 found that three genes located in the adjacent loci of the cromosome control the skin colour
in man. These genes are ABC and abc. Each gene can produce some amount of melanin
pigment in the body. A Negro has all dominant genes and hence maximum melanin (around 80
%). But the white has all recessive genes and can produce only 6 % melanin. A cross between
Negro and White will produce an intermediate called Mulatto with 38% melanin. The genotype of
mulatto is AaBbCc. Crossing of two mulattos will produce individuals with skin colours like Black,
Intermediate, Light etc in addition to Negro and White.
NEGRO
X
AABBCC
WHITE
aabbcc
Aa Bb Cc
Mulatto
Other examples 1.Corolla length in Nicotiana longiflora 2.Kernel colour in wheat 3.Height in man
EPISTASIS AND HYPOSTASIS
This is a gene behavior in which the presence of a gene (not allele) will suppress the expression
of another gene for the same character. The gene, which is suppressing is called Epistatic gene
and the gene that is undergoing suppression as Hypostatic gene.
Example 1.In Dogs coat colour Black is controlled by the gene B and brown by b. Another allele I
inhibits the expression of B and colour becomes white. Here I gene is epistatic and B hypostatic
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Example 2. Coat colour in Mice. In mice the wild coat colour Agouti is controlled by the gene A.
Its allele a is responsible for albino. Another gene c controls the spreading of melanin. This gene
has a recessive epistatic effect and if if comes with A gene,the colour will be albino instead of
agouti.
PLEIOTROPISM
It is a phenomenon in which a gene has multiple phenotypic effects. Such genes are called
pleiotropic genes.
Example: Gene responsible for sickle cell anemia. A mutant recessive gene Hbs in the
homozygous condition produces sickle cell anemia. In sickle cell anemia the haemoglobin
crystallizes in the RBC when the partial pressure of oxygen decreases. This changes the shape
of RBC to sickle form, which will block blood vessels causing anemia and haemorrhage. Normal
gene for hemoglobin is HbA .The genotype of sickle cell anemia is Hbs / Hb s. The heterozygotes
with genotype Hb A / H b s are called Carriers and are showing Sickle Cell Trait. So the same
gene produces three phenotypes namely Normal, Sickle cell trait and sickle cell anemia.
Other Examples
1.In Drosophila the eye colour gene in different flies produce different eye colours like red, white
honey, ebony etc. The same gene will also control sperm storage organs in females.
2.In White tiger the gene for fur colour also control the connection between eye and brain.
LETHAL GENES
These are harmful genes, which will destroy the possessor either in the dominant or recessive
conditions.
Example 1.A dominant gene Y in the homozygous condition will causes the death of mice. The
gene controls yellow coat colour in mice and will be normal in the heterozygous condition and
produce yellow coat colour ( Yy ) .In the homozygous recessive condition ( yy ) it produces brown
coat. Only in the homozygous condition ( YY ) the gene becomes lethal.
Other examples. 1.Leaf colour in Snapdragon – Antirrhinum majus
2.Thalassemia in man
3.Huntington’s chorea in man
Some times a gene in the homozygous recessive condition will become lethal. Sickle cell anemia
in man is caused when the gene for hemoglobin becomes recessive due to mutation.
Hbs / Hbs
Other examples 1.Congenital ichthyosis in man – skin disease
2.Amaurotic idiocy in man – mental retardation
COMPLEMENTARY GENES
These are genes, which complement other genes to produce a character. The two genes are
necessary to produce a particular phenotype. Bateson and Punnet observed this phenomenon
in pea plant Lathyrus odoratus. In this plant the genes for purple and white colours of flower are P
and p. But the purple or white colour is produced only when another gene C is present. In the
dominant form
PpCc Purple
PPcc / ppCC White
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PEDIGREE ANALYSIS
Pedigree is the history of inheritance of characters for many generations. Pedigree analysis is the
method to trace back the ancestral characters by taking one or more the characters from an
individual and the family tree or pedigree tree is constructed. Squares represent males and
circles represent females. Shaded squares or circles indicate dominant traits and open squares
or circles indicate recessive traits.
Pedigree tree
PROBABLE QUESTIONS
DO IT YOURSELF
1. Some plants occur in of the two sizes- tall or dwarf. Tallness is dominant to
shortness. Choose suitable letters for the gene.
2. Why are there two genes controlling one character? Do the two genes affect the
character in the same way as each other?
3. The gene for red hair is recessive to the gene for black hair. What colour hair will a
person have if he inherits a gene for red hair from his mother and a gene for
black hair from his father?
4. Read the above question carefully and choose letters for red hair and black hair
and write down the gene combinations. Would you expect a red haired couple
to breed true? Could a black haired couple a red haired baby?
5. Use the words homozygous, heterozygous, dominant and recessive to describe
the following gene combinations- Aa , AA, aa
6. A plant has two varieties, one with red petals and one with white petals. When
these two varieties are cross-pollinated, all the offspring have red petals. Which
gene is dominant? Choose suitable letters to represent the two genes?
7. Two black guinea pigs are mated together on several occasions and there off
springs are invariably black. However when their black offspring are mated with
white guinea pigs, half of the matings result in all black litters and the other half
produced litters containing equal number of black and white babies. From these
results deduce the genotypes of the parents and explain the results of the various
matings assuming a single pair of alleles in this case determines that colour.
8. Two black rabbits thought to be homozygous for coat colour were mated and
produced a litter, which contained all black babies. The F2 however resulted in
some white babies, which meant that one of the grand parents was
heterozygous for coat colour. How would you find out which parent was
heterozygous?
9. What are the possible blood groups likely to be inherited by children born to a
group A mother and a group B father? Explain your reasoning?
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10. A red cow has a pair of alleles for red hairs. A white bull has a pair of allele for
white hairs. If a red cow and white bull are mated, the off springs are Roan. That is
they have red and white hairs equally distributed over their body.
Is this an example of co-dominance or incomplete dominance?
What coat colours would you expect among the offspring of a mating between
two Roan cattle?
11. A married couple has four girl children, but no boys. This does not mean the
husband produces only X sperms. Explain why not
12. A woman has sickle cell trait. What are the chances of her children inheriting –a.
Sickle cell trait b. Sickle disease if she marries a normal man or a man with sickle
cell trait or a man with sickle cell disease?
13. Which of the following do you think are – a. mainly inherited characters b. mainly
acquired characters or more or less equal mixture?
Manual skills, Facial features, Body build, Language, athletism, Ability to talk
14. What new combinations of characters are possible as a result of crossing a tall
plant
With yellow seeds? (TtYy) with a dwarf plant with green seeds (ttyy)
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CHROMOSOMES AND HEREDITY
Chromosomes are structures formed in the cells during cell division. They are visible only in a
dividing cell. During
inter phase stage chromosomes remain as chromatin reticulum.
Chromosomes are short and stumpy structures that form the physical basis of heredity. They
carry hereditary units called genes.
Chromosomes were first reported by Hofmeister in 1849.The details of the behaviour during cell
division were worked out only at the end of 19th century.Experiments conducted by scientists
proved that both chromosomes and genes play an important role in heredity and both shows
parallelism.
Chromosomes and genes exhibit parallelism
1.Genes and chromosomes exists in pairs.
2.Genes and chromosomes segregate during meiosis.
3.Paired condition of both chromosomes and genes will restore during fertilization.
4.Genes and chromosomes exhibit independent assortment.
CHROMOSOMES THEORY OF HEREDITY.
alter Sutton and Theodore Boveri in 1902 independently proposed the chromosome theory of
heredity. According to this theory hereditary factors are located in the chromosomes.The
segregation and independent assortment of genes depends on the segregation and independent
assortment of chromosomes.
Structure of chromosome
Chromosomes are rod shaped structures present in the dividing cells.They are stainable and
become purple coloured when stained using Acetoorcein ( animal chromosome ) or Acetocarmine (
plant chromosome ).A typical chromosome is called metacentric chromosome.It has two equal
chromosome arms connected by a centromere or kinetochore.The centromere is the primary
constriction of the chromosome.Besides this some chromosome has a secondary constriction at
one end.The part above the secondary constriction is called Satellite.The chromosome with a
satellite is called SAT chromosome.The secondary constriction is also called nucleolar organizer
because the nucleolus is formed in this region.
The chromosome is covered by a pellicle.Inside the chromosome is a matrix in which the thread
like chromonema are present.The chromonema bears equally spaced spherical bodies called
chromomeres.They are treated as the genes.
A typical Meta centric chromosome
TYPES OF CHROMOSOMES
Chromosomes are classified in to four types based on the position of centromere. These are
1.Metacentric chromosome :
The centromere is present exactly in the middle of the
So the chromosome arms are equal in length.
2.Sub meta centric
The centromere is slightly placed towards one arm. So the
chromosome appears as L shaped during cell division.
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3.Acro centric
4.Telocentric
The centromere is towards the end of the chromosome. The
chromosome appears as J shaped.
The centromere is at the tip of the chromosome. Usually such
a chromosome does not exist and is formed when on arm
breaks.
Other chromosome types
1.Acentric
2.Giant chromosomes.
A chromosome without centromere. This is fomed when one arm
of the chromosome breaks leaving the arm without a centromere.
These are large chromosomes present in some insects and
amphibian oocytes. They are of two types.
A. Polytene chromosome - These are giant chromosome present
In the salivary gland cells of Drosophila. These are formed by
Endo duplication of chromatin. The chromatin duplicate
continuously with out cell division. This leads to the formation of
a large chromosome.At some points the chromosome has swollen
parts called chromosome puffs or Balbiani rings. These are sites
of high gene activity.
B. Lamp brush chromosome - These are giant chromosomes
present in the oocytes of some amphibians. The chromosome
looks like bottle brush with numerous filaments arranged
around a central filament.
3.SAT chromosome
Usually one or two chromosome in a cell has a secondary
constriction for accomodationg the nucleoli.. The secondary
constriction is called as nucleolar organizer because the
nucleolus is supposed to be formed from this region.
The part above the secondary constriction is swollen and
is called Satellite body and the chromosome as SAT chromosome.
SEX DETERMINATION
In higher organism males and females are found and the maleness and femaleness are
determined by various factors. The process by which the sex of an individual is established is
called sex determination. Chromosomes , hormones , environment etc. are various factors that
determines the sex of the organisms. Various mechanisms of sex determination are found among
organisms and theories have been put forwarded to explain the mechanism of sex determination.
Some of the important theories are :
1.Chromosome theory of sex determination :
Chromosomes play an important role in sex mination. Egs. Man , Drosophila etc.
2.Gene balance theory : Put forwarded by Bridges.This theory says that the balance between the
genes present in the sex chromosomes and autosomes determine the sex.
3.Environmental theory : The environment of the developing organism influence the expression of
sexual characters.
1.XX Femele XY Male mechanism or Homogametic female Heterogametic male method.
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Y-LINKED INHERITANCE
The somatic genes present in Y chromosome are called Y linked genes or holandric genes. They
express only in males and no counter part allele is present in the X chromosome.
Eg.Hypertrychosis – hair in the ear pinna
SEX LIMITED CHARACTERS
These are characters express only in one sex , either in male or female. It is not expressed in the
other sex even though the gene is present.
Example 1.Feather pattern in poultry. In poultry two feather types are seen. Males have cock
feathering and females have hen feathering .The cock feathering is limited to males only.
Females will not develop cock feathering even though the gene is present. The hen feathering is a
dominant character controlled by the gene h+ .Females show hen feathering both in the h+h+ and
H+h conditions. Cock feathering appears in males only when both the genes becomes recessive
hh .Cock feathering will not develop in females if hh genes are present. The hormone from the
ovary of female inhibits the expression of the genes in females.
Genotype
h+h+
h+h
hh
Phenotype
Male
Hen feathered
Hen feathered
Cock feathered
Female
Hen feathered
Hen feathered
Hen feathered
Example 2. Premature baldness in human males
Example 3. Milk production in cattles. Gene express only in females.
SEX INFLUENCED CHARACTERS
These are characters behave as dominant in one sex and recessive in the opposite sex. So the
phenotype is different even though the genotype is same. This is mainly due to the influence of
sex hormones.
Example 1.Horn development in sheep : In sheep, horned character is dominant in male and
recessive in female. In the heterozygous condition horn appears in males and not in females.
Genotype
h+h+
h+h
hh
Phenotype
Male
Horned
Horned
Hornless
Female
Horned
Hornless
Hornless
Example 2.Baldness in man.- This character is more frequent in males.
Example 3.Hare lip
- More frequent in males
Example 4.Spina bifida
– forked spinal cord – more frequent in females.
MUTATION
Mutations are sudden or spontaneous heritable changes in the genotype of an organism. The
mutation was observed first time by a Duch botanist Hugo de Vris in the evening prim rose
Oenothera lamarkiana. This plant has narrow leaves and yellow flowers. Hugo de Vris got a new
variety of oenothera with broad leaves and large flowers. He named the plant as Oenothera gigas.
Experiments conducted in O.gigas revealed that it was formed as result of a sudden change in the
genes of Oenothera lamarkiana. This lead to the formulation of the Mutation theory. According
to the mutation theory , mutations are sudden changes in the genotype and are responsible for
evolution. Thus evolution is a sudden change without intermediate stages.
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TYPES OF MUTATIONS
Mutations are classified in to various types based on the site of occurrence and size.
Somatic mutation
- Mutation in the somatic or body cells – non-heritable – no role in evolution
Germinal mutation – Mutation in the germ cells – heritable – form the raw materials of evolution
Germinal mutation may be Gametic (in gametes) or Zygotic (in zygote)
Natural mutation
- Mutations that develop naturally.
Induced mutations – Artificially produced mutation by mutagens.
Gene mutation
- Mutation in the DNA or gene
Chromosome mutation - Mutation in the chromosomes – It may be structural changes
(Aberrations) or numerical changes (ploidy)
MECHANISM OF MUTATION
Mutations are of two types based on the effect on the genotype. These are gene mutations and
chromosome mutations.
MUTATIONS
GENE MUTATION
CHROMOSOME MUTATION
1.Deletion
2.Addition or Insertion
3.Substitution
1.Aberrations
a.Deletion
b. Addition
c. Inversion
d. Translocation
a. Transitions
b. Transversions
2.Ploidy
a. Aneuploidy
1.Monosomy
2.Nullisomy
3.Trisomy
b.Euploidy
1.Haploidy
2.Diploidy
3.Polyploidy
GENE MUTATIONS
These are changes in the structure or chemical nature of genes. Genes are made up of DNA and
any change in the DNA will affect the hereditary characters of the organisms. Gene mutations will
reflect as a defective phenotype. Since gene mutations affect only smaller regions of DNA, they are
called as Point mutations. The following mechanisms are responsible for gene mutations.
1.Deletion.These are removal of one or more nitrogen bases or nucleotides from the DNA. Certain
Ionizing radiations, alkylting agents cause deletion of nitrogen bases.
2.Addition When one or more nitrogen bases or nucleotides are added to the DNA, the mutation is
called addition. Some mutagens like Acridine dyes (Acridine orange) causes additions.
Acridine dye gets incorporated between the nucleotides and makes a gap between them.
in to the gap additional base or nucleotide will be inserted.
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3.Substitution :When a nitrogen base, either purine or pyrimidine is replaced by another
`
base , then it is called substitutions. These are of two types: Transition and
Transversion
Transition
When a purine is replaced by another purine or a pyrimidine is replaced by
another pyrimidine, the substitution is called transition. Transitions are mainly
due to the following mechanisms.
A.
Tautomerization - The rare forms of nitrogen bases are called Tautomers.
The normal Nitrogen bases undergo tautomeric shifts – shifting of protons
from one position to another – and becomes the tautomers. Such changes
will alter the normal bases and the abnormal base will mis- pair with an
unusual base leading to substitution.
B. Base analogoue - These are chemicals, which have structure identical to
the bases. For example the 5-bromo uracil – 5BU – is a base analogue of
thymine In the presence
5BU the adenine pairs with 5 BU instead of pairing with thymine. This
changes the base sequences and causes substitution.
C. Deamination - The removal of amino group - NH2 group - from the nitrogen
base is called deamination. Nitrous acid – HNO2 – remove amino group from bases
and causes substitution.
Transversion
When a purine is replaced by a pyrimidine or vice versa, then it is called
tranversion.
GENE MUTATION AND FRAME SHIFTS
The most important effect of gene mutation is the Frame shifting. In the DNA molecule the
nucleotides are arranged in a linear fashion. A group of three nitrogen bases form a genetic code
that is transcribed to the m-RNA during protein synthesis. The code will rearrange after the base
change and new codes are formed. These new codes will represent new amino acids and the
proteins thus formed will be an abnormal one. This change in the reading frame of DNA will
reflect as a defective phenotype. This is called frame shifts and the mutations as frame shift
mutations.
Mis sense mutations : Mutations resulting in a different amino acid in the protein
Non sense mutations : Mutations resulting in termination codon . Protein synthesis stops in
the termination codon
Silent mutations
: These are mutations that do not alter the amino acid sequence.
CHROMOSOME MUTATIONS
These are mutations affecting the structure or number of chromosomes. Structural changes are
called aberrations and numerical changes are called Heteroploidy.
CHROMOSOME ABERRATIONS.
These are deletions , additions and translocations.
Deletion: Loss of a segment from the chromosome is called deletion. Some mutagens causes
breaks in the chromosomes. This will result in the loss of many genes so that the characters
controlled by the deleted part of the chromosome will be absent in the organism. Loss of the
segment from the tip of the chromosome is called Terminal deletion and between the centromere
and tip is called intercalary or interstitial deletion.
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ABC D
A D
Segment with genes B and C deleted
Clastogenic agents : These are chromosome breaking agents cause deletions. Egs. X ray Gamma
rays.
Cri–du–chat syndrome : It is a defect caused due to chromosome deletion in the short arm of the 5
th chromosome .Children with this defect shows mental retardation and physical defects. The cry
of such children is cat like.
Addition or Duplication : When a new segment is added to the chromosome , the mutation is
called addition. This causes repetition of genes in the chromosome and hence it is called
duplication.
ABCBCD
ABCD
Tandem duplication
Reverse tandem duplication
: The repeated genes may in the same order.
: The repeated genes may be in the reverse order.
Duplication may increase the dose of genes which may beneficial or harmful.
Example : In Drosophila the duplication in the 16 – A locus causes bar eye - narrow eye with
reduced facets.
Inversion
: If a chromosome segment is brocken and rejoined in the reverse order, the
mutation is called Inversion.
Paracentric inversion – Centromere is not included in the inverted segment.
Pericentric
- Centromere is included.
Inversition causes position effect, suppress crossing over etc.
Position effect : The inversition affect the mutual relationship between the genes. This is called
position effect.
: It is a method used to study the inversions in man. The chromosome is
karyotyped and stained using Giemsa stain. The different regions of the chromosome stain
differently.
G-banding
Translocation :
This the breakage of chromosome segment followed by its transfer to a non
homologous chromosome.
Simple translocation
Reciprocal translocation
: Translocation of only one segment.
: Mutual transfer of chromosome segments between the non
homologous chromosomes.
Translocation may affect the phenotype because of the change in gene relations. It may lead to
sterility in some plants. Example – Translocation in plants like Rhoeo and Oenothera.
:Recoprocal translocation in man produces a smaller 22nd
chromosome called Philadelphia chromosome .It is due to the translocation between
chromosome 9 and 22.It is seen in patients with Chronic myeloid leukemia ( CML )
Philadelphia chromosome
All chromosome aberrations play an important role in evolution because gene rearrangements
and variations are produced by aberrations.
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HETEROPLOIDY
These are numerical changes in chromosome make up. The normal number of chromosome is
diploidy and any variation from the normal number leads to mutations.
Ploidy can be classified into Aneuploidy and Euploidy.
Aneuploidy : Numerical changes involving addition or deletion of one or more individual
chromosomes is called aneuploidy. Full chromosome set is not involved.
1.Monosomy
Deletion of only one chromosome from the diploid set.
2.Nullisomy
Deletion of two chromosomes from the diploid set.
3.Trisomy
Addition of one chromosome to the diploid set.
4.Tetrasomy
Addition of two chromosomes to the diploid set.
It may be
2n-1
2n-2
2n+1
2n+2
Aneuploidy is the result of genetic non-disjunction.It is the abnormal separation of
chromosomes at the time of cell division. Due to the abnormal spindles , some chromosomes may
lag during the movement .This may lead to changes in the chromosome number in the new cells.
Examples
1.In man Turner’s syndrome is due to monosomy in the X chromosome
2.Down’s syndrome Or Trisomy 21 is due to the addition of one chromosome
in the 21st pair.
3.In Datura aneuploidy produces changes in fruit shape
EUPLOIDY
This is the numerical change due to addition or deletion of entire set of chromosomes ( n )
Euploidy is classified in to haploidy, diploidy, and polyploidy.
Haploidy
This is the conditition in which the organism has only one set of chromosomes.
The haploid condition of gametes is normal but in a diploid organism the haploidy is abnormal
and produce mutation. Haploidy is produced naturally in some plants. They are usually weak and
sterile. Haploidy can be induced in plants artificially by anther culture. Haploids produced from
pollen grains are called Androgenic haploids. Datura plant can be produced by anther culture
method. Honey bee male is a natural haploid. It is produced from unfertilized egg by
parthenogenesis. Its chromosome number is 16 while that of female is 32.
Diploidy This is the normal condition found in all most all organisms. In this condition two sets of
chromosomes (paternal and maternal sets ) are present. This represents the 2n number. Each set
is called a genome ( n ).
Polyploidy
Presence of more than two sets of chromosomes in the cell is called polyploidy.
This may be triploidy ( 3n ),tetraploidy ( 4n ), pentaploidy (5n ), hexaploidy ( 6n )etc.
Polyploidy is rare in animals but is a common phenomenon among plants. Polyploidy arises due
to abnormalities in the cell division. Sometimes the cell division stops after the chromosome
doubling. This converts a normal cell in to a polyploid cell. Polyploids may be autopolyploid or
allopolyploid .In Autopolyploid the same genome is multiplied .For example potato is a
autotetraploid. ( 4n =48).When different genomes in a hybrid are multiplied then it is
Allopolyploid.For example the common wheat is allohexaploid .A new genus Tricale is produced
by crossing Wheat with Rye.The hybrid is then subjected to induced polyploidy to produce an
allopolyploid.
Wheat
Rye
Hybrid ( sterile )
Induced polyploidy
Tricale ( allopolyploid )
Tricale is the first man made cereal.
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Induced polyploidy is the artificially produced polyploidy. Polyploidy can
be induced in many plants for agricultural purposes. A number of vegetables like tomato, potato,
wheat, banana etc are polyploids .Banana is a sterile tatraploid. Colchicine is used induce
polyploidy. It is a Mitotic Poison that can destroy the spindle fibres. If colchcine is applied to the
growing seed or shoot , the mitotic division stops at the metaphase stage and the cells become
polyploids. Colchicine is an alkaloid obtained from a plant Colchicum autumnale of Liliaceae.
Induced polyploidy
MUTAGENS
Agents either physical or chemical that can produce a mutation is called Mutagen or Mutagenic
agent. Mutagens are classified in to physical and chemical mutagens.
High energy radiations like electromagnetc or particulate radiations are
capable of producing mutations. Most of the high energy radiations are ionizing radiations. They
produce ion pairs in the genes and produce mutations. Ultra violet is a non ionizing radiation
that can cause mutations.
Physical Mutagens
Radiation
Physical property
Effect
X-ray
Gamma rays
Alpha rays
Beta rays
Neutrons
Ultraviolet
Electromagnetic
Electromagnetic
Particulate
Particulate
Particulate
Electromagnetic
Ionizing
Ionizing
Ionizing
Ionizing
Ionizing
Excitation
Chemical Mutagens
Several chemicals act as mutagens. The first chemical mutagen identified
is mustard gas. Many of the chemical mutagens can alter the chemical nature of genes and
produce mutations.
Chemical mutagen
Type
Effect
Nitrous acid
Ethyl Methane
Deaminating agent
Deamination of base
Sulphonate ( EMS )
Ethyle Ethane
Alkylating
Add alkyl groups to bases
Sulphonate (EES )
Alkylating
Add alkyl groups to bases
5-Bromo uracil
Base analogue
Substitution
Acridine orange
Fluorescent dye
Additions
This is the modern technique to induce mutation in plant
breeding., Molecular method is used to induce mutation only at the desired sites of the DNA.
Site directed mutagenesis
Sharbati Sonora
A variety of Wheat produced by induced mutation.
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