DAY TWENTY NINE
Genetics
Learning & Revision for the Day
u Genetic Terms u Chromosomal Theory of u Sex-Determination
u Mendel and his Experiments Inheritance u Sex-Linked Inheritance
u Deviation from Mendelism u Linkage u Mutation
u Interaction of Genes u Crossing Over u Genetic Disorders
The study of heredity and variations is called genetics.
Heredity (e.g. like begets like) is the transmission of characteristics from parent to
offsprings. Such transmissible characters are called hereditary characters.
Variations are differences in morphological, physiological, cytological and behavioural
characters shown by the individuals of the same species, race and family. The main
sources of variations are crossing over, chance combination of chromosomes during
meiosis, fertilisation and mutations. Mutation is the ultimate source of variations.
Archibald Garrod is considered as the Father of Experimental Genetics. The term
‘Genetics’ was coined by W Bateson in 1905.
Genetic Terms
Different genetic terminology are as follows
1. Allele is one of two or more alternative forms of a gene, in the gene pool that
occupies same locus on homologous chromosomes.
2. Dominant allele is an allele which is expressed in metabolism or the appearance
(phenotype) of heterozygotes. One allele is said to be dominant over another if a
heterozygous individual with one copy of that allele has the same appearance as a
homozygous individual with two copies of it.
3. Recessive allele is an allele whose expression is masked in heterozygotes by the
expression of a dominant allele.
4. Wild or mutant alleles The allele, which was originally present in the population
and is usually most common and dominant is called wild allele; whereas recessive
allele is less common and forms from wild allele through mutation. It is also called
mutant allele.
5. Gene is the basic unit of heredity; a sequence of DNA nucleotides on a chromosome
that encodes a polypeptide or RNA molecule and so determines the nature of an
individual's inherited traits.
6. Locus is the location of a gene on a chromosome.
7. Genotype is the total set of genes present in the cells of an organism. This term is
often used to refer the set of alleles of a single gene.
8. Phenotype is the physical appearance or metabolic expression of the genotype; the
observable manifestation of a trait (affecting an individual's structure, physiology or
, behaviour) that results from the biological activity of the 22. Genome is the complete set of chromosome where every
DNA molecules. gene chromosome is represented singly as in gamete. A
9. Haploid condition is characterised by the presence of single genome is present in haploid cells, two in diploid
only one set of chromosomes. It is found in gametes, cells and many in polyploid cells.
certain animals like protists and fungi and at certain 23. Pedigree analysis is the analysis of traits in several
stages in the life cycle of plants. generations of a family. The inheritance of a particular
10 Diploid condition is characterised by the presence of trait is represented in the family tree over generations. It
two sets of chromosomes, which are referred to as is a strong tool to trace the inheritance of a specific trait,
homologous. Higher animals and plants are diploid in abnormality or disease.
the dominant phase of their life cycles, so are some Some specific symbols are used to show pedigree in a
protists. pedigree chart
11. Heterozygote is a diploid individual carrying two Male
different alleles of a gene on two homologous Female
chromosomes. Most human beings are heterozygous for Sex unspecified
many genes.
Affected individuals
12. Homozygote is a diploid individual carrying identical
alleles of a gene on both homologous chromosomes. Mating
13. Hybrid is an organism produced after crossing two Mating between relatives
(consanguineous mating)
genetically different individuals, such process of
obtaining hybrids is known as hybridisation. Parents above and children below
(in order of birth-left to right)
14. Monohybrid cross is made to study the inheritance of a
single pair of allele. Parents with male child
15. Dihybrid cross is made to study the inheritance of two affected with disease
pairs of factors or alleles of two genes. 5 Five unaffected offsprings
16. Trihybrid ratio is the ratio obtained in
F2 -generation raised from a trihybrid cross followed
by selfing or inbreeding of F1 individuals. Eight Mendel and his Experiments
phenotypes are formed. The phenotypic ratio is l
Gregor Johann Mendel, an Austrian monk performed
27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. breeding experiments on the garden pea (Pisum sativum),
17. Reciprocal cross is a cross involving two types of for more than 8 years between 1856-1863. His experimental
individuals, where the male of one type is crossed with data was published in 1865.
female of the second type and vice-versa. l
Mendel used garden pea for his experiments due to its
18. Back cross is performed between F1 progeny and any well-defined characters, hermaphroditism, predominance
of self-fertilisation, easy hybridisation and emasculation
one of its parents.
(i.e. removal of anthers before maturity).
19. Test cross is performed between F1 progeny with its l
Mendel’s success was mainly based on the fact that he
recessive parent. The test cross ratio in monohybrid
considered a single character at one time.
cross is 1 : 1 and in dihybrid cross, ratio will be
1 : 1: 1 : 1. Seven Pairs of Contrasting
Characters Studied by Mendel
20. Pureline is a strain of genetically pure, true
breeding individuals, which have been derived Chromosome Parental Appearance
from a single self-fertilised homozygous ancestor. Characteristic Location
(Known now) Dominant Recessive
It always produces offsprings which are true for its
characters. Length of stem 4 Tall Dwarf
21. Checker board Punnett square is a square divided into Shape of seed 7 Round Wrinkled
smaller squares, which show the mathematical Colour of seed 1 Yellow Green
(probable) result of a cross, both phenotypic and
Shape of pod 4 Inflated Constricted
genotypic. It is of three types, i.e. gametic, phenotypic
and genotypic. Forked line or branching system is also Colour of pod 5 Green Yellow
used to know phenotypic and genotypic probabilities. Position of 4 Axial Terminal
It was devised by a British geneticist, Reginald Punnett flower
in 1927. Colour of flower 1 Violet White
, l
Genes controlling seven traits (characters) in pea studied When a dihybrid cross is made and the offsprings of
by Mendel are now known to be located on only four F1 -generation are self-bred, dihybrid ratio
chromosomes out of seven. 9 : 3 : 3 : 1 (phenotypic ratio) is obtained, where 9/16
l
Mendel’s work was rediscovered by Hugo de Vries (Dutch individuals carry both the dominant traits, 3/16 carry first
biologist), Carl Correns (German botanist) and Erich von dominant and second recessive trait, 3/16 carry first
Tschermak (Austrian botanist) in 1900. recessive and second dominant trait, where as, 1/16 carry
both the recessive traits.
Three Laws of Inheritance
P-generation
Mendel proposed three laws of heredity which are as follows
(Round yellow) (Wrinkled green)
1. Law of dominance It states that only one of the forms RR YY rr yy
of parental traits is expressed in the F1 , i.e. the
F1 -generation always displays only one of the parental
RY ry Gametes
traits. He described that the trait which were always
seen in F1 -generation are dominant traits and the trait
that disappeared are recessive.
F1-generation
(Red flower) (White flower) Parents (Round yellow)
RR rr Rr Yy
Selfing
R R r r Gametes
RY RY
F1-generation Gametes Gametes
rY rY
Rr Rr Rr Rr RRYY
(Red flower) (Red flower) (Red flower) (Red flower)
Ry Ry
Monohybrid cross RrYY RrYY
ry ry
2. Law of segregation It states that two alleles of each RRYy RRYy
rrYY
gene pair separate when gametes are produced during
meiosis. RrYy RrYy RrYy RrYy
Mendel’s law of segregation is universal and without
F2-generation
any exception. It is also called law of purity of rrYy RRyy rrYy
gametes or law of splitting of hybrids because gametes
always remain pure and may carry either the dominant Rryy Rryy
or the recessive factor of a single gene but never both.
(Tall plants) (Dwarf plants) rryy
TT tt Parents
Phenotypic ratio Round yellow : Round green
9 3
T t Gametes : Wrinkled yellow : Wrinkled green
3 1
Genotypic ratio 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1
Tt × Tt F1-generation
Results of a dihybrid cross where the two parents differed in
T t
two pairs of contrasting traits : seed colour and seed shape
F2-generation T TT Tt
t Tt tt
Phenotype ratio 3(Tall) : 1 (Dwarf) Deviation from Mendelism
1(TT) : 2 (Tt) : 1(tt) There are many exceptions and deviations of Mendel’s laws
Genotype ratio
Monohybrid cross
such as
l
Mendel observed that one allele dominates over the other
3. Law of independent assortment It states that pairs of but incomplete dominance in 4 O’clock plant (Mirabilis
alleles separate independently of each other during jalapa) and snapdragon are some exceptions.
gamete formation. This law was deduced from dihybrid
cross.
Genetics
Learning & Revision for the Day
u Genetic Terms u Chromosomal Theory of u Sex-Determination
u Mendel and his Experiments Inheritance u Sex-Linked Inheritance
u Deviation from Mendelism u Linkage u Mutation
u Interaction of Genes u Crossing Over u Genetic Disorders
The study of heredity and variations is called genetics.
Heredity (e.g. like begets like) is the transmission of characteristics from parent to
offsprings. Such transmissible characters are called hereditary characters.
Variations are differences in morphological, physiological, cytological and behavioural
characters shown by the individuals of the same species, race and family. The main
sources of variations are crossing over, chance combination of chromosomes during
meiosis, fertilisation and mutations. Mutation is the ultimate source of variations.
Archibald Garrod is considered as the Father of Experimental Genetics. The term
‘Genetics’ was coined by W Bateson in 1905.
Genetic Terms
Different genetic terminology are as follows
1. Allele is one of two or more alternative forms of a gene, in the gene pool that
occupies same locus on homologous chromosomes.
2. Dominant allele is an allele which is expressed in metabolism or the appearance
(phenotype) of heterozygotes. One allele is said to be dominant over another if a
heterozygous individual with one copy of that allele has the same appearance as a
homozygous individual with two copies of it.
3. Recessive allele is an allele whose expression is masked in heterozygotes by the
expression of a dominant allele.
4. Wild or mutant alleles The allele, which was originally present in the population
and is usually most common and dominant is called wild allele; whereas recessive
allele is less common and forms from wild allele through mutation. It is also called
mutant allele.
5. Gene is the basic unit of heredity; a sequence of DNA nucleotides on a chromosome
that encodes a polypeptide or RNA molecule and so determines the nature of an
individual's inherited traits.
6. Locus is the location of a gene on a chromosome.
7. Genotype is the total set of genes present in the cells of an organism. This term is
often used to refer the set of alleles of a single gene.
8. Phenotype is the physical appearance or metabolic expression of the genotype; the
observable manifestation of a trait (affecting an individual's structure, physiology or
, behaviour) that results from the biological activity of the 22. Genome is the complete set of chromosome where every
DNA molecules. gene chromosome is represented singly as in gamete. A
9. Haploid condition is characterised by the presence of single genome is present in haploid cells, two in diploid
only one set of chromosomes. It is found in gametes, cells and many in polyploid cells.
certain animals like protists and fungi and at certain 23. Pedigree analysis is the analysis of traits in several
stages in the life cycle of plants. generations of a family. The inheritance of a particular
10 Diploid condition is characterised by the presence of trait is represented in the family tree over generations. It
two sets of chromosomes, which are referred to as is a strong tool to trace the inheritance of a specific trait,
homologous. Higher animals and plants are diploid in abnormality or disease.
the dominant phase of their life cycles, so are some Some specific symbols are used to show pedigree in a
protists. pedigree chart
11. Heterozygote is a diploid individual carrying two Male
different alleles of a gene on two homologous Female
chromosomes. Most human beings are heterozygous for Sex unspecified
many genes.
Affected individuals
12. Homozygote is a diploid individual carrying identical
alleles of a gene on both homologous chromosomes. Mating
13. Hybrid is an organism produced after crossing two Mating between relatives
(consanguineous mating)
genetically different individuals, such process of
obtaining hybrids is known as hybridisation. Parents above and children below
(in order of birth-left to right)
14. Monohybrid cross is made to study the inheritance of a
single pair of allele. Parents with male child
15. Dihybrid cross is made to study the inheritance of two affected with disease
pairs of factors or alleles of two genes. 5 Five unaffected offsprings
16. Trihybrid ratio is the ratio obtained in
F2 -generation raised from a trihybrid cross followed
by selfing or inbreeding of F1 individuals. Eight Mendel and his Experiments
phenotypes are formed. The phenotypic ratio is l
Gregor Johann Mendel, an Austrian monk performed
27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. breeding experiments on the garden pea (Pisum sativum),
17. Reciprocal cross is a cross involving two types of for more than 8 years between 1856-1863. His experimental
individuals, where the male of one type is crossed with data was published in 1865.
female of the second type and vice-versa. l
Mendel used garden pea for his experiments due to its
18. Back cross is performed between F1 progeny and any well-defined characters, hermaphroditism, predominance
of self-fertilisation, easy hybridisation and emasculation
one of its parents.
(i.e. removal of anthers before maturity).
19. Test cross is performed between F1 progeny with its l
Mendel’s success was mainly based on the fact that he
recessive parent. The test cross ratio in monohybrid
considered a single character at one time.
cross is 1 : 1 and in dihybrid cross, ratio will be
1 : 1: 1 : 1. Seven Pairs of Contrasting
Characters Studied by Mendel
20. Pureline is a strain of genetically pure, true
breeding individuals, which have been derived Chromosome Parental Appearance
from a single self-fertilised homozygous ancestor. Characteristic Location
(Known now) Dominant Recessive
It always produces offsprings which are true for its
characters. Length of stem 4 Tall Dwarf
21. Checker board Punnett square is a square divided into Shape of seed 7 Round Wrinkled
smaller squares, which show the mathematical Colour of seed 1 Yellow Green
(probable) result of a cross, both phenotypic and
Shape of pod 4 Inflated Constricted
genotypic. It is of three types, i.e. gametic, phenotypic
and genotypic. Forked line or branching system is also Colour of pod 5 Green Yellow
used to know phenotypic and genotypic probabilities. Position of 4 Axial Terminal
It was devised by a British geneticist, Reginald Punnett flower
in 1927. Colour of flower 1 Violet White
, l
Genes controlling seven traits (characters) in pea studied When a dihybrid cross is made and the offsprings of
by Mendel are now known to be located on only four F1 -generation are self-bred, dihybrid ratio
chromosomes out of seven. 9 : 3 : 3 : 1 (phenotypic ratio) is obtained, where 9/16
l
Mendel’s work was rediscovered by Hugo de Vries (Dutch individuals carry both the dominant traits, 3/16 carry first
biologist), Carl Correns (German botanist) and Erich von dominant and second recessive trait, 3/16 carry first
Tschermak (Austrian botanist) in 1900. recessive and second dominant trait, where as, 1/16 carry
both the recessive traits.
Three Laws of Inheritance
P-generation
Mendel proposed three laws of heredity which are as follows
(Round yellow) (Wrinkled green)
1. Law of dominance It states that only one of the forms RR YY rr yy
of parental traits is expressed in the F1 , i.e. the
F1 -generation always displays only one of the parental
RY ry Gametes
traits. He described that the trait which were always
seen in F1 -generation are dominant traits and the trait
that disappeared are recessive.
F1-generation
(Red flower) (White flower) Parents (Round yellow)
RR rr Rr Yy
Selfing
R R r r Gametes
RY RY
F1-generation Gametes Gametes
rY rY
Rr Rr Rr Rr RRYY
(Red flower) (Red flower) (Red flower) (Red flower)
Ry Ry
Monohybrid cross RrYY RrYY
ry ry
2. Law of segregation It states that two alleles of each RRYy RRYy
rrYY
gene pair separate when gametes are produced during
meiosis. RrYy RrYy RrYy RrYy
Mendel’s law of segregation is universal and without
F2-generation
any exception. It is also called law of purity of rrYy RRyy rrYy
gametes or law of splitting of hybrids because gametes
always remain pure and may carry either the dominant Rryy Rryy
or the recessive factor of a single gene but never both.
(Tall plants) (Dwarf plants) rryy
TT tt Parents
Phenotypic ratio Round yellow : Round green
9 3
T t Gametes : Wrinkled yellow : Wrinkled green
3 1
Genotypic ratio 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1
Tt × Tt F1-generation
Results of a dihybrid cross where the two parents differed in
T t
two pairs of contrasting traits : seed colour and seed shape
F2-generation T TT Tt
t Tt tt
Phenotype ratio 3(Tall) : 1 (Dwarf) Deviation from Mendelism
1(TT) : 2 (Tt) : 1(tt) There are many exceptions and deviations of Mendel’s laws
Genotype ratio
Monohybrid cross
such as
l
Mendel observed that one allele dominates over the other
3. Law of independent assortment It states that pairs of but incomplete dominance in 4 O’clock plant (Mirabilis
alleles separate independently of each other during jalapa) and snapdragon are some exceptions.
gamete formation. This law was deduced from dihybrid
cross.
