Chromosomes and Mechanism of inheritance.
The
transmission of genetic information from one generation to other generation is
known as heredity or inheritance.
1.
The mechanism of inheritance was successfully investigated before
the study of chromosomes or genes
2.
Gregor Mendel, son of the peasant farmer, was born in Moravia in
1822.
3.
Gregor Mendel first gave
the accurate explanation for the mechanism of inheritance by using
hybridization technique.
4.
Mendel studied seven traits in garden pea plant individually one
at a time or in combination of two or three character at a time.
5.
These characters are -
|
Sr. No.
|
Character
|
Dominant
|
Recessive
|
|
1
|
Height
of stem
|
Tall
(TT)
|
Dwarf
(tt)
|
|
2
|
Colour
of flower
|
Coloured
(CC)
|
White
(cc)
|
|
3
|
Position
of flower
|
Aerial
(AA)
|
Terminal
(aa)
|
|
4
|
Colour
of pod
|
Green
(GG)
|
Yellow
( gg)
|
|
5
|
Shape
of pod
|
Inflated
(II)
|
Constricted
(ii)
|
|
6
|
Shape
of seed
|
Round
(RR)
|
Wrinkled
(rr)
|
|
7
|
Seed
colored
|
Yellow
(Y)
|
Green
(y)
|
6.
He processed the data
mathematically and statistically.
7.
Mendel postulated the
principles of heredity which are known as fundamental laws of heredity, as
proposed by Correns (1900).
8.
According to Mendel, transmission of characters due to ‘something’
present inside the gametic cell.
9.
To this ‘something’, he coined term ‘factors’ that are
responsible for expression of a particular trait/ character.
10. He proposed that factors are particulate in
nature.
11. The term the factor is now known as gene
which is given by Johannsen.
12. These factors occur in
pairs in the parents and segregate from each other during gamete formation
without blending/ mixing.
Reasons for Mendel’s Success:
1)
Mendel
chose garden pea plant for his experiment which was an annual, naturally self-pollinating
plant with several pairs of contrasting character.
2)
experiments were carefully planned and involved large sample.
3)
Mendel
used pure breeding varieties which are verified personally.
4)
He
considered contrasting characters for his experiment.
5)
Mendel
considered only one character at a time
6)
Each character in pea plant was controlled by a single factor.
7)
These factors are located on separate chromosomes and these
factors are transmitted from generation to generation
8)
He introduced the concepts of dominance and recessive.
9)
He
kept accurate records.
10) He used statistical method for analyzing
of the results.
11) The characters selected by Mendel where
present on different chromosomes.
Genetic Terminology:
1. Character: It is a specific
feature of an organism e.g. height of stem.
2. Trait: An inherited character and its detectable variant e.g. Tall or
dwarf.
3. Factor: According to Mendel, it is
a unit of heredity, a particle present in the organism which is responsible for
the inheritance and expression of a character. (factor is passed from one generation to the
next through gametes). Factor determines a genetical (biological) character of
an organism.
4. Gene: It is a particular
segment of DNA which is responsible for the inheritance and expression of that
character.
5. Alleles or Allelomorphs: The two or more
alternative forms of a given gene (factor) present on identical loci
(positions) of homologous chromosomes is known as allele.
Allele is a short form of Allelomorph.
6. Dominant: It is an allele that expresses
its trait even in the presence of an alternative allele i.e. in heterozygous
condition only.
The allele that expresses in F1 is called dominant. (It is an
allele of a pair that hides the expression of other allele in F1 generation.)
7. Recessive: This allele is not
expressed in the presence of an alternative allele (in heterozygous condition).
It expresses only in the presence of another
identical allele.
It is an allele that does not express in F1
hybrid.
8. Phenotype: The external apperance
of an individual for any trait is called phenotype for that trait.
It is observable and is
determined by different combinations of alleles. e.g. In pea, for the height of
stem (plant) tall and dwarf are the two phenotypes (Tall is determined by TT or
Tt and dwarf by tt).
9. Genotype: Genetic constitution or genetical
make up of an organism with respect to a particular trait.
It is representation of the genetic constitution of an
individual with respect to a single character or a set of characters. e.g. pea
tall plants can have genotype TT or Tt and dwarf has tt.
10. Homozygous (pure): An individual possessing
identical alleles for a particular trait, is called homozygous or pure for that
trait.
Homozygous breeds true to the trait
and produces only one type of gametes e.g., tall with TT and dwarf with tt.
11. Heterozygous: An individual
possessing contrasting allele for a particular trait, is called heterozygous.
Heterozygous does
not breed true for that trait and produces two types of gametes e.g. F1
generation hybrids (Tt).
Heterozygous
individual is also called hybrid.
12. Pure line: An individual or a group
of individuals (population) which is homozygous or true breeding for one or
more traits, constitutes pure line i.e. plant which breeds true for a
particular character. It is a descendent of a single homozygous parent produced
after self-fertilization.
13. Monohybrid: It is heterozygous for one
trait and is produced from a cross between two pure parents differing in single
pair of contrasting characters
e.g. Hybrid tall produced in a
cross between pure tall and pure dwarf parents. It is a heterozygote for a
single pair of alleles.
14. F1 generation: It refers to the first
filial generation. It consists of all off-springs produced from a parental
cross.
15. F2 generation: The second generation
(progeny) produced by selfing (inbreeding) of F1 generation
offsprings is called second filial generation.
e.g. Progeny produced from a cross between two
F1 individuals (e.g. Tt × Tt).
16. Punnett square/checker
board :
It is a probability table representing different permutations and combination
of fertilization between gametes of the opposite mating types.
In short,
it is a diagrammatic representation of a particular cross to predict the
progeny of a cross.
17. Homologous Chromosomes: The morphologically,
genetically and structurally essentially identical chromosomes present in a
diploid cell, are called homologous chromosomes.
Such chromosomes synapse during
meiosis.
18. Back cross: It is a cross of F1
progeny with any of the parents (e.g. F1 tall, pure tall× F1
tall, pure dwarf (Tt,TT×tt).
19. Test cross: It is a cross of F1 progeny
with homozygous recessive parent (e.g. F1 tall ×pure dwarf (Tt × tt ).
It is used to test the homozygous/
heterozygous nature of hybrid. It is a kind of back cross.
20. Phenotypic ratio: It is the ratio of the
offsprings produced in F2 and subsequent generation with respect to their
physical appearance
e.g. 3Tall: 1 dwarf, is F2 ‘Phenotypic
ratio’ in monohybrid cross.
21. Genotypic ratio: It is the ratio of the
offsprings produced in the F2 and subsequent generation with respect to their
genetical makeup
e.g. 1 TT: 2 Tt: 1 tt, is F2
genotypic ratio in monohybrid cross.
