INHERITANCE AND VARIATION
2)The mechanism of inheritance was successfully
investigated before the study of chromosomes or genes.
3)Gregor
Mendel, son of the peasant farmer, was born in Moravia in 1822.
4)Gregor
Mendel first gave the accurate explanation for the Mechanism
of inheritance by using hybridization
technique.
5) Inheritance
of seven traits in garden pea plant
individuaIy one at a time or in combinationof two or three character at
a time.
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)He
visualized that the traits as such are not inherited physically but by
'something' present inside the gametic cell.
10)To
this 'something', he coined term 'factors' that are responsible for expression
of a particular trait! character.
11)He
proposed that factors are particulate in nature.
12)The
term the factor is now known as gene which is given by Johannsen.
13)These
factors occur in pairs in the parents and segregate from each other during
gamete formation without blending/ mixing.
Reason
of Mendel Success:
1)His
experiments were carefully planned and involved large sample.
2)He
carefully recorded the number of plants of each type and expressed his results
as ration.
3)In
pea plant contrasting characters can be
easily recognized..
4)The
seven different characters in pea plant were controlled by a single factor each.
5)The
factors are located on separate chromosomes and these
factors are transmitted from
generation to generation.
6)He
introduced the concepts of dominance and recessiveness.
7)Before
learing about Mendel’s experiments let us get acquainted with genetic terms and symbols
8)
Mendel used pure breeding varieties which are verified personally
9)
He used statistical method for analyzing of the results.
10)The
characters selected by Mendel where present on different chromosomes
11)Mendel
chose garden pea plant for his experiment which was an annual, naturally self-
pollinating plant with several pairs of contrasting character
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. Tailor 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 a given gene (factor) are called allele of each other.They
occupy identical loci (positions) of
homologous chromosomes . 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 d termined 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
coostitution or genetical make up of an organism with respect to aparticular
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 heterozy.gote 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 F 1 generation offsprings is called second filial generation.
e.g.
Progeny produced from a cross between two F1 individuals (e.g. Tt x 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 tallx F1
tall, pure dwarf (Tt.Tl xtt).
19)Test
cross: It
is a cross of F1 progeny with homozygous recessive parent (e.g. F1 tall xpure
dwarf (Tt x 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 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
A
cross between parents differing in only one heritable traits, is called monohybrid cross.
in
a cross between pure tall and pure dwarf parents. Mendel performed
the monohybrid cross between
two pea plant with only
one pair of contransting character.
Phenotypic ratio- 3:1 Genotypic ratio- 1:2:1
A cross between parents differing in two heritable traits, is called dihybrid cross.
e.g. cross of pure tall, round . seeded plant with dwarf, wrinkled seeded plant. Mendel also performed the dihybrid cross between pea plants that differed in two pairs of contrasting characters.
F2
generation shows four types of
offspring
.Round
,Yellow seed -9
.Round
,Wrinkled seed -3
.Round
,Green seed -3
.Wrinkled,Green
seed -1
Phenotypic ratio- 9 : 3
: 3 : 1
Genotypic
ratio -1 : 2 : 2 : 4 : 1 : 2 :1: 2
: 1
RRYY : RRYy : RrYY : RrYy : Rryy : Rryy : rrYY : rrYy : rryy.
Mendel proposed
three three laws below-
In
monohybrid and dihybrid crosses, the phenotypic characters are controlled by
discrete units, called factors. In a dissimilar pair of factors, one
member of the pair dominates (i.e. dominant) over the other (i.e. recessive).
The
law of dominance is used to explain the expression of only one of the parental
characters of a monohybrid cross in F1 and the expression of both in F2.
Statement of Law of Dominance:
"When
two homozygous individuals are crossed with each other considering one or more
sets of contrasting characters, the alleles (characters) that appear in F1
are dominant and those which do not appear in F1 are recessive".
2.)Law of segregation (Law of purity of gametes):
1)This
law is based on the fact that the alleles do not show any blending / mixing and
both the alleles (characters) are recovered as such in the F2 generation,
though one of these is not seen at the F1 stage.
2)During formation of gametes, these two alleles
(factors) obviously separate or segregate, otherwise recessive type will not
appear in F2 generation.
3)The
gametes which are formed are always pure for a particular character (trait).
4)
A gamete may carry either dominant or recessive factor but not both. That's why
it is also called as law of purity of gametes.
Statement of Law of Segregation:
The
law states that "When hybrid (F1) forms gametes, the alleles segregate
from each other and enter in different gametes".
The
gametes formed are pure in that they carry only one allele each (either
dominant allele or recessive allele).
Hence, this law is also described as "Law of purity of gametes .
3)Law of
Independent Assortment :
1)This
law is based on dihybrid cross.
2)It
is basic principle of genetics developed by a Mendel. .
3)It
describes how different genes or alleles present on separate chromosomes
independently separate from each other, during formation of gametes.
4)These
alleles are then randomly united in fertilization.
5)In
dihybrid cross, F2 phenotypic ratio 9:3:3:1 indicates that the two pairs of
characters behave independent of each other.
6)
It can be concluded that the two characters under consideration are assorted
independently giving rise to different combinations.
Statement of Law of Independent Assortment:
The law states that "When hybrid
possessing two (or more) pairs of contrasting factors (alleles) forms gametes,
the factors in each pair segregate independently of the other pair".
Back Cross and Test Cross:
a)Back cross:
The
F1 individuals obtained in a cross are usually selfed to get the F2 progeny.
They
can also be crossed with one of the two parents from which they were derived
(either recessive or dominant). Such a cross is known as back cross.
b)Test cross:
The
cross of F1 hybrid with the homozygous recessive parent is known as a test
cross.
Test
cross is use to find out whether an individual is homozygous (pure) or heterozy
gous( hybrid).
Test
cross can be used to find out genotype of any plant with dominant expression.
But it is not known whether it is homozygous
(pure) or heterozygous for that trait.
For
example, of Test cross:
A
pea plant having violet (purple) flowers is crossed with a pea plant
with white flowers. .
If
all flowers produced are violet, then the plant is pure or homozygous
and if flowers are violet and white flowers in 1:1 ratio, then the plant
is heterozygous.
Test
cross is also used to introduce useful recessive traits in the hybrids of
self-pollinated plants during rapid crop improvement programs.
Following is the graphic representation of test cross. Recessive parent is crossed to find out unknown genotype.
Deviations from
Mendel's findings:
On
the basis of study of Mendel experiments various conclusions are given such as-
i) Single trait-- Single gene--Two alleles.
ii)Two
alleles show interaction in which one is completely dominant.
iii)
Factors (genes) for different traits
present on different chromosomes assort independently.
With the passage
of time, number
of deviation were
observed/ identified in the
post –medelian era.that gave additional information on the patterns of
inheritance. These
deviationare then described as Neo-Mendelism.
It was
observed that the phenotypic expression of a
gene can be modified
or influenced by the
other gene.
These gene are interaction areof two types. i)Intragenic and ii)Intergenic
i) Intragenic interactions:
Occur
between the alleles of same gene.
Eg.incomplete
dominance and co-dominance.
It also occurs between the multiple allele
series of a gene
ii)Intergenic
(non-allelic) interactions:.
Occur between the alleles of different-genes
present on the same or different-chromosomes. Pleiotropy, polygenes, epistasis
supplementary genes, complementary genes. Etc.
Some of thesenteractions discussed below.
a)Incomplete
dominance:
Incomplete
dominance both the alleles (gene) Of an
allelomorphic pair express themselves partially.
One alleles (gene) can not supress the
expression of the
other allele (gene) completely.
In such
case there is an intermediate
expression in the F1
hybrid.
b) Co-dominance:
In
Codominance is both the alleles (genes ) of an allelomorphic pair expressed themselfs equally in f1 hybrid such
allele'swhich are able equally independently in hybrid are called
co-dominance alleles. Thus in co-dominance both alleles are expressed.
Classic
example of co-dominance is coat colour in cattle .
There are two types of cattle:
1)One
with red coat (skin) with red colored hairs ) and another with white coat (with
white hairs)
2)When
red cattle (RR) is cross with white cattle (WW)
f1 hybrids are roan colored (RW). .
3)Roan
colour is the mixture of red & white hairs.
4)Thus,
both the traits are expressed equally .
5)In
f2 generation one red (RR),
Roan two (RW) & one white (WW) individuals are produce in the ratio (1
:2:1).
6)Thus,
co dominance shows the qenotyoic or
phenotypic ratio same or
identical.
c) Multiple alleles:
More
than two alternative forms (alleles) of a gene in a population occupying the
same locus on a chromosome or its homologue, are known as multiple alleles.
1)Multiple
alleles arise by mutations of the wil type of gene.
2)A
gene can mutate several times producing a series of alternative expression.
3)Different
alleles in a series show dominant- recessive relation or may show co-dominance
or incomplete dominance among themselves.
4)Wild
type is dominant over all other mutant alleles
5)In
Drosophila, a large number of multiple alleles are known.
6)
e.g. In drosophila large no f multiple allele's are observe the one example of
multiple alleles
7)The
size of wings i.e. just stumps, is due to one allele (vg) in homozygous
condition.
8)The
normal wing is wild type while vestigial wing is recessive type.
d)Pleiotropy:
When
a single gene controls two or more different traits it is called as pleiotropic
gene & this phenomena is called as pleiotropy or Pleiotropism.
1)The
Phenotypic ratio is 1:2 the instead
of 3:1 beacause of the death of
homozygous .
2)The
disease sickle-cell anemia iscaused by a gene Hbs.
nolmal or healthy gene HbA is dominant.
3)The carriers
(heterozygotes HbA/Hbs) show
signs of mild anemia as their
RBCs become Sickle-shaped
4)i.e. half-moon- shaped only under abnormally low O2
concentration
5)The
homozygotes with recessive gene Hbs however, die of total anaemia.
6)Thus,
the gene for sickle- cell anaemia is lethal in homozygous condition and
produces sickle cell trait in heterozygous carrier..
7)
Two different expressions are produced by a single gene.
8) A marriage between two carriers will produce normal, carriers and sicklecell anemic children in
1 :2:1 ratio.
9)
Sickle cell anemics die leaving carriers and normal in the ratio 1 :2.
10)The
heterozygotes or carriers can be identified by microscopic examination of
blood. Chromosom.
