This PowerPoint presentation intends to explore the thought and development of genetics after G.J.Mendel along with the factor hypothesis in this new line of research during the contemporary.

1. Post- Mendelian Genetics & Factor
Hypothesis w.s.r. Dihybrid Cross
Presented By
N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia
D. B. Road, Purulia (W.B) India
GENETICS

2. POST-MENDELIAN GENETICS
Mendel reached to his findings due to his choice of
experimental materials, garden pea where each character was
governed by a single factor or gene and by dint of his
mathematical inquisitiveness, he reached to the principles as
stated earlier .This laid the foundation of experimental genetics.
But later on during the early of the 20th century, it was found
that the expression of many characters in almost all the
organisms except pea is governed by two or more factors and
the effect of the development of the concerned characters in
various ways as an outcome of the interaction of the concerned
factors in this regard. The modification of phenotypic ratio of
3:1 or the dihybrid ratio 9:3:3:1 is an interesting episode in
genetics –called Post- Mendelian genetics or factor Hypothesis.

3. TYPES OF GENE INTERACTION
Gene interaction among two or more genes for the regulation of
character became a new domain of research in this field. A number
of gene interaction was observed but the most common and
familiar issues to be addressed in this regard are as follows:
➢Typical Dihybrid ratio for a single trait
➢Duplicate gene action
➢Complementary gene action
➢Supplementary gene action
➢Inhibitory gene action
➢Masking gene action
➢Polymeric gene action
➢Additive gene action along with the test cross ratio to find out
whether the phenotype is the outcome of homozygous or
heterozygous gene interaction.

4. DIHYBRID TEST CROSS
Test cross is the cross of the determination of the genotype of a
dominant character bearing traits in the subsequent generation.
Mendel tested his theory by crossing the F1 plant (double
heterozygote) to a completely recessive . If Mendel's
hypothesis was correct, the progeny would be of four kinds-
wrinkle green, wrinkle white, round yellow and round green in
the ratio of 1:1:1:1 as expected from a dyhybrid back cross top
the double recessive parent. Mendel obtained , in the test cross
progeny , 55 round yellow, 51 round green, 53 wrinkle green
and wrinkle yellow 49. This is approximately 1:1:1:1. A cross
of this type is used in practical breeding programmes
undertaken for the assessment of the desired attributes for
further designing of the hybrid plants in the subsequent
generation. As it is a kind of back cross but test cross by
objectives to know the genotype of the unknown one.

6. TYPICAL DIHYBRID RATIO
Mendel initial findings was the regulation of character by one
factor or gene and two genes control two different traits if they
remain present together. But later on , it was confirmed that one
trait may be controlled by more than one factors but the presence
of the two factors in dominant form may produce quite different
traits which do not match with the expected Mendelian dihybrid
result in F1 generation. The concerned single character is
controlled by two genes exhibiting complete dominance. The
dominant alleles of each of the two genes produce the separate
forms of the character (phenotypes) when they are alone i.e. when
the dominant allele of one gene is present with the homozygous
recessive allele of the other gene. But when the dominant alleles
of both the genes present together, they produce distinct
phenotype.

7. Let for give an example of the shape of comb of chickens
governed by two genes- p& r. The dominant allele of gene p alone
produce pea comb while that of r alone produces rose comb. Thus,
the genotype PPrr produces pea comb while ppRR gives rise to rose
comp. But when both the genes remain in dominant form-P & R
together i.e PPRR, such individual has walnut comb. Recessive
condition of both the loci i. e pprr gives rise to distinct shape called
single. When both breed of poultry homozygous for pea comb
(PPrr) is crossed with another breed of homozygous for rose comb(
ppRR), the F1 individual have walnut comb as they bear both the
genes in dominant forms. Segregation of the genes produce 16
possible combination in F2.Nine of these 16 combinations have at
least one P and one R giving rise to 9 walnut comb. Out of 16, at
least three have one or two P alleles but homozygous rr, giving rise
to rose comb. The remaining individuals is single. Thus, interaction
of two dominant genes produce different traits contradicts to
Mendelian expected ration-a kind of deviation.

8. COMPLEMENTARY GENE ACTION
Mendel was always in favor of individual gene action for
individual trait but post- Mendelian exploration found another
type of gene action where an individual trait is the outcome of
two dominant genes. Absence of one in dominant form can
modify the trait. That is production of one phenotype requires
the presence of dominant alleles of both the genes controlling
character. When any one of the two or both the genes are present
in the homozygous recessive state, the contrasting phenotype is
produced. Thus any of the dominant genes is unable to produce
the phenotype when it is alone, but dominant alleles of the two
genes complement to each other to produce the concerned
phenotype when they are together. Such type of gene action is
regarded as complementary genes .

9. Let us take an example to establish the complementary gene
action. In sweet pea, Lathyrus sativus, the development of purple
flowers require the presence of two dominant genes-C & R, i.e
CCRR. When either C ( cc RR) 0r R (CCrr) or both of them (ccrr)
are present in homozygous recessive form, purple color flower
can not be produced; as a consequence, white flowers are formed
due to lack of the synthesis of anthocyanins. When a purple –
flowered variety of sweet pea (CCRR) is crossed with white
(ccrr), the F1 becomes purple color due to CcRr genotype derived
from the fusion of the subsequent gametes. I F2 generation, on an
average, 9 plants will have at least one dominant alleles of both
the genes C & R. Three out of the 16 plants will have dominant C
bit with homozygous rr. Three others will have dominant R but
will be homozygous cc. The remaining one plant will have both
the genes in homozygous recessive state (ccrr) and all the 7 will
have white flowers. The typical dyhbrid ratio is modified into 9:7

11. SUPPLEMENTARY GENE ACTION
Mendel did not consider the modification of one factor or gene
by the another one because of his choice of materials where one
character is regulated by one factor. But gene action explores
another magic of reality which is the appetite of other geneticists
during post- Mendelian era. The dominant allele of one gene
produces a phenotype effect. The dominant allele of the other
gene does not produce any phenotypic effect on its own. But
when it is present with the dominant allele of the first gene, it
modifies the phenotype effect produced by the first gene. This
kind of interaction is called supplementary gene action., the
dominant allele one gene is necessary for the development of the
concerned genotype while that of other gene modifies the
phenotypic effect of the first gene. The following example can
explain the supplementary gene action And it add an another
stomach for the domain of genetics.

12. The development of aleurone (grain) color in maize is governed by
two completely dominant genes-R & Pr. The dominant allele R is
essential for color production. Homozygous state of the recessive
allele rr prevents the production of red color The gene Pr is unable
to produce any color of its own. But it modifies the color produced
by the gene R from red to purple The recessive allele pr has no
effect on grain color. When inbreed purple maize grains (RRPrPr0
are crossed with inbred white (prpr), the F1 (RrPrpr) plants
produce purple grains. In the F2, 9/16 will have dominant alleles
of both the genes R & Pr; they therefore develop into purple
grains. 3/16 will have dominant allele of the gene R but
homozygous for pr (RRprpr). These grains will develop red color
since the recessive allele pr has no effect on color production
.Three other zygotes will be homozygous rr but will have
dominant allele of Pr (rrPrPr), these seeds will be white since rr
prevents the production of any color and Pr also does not produce
any color. The remaining one zygote will become white modifying
the ratio 9:3:4

14.  Another interesting phenomenon observed as far as the expression
of the one dominant gene while the presence of recessive allele
produces another phenotypic trait. One dominant gene or factor
produces the concerned phenotype or the character while its
recessive allele produces the contrasting phenotype. The second
dominant gene has no effect of its own on the character in question,
but it stops the expression of the dominant allele of the first gene.
As a consequence, when the two dominant genes are present
together, they produce the same phenotype as that produced by the
recessive homozygote of the first gene. The recessive allele of the
second gene does not affect the development of the character in any
way. Thus, in inhibitory gene action, one dominant gene is capable
of producing a phenotype only it its expression is not prevented by
another dominant gene. The gene that stops the expression of
another gene is called inhibitory gene (I) , called epistatic gene and
the mask the effect of the another gene(hypostatic gene) modified
the 9:3:3:1 into 13:3.

15.  An example of the inhibitory gene action occurs in the development
of the comb color of the hen. The dominant R produces red color
while it recessive allele r produces no (white) color. Another
dominant gene I does not produces any color by itself but it prevents
color production by the gene R when both I & R present together.
The recessive allele I does not affect the color production in any
way. As a result, red color in the aleurone is produced only when R
is present with the homozygous recessive allele of the inhibitory
gene ( Rrii). When a maize inbred with red aleurone (Rrii) is
crossed with another inbred having the genotype rrII and white
grains, the F1 (RrIi) has white grains. This is because the gene I
stops the color production by R. on an average , nine out of sixteen
will have at least one dominant allele of both R and I. These seeds
will develop no color due to inhibitory action of I on the color
production by R and (Rrii) or in heterozygous state (Rrii) but will be
homozygous for the recessive allele of I, the dominant allele of I.
So, 13 will be white and three will be red as a matter of inhibitory
gene action.

16.  A miracle happen very often in the action of genes for the regulation
of phenotype of different individual that does not support the
conventional Mendelian traits. In one kind of interaction, dominant
alleles of the two genes affecting a character produce distinct
phenotypes when they are alone . But when dominant alleles of both
the genes are present together, the expression of dominant allele of
one gene masks the expression of the other. When both the genes
are present in recessive state, a different phenotype is produced. It
should be noted that this type of gene interaction is distinct from
that found in case of inhibitory gene action in the following types.
 1. One gene doers not inhibit the expression of the other gene,
which is in case of inhibitory gene action,
 2. In fact, both the gene express themselves when their dominant
alleles are present together,
 3. But the expression of one gene is so intense and strong that the
expression of the other gene cannot be observed---masking gene
action.

17.  Seed coat color in barley is governed by two dominant genes-
B & Y. The B produces black while b produces white and the
dominant allele Y produces yellow seed coats while it
recessive allele produces white color. When both B & Y
present together, both of them express themselves and intense
black color is produced that suppress the yellow. When a
barley strain with black seed coat having genotype Bbyy is
crossed with another strain of yellow coat having genotype
bbYY, the F1 seeds have black seed coat (BbYy). In the F2,
the average 9 seeds will have at least of one dominant allele of
both the genes, B and Y. In three other seeds, the recessive
allele b will be in homozygous state while one or two
dominant alleles of both Y will be present (bbYY ,BBYy). The
remaining one zygote (bbyy) will develop white coat and the
(:3:3:1 has been modified into 12:3:1- a case of masking gene
action.

18.  Here the marvels of genetics is exhibited. Two completely
genes controlling a character produce the same phenotype
when their dominant alleles are alone. But when the dominant
alleles of both the genes are present together, their phenotypic
effect is enhanced as if the effect of the two genes were
cumulative or additive. In barley, two completely dominant
genes A and B affect the length of awns ( the thin needle like
extensions of lemma). Dominant allele of the gene A or B
alone ( Aabb, Aabb, aaBb, aaBB) give rise to the awns of
different lengths. The phenotypic effect of A or B are equal
because of the general rule but when both the genes of A & B
stay together, they produce long awns. Individuals
homozygous recessive alleles of both the genes are awnless.

20.  Thus from the above explanation, it becomes crystal clear that
after the foundation principles as laid down by Mendel on the
basis of his experimental outcome with respect to pea, Pisum
sativum but during the post-Mendelian period, the result
obtained thereafter did not confirm the principles of Mendel in
detail although the experimental outcome was interpreted in
the light of the Mendelian concept. As previous told, Mendel
was quite lucky enough to have the magnificent result of the
experiments due to lot of positive features of the experimental
material but it can be conclusively stated that Mendel opened a
new road of research to interpret the inheritance of the
acquired character and its possible reasons behind the marvels
of the nature. In a word, the Neo-Mendelism is a new recipe of
thought to the geneticist to think in much more elaborated
form for the beauty of the biology in general and genetics in
particular.

21. References:
1. Google for images,
2. Principles of Genetics- Basu & Hossain,
3. A textbook of Botany (Vol III) Ghosh,
Bhattacharya, Hait
4. Fundamentals of Genetics- B.D. Singh,
5.A Textbook of genetics- Ajoy Paul
DISCLAIMER:
This presentation has been made to enrich
open source of information without any
financial interest. The presenter
acknowledges Google for images and other
open sources of knowledge to develop this
PPT.