This presentation offers an interaction about Post-Mendelian genetics overview with special refrence4 to Mono-hybrid interaction.

1. GENETICS
POST-MENDELIAN GENETICS AND
FACTOR HYPOTHESIS
PRESENTED BY
N. Sannigrahi, Associate Professor
Department of Botany, Nistarini College,
D. B. Road, Purulia (W.B) 723101 , India

2. INTRODUCTION
Mendel was able to reach to the conclusion of the laws of
inheritance pattern & its transmission like law of segregation
and law of independent assortment due to his mathematical
interpretation along with the virtue of the experimental
material, Pea plant for its unique genetic nature. The seven
contrasting characters chosen by him was located in seven
different non-homologous chromosome that helped him to
draw the conclusion. But after Mendel’ discovery, the other
successors in the field of genetics tried to explore but got
different result which was not at per the Mendelian findings
irrespective of the Monohybrid or dihybrid experiments
conducted on the other experimental plants materials. These
findings of successors geneticists published as post-
Mendelian hypothesis or Factor hypothesis.

3. INCOMPLETEDOMINANCE
Mendel had shown that each character was governed by a
single gene and only one gene will express irrespective its
homozygous or heterozygous in nature. In presence of Tt, pea
plant will be tall. But deviations were discovered soon after the
experiments conducted on another plant, four-o’ clock,
Mirabilis jalapa. When a pure red colored flower plant
(dominant) was crossed with white (dwarf) one, it become
pink, neither red nor white, the intermediate between the two
color and it was due to expression of the both the factors or
alleles. It was treated as Incomplete dominance. The alleles of
one gene can interact among themselves at the functional level
resulting in variations in the expression of dominance and
marked phenotypes occur in different allelic combinations.

5. MULTIPLE ALLELES & CODOMINANCE
Generally, a gene has two alternative forms called alleles.
Usually, one of the two alleles of a gene dominant over the
other. The two alleles of a gene determine the two contrasting
forms of a single character e.g., round and wrinkled seed shapes
in Pea. But in many cases, several alleles of a single gene
determine are known each governing a distinct form of the
concerned trait. Such a situation is known as multiple allelism,
and the many alleles of a single gene are called multiple alleles.
In the case of multiple alleles, only the monohybrid ratio is
observed in the F2 generation. Many genes both prokaryotes and
eukaryotes (both plants & animals) show multiple alleles. Some
examples of such genes are Human ABO, Rh, Other blood
groups, human hemoglobin, self incompatibility in plants,
notched wing, white eye , color genes of drosophila , waxy gene
of maize , .

6. MULTIPLE ALLELES & CODOMINANCE
Blood groups of human generally determined by the antigenic
properties remain in the erythrocytes molecules of RBC. An
antigen is a molecule which specifically interacts with an
antibody, a kind of protein forms in presence of antigen. Most
antigens are immunogenic. An immunogenic is a molecule
which when introduced into the system of animal induces the
production of a class of proteins called antibodies. Antigen and
antibody are very much specific to each other. When molecules
of an antigen come in contact with the molecules of antibody
specific to this antigen, several antigen and antibody molecules
bind together and undergoes different interaction- agglutination,
precipitation, adherence, lyses , neutralization etc. This
interaction is highly specific and forms a very sensitive and
potent tool in biological study. Such process is called
serological tests.

7. ABO ANTIGEN & BLOOD GROUPS
Based upon the ability of antigens to carry out their functions,
antigens are of two types: complete antigens and incomplete
antigens (haptens). A complete antigen is able to induce antibody
formation and produce a specific and observable reaction with
the antibody so produced. Haptens (Gr. hapten to grasp; partial
antigens) are substances which are incapable of inducing
antibody formation by themselves, but can be capable of inducing
antibodies on combining with larger molecules (normally
proteins) which serve as carriers. Antigens which are present on
the body’s own cells are called the auto-antigens or self antigens.
The antigens on the non-self cells are known as foreign antigens
or non-self antigens. Red blood corpuscles of all ABO blood
groups possess a common antigen, the H antigen, which is a
precursor for the formation of A and В antigens. Due to universal
distribution, H antigen is not ordinarily important in grouping or
blood transfusion.

8. CODOMINANCE
Mendel did not anticipate the expression of the two factors
simultaneously. But in certain cases, both the alleles of a gene
express themselves in the heterozygote. This is called co
dominance. As a consequence, heterozygote for such genes
possess the phenotypes produced by both the concerned alleles.
Blood group antigens of man present excellent examples of co
dominance. One of the most widely known , and the earliest
recognized , human blood group is the ABO blood group. These
blood group arise due to the presence of antigen on the inner
surface of RBC. These antigens are produced by a gene i. One
dominant allele of this gene viz. IA allele produces the antigen
A, turns blood group A, another dominant allele is IB that
produces the antigen B and the blood group is B. If both the Ia &
Ib present, the blood group becomes AB and the absence of both
makes the blood group O.

9. CODOMINANCE
Marriages between heterozygous IA IB, having blood group
AB, produces three types of progeny. One fourth of their
progeny are homozygous for the allele Ia (IaIA) and have A
type of blood group, while another one fourth have B
blood group since they are homozygous for the IB
allele(IBIB). The remaining one half of the progeny have
AB blood group as they have heterozygous (IAIB) and
possess both the antigens, A & B. The co-dominance also
produces 1:2:1 ratio in F2 generation. Sickle cell anemia is
a kind of human disease and it is also the outcome of the
co-dominance of two different alleles in this regard. The
ladybird beetle, Harmonia axyridis, differ in the
pigmentation pattern found in the elytra or wing cover.
Some have black dots on a yellow background whereas the
others have broad black bands on a yellow black cover. It
is also due to co-dominance.

11. LETHAL FACTORS
A lethal factor cause death of all the individuals carrying this
gene in the appropriate genotype before they reach adulthood.
It may be either dominant or recessive lethal. It has been
observed that all genes or genetic factors are not useful to the
organism. There are some genetic factors or genes, when
present in any organism cause its death during early stage of
development. They may even cause death of the individual
either in homozygous dominant or homozygous recessive
condition. E. Baur (1907) observed lethal gene in
Snapdragon (Antirrhinum) and found that it is characterized
by variegated leaves. The “golden” variety on selfing gives
rise to 2 types of offspring, golden and green in the ratio of 2:1
instead of 3: 1. The golden ones are heterozygous and the
green ones breed true being recessive homozygous.

12. LETHAL FACTOR
In animal world, such type of interaction is also observed. A
French geneticist L. Cuenot (1905) reported on the
inheritance of mouse body color. He found that “yellow”
body color was dominant over normal “brown” color and
was governed by single gene “Y”. It was observed that
yellow mice could never be obtained in homozygous
condition. When yellow coated mice was crossed with
another yellow coated mice, segregation for yellow and
brown body color was obtained in 2: 1 ratio. The brown
individuals were pure and homozygous where as yellow
individuals were heterozygous. These results may be
explained on the assumption that the dominant allele for
yellow body color is lethal in homozygous condition. Lethal
genes may be of 5 types-Recessive, dominant, Conditional,
Balanced & Gametic lethal.

13. LETHAL FACTOR
On the basis of survivality, the genes may be of 5 types- Vital
genes, Lethal genes, Sub-lethal genes, Sub-vital genes & super-
vital genes.
Vital genes-The genes which do not affect the survival of the
individuals in which they are present are said as vital genes. It
does not mean that these genes are necessary for the survival of
the concerned individual. Wild type alleles of all the genes of an
organism are said as vital genes. In other words, the survival of
the organism is not influenced by the vital genes, whether may
be present in homo or heterozygous condition.
Sub-lethal genes- Such genes do not lead the organism to the
death that carry them in appropriate genotype. 90% of the
individuals die, however, only less than 10% of the individuals
survive. Some Xantha mutants of several plants are sub-lethal or
semi-lethal in the homozygous state.

14. LETHAL GENES
Most of the mutant genes increase the survival of such
individuals which carry them in appropriate genotype as
compared to that of wild type allele. Such genes are called as
super-vital genes .Super vital genes protect the individuals
carrying them against the various disease thus increasing the
chance of their survival. Likewise, the genes providing
resistance or tolerance to different environmental pressure or
strain like high and low temperature, low and high light
intensity, drought, salinity, alkanity etc. Most of the mutant
genes reduce the viability of individuals having them in
appropriate genotype as compared to that of normal
individuals. Most of the mutant genes are sub-vital in their
effect and kill less than 90% of the individuals which carry
them. The examples are some virdis mutants of barley,
miniature wings in Drosophila.

15. PLEIOTROPY
As per Mendel’s rule, one gene affects a single character. But
genes are known to affect more than one character; Such genes
are known as pleiotropic genes and the condition is known as
Pleiotropy. The most common example is found in human
beings. Gene s or Hbs which produces sickle cell anemia in the
ss homozygous condition. More than 50% of individuals for
this gene(ss) die before the age of 20 years. The primary effect
of this gene is the substitution of a valine molecule for a
glutamic acid molecule at the position 6 of the β-chain of
hemoglobin. The mutant (Sickle cell) hemoglobin remains in
solution as long as the solution conc. of oxygen is very high.
However, at low oxygen concentration, filamentous aggregates
of the sickle cell hemoglobin precipitate causing the red blood
cell assume the characteristic sickle shaped.

16. PllEIOTROPY
In addition to the morphological change of shape, the person
suffer from this genetic issue suffer multiple problems like
dilation of heart and heart failure, poor physical development,
impaired mental function, pneumonia due to lung damage ,
rheumatism, kidney damage and its defective damage. Different
other syndromes like Hunter’s syndrome, Huntington’s chorea,
cystic fibrosis etc also happen and all these health issues are very
serious concern in this field. Genes showing pleiotropy produce
a single polypeptide just like other nonpleiotropic genes. But
their polypeptides govern such a biochemical reaction, which is
basic to many biochemical events. As a result, the impairment of
this function interferes with a number of developmental events,
which in turn leads to the pleiotropic expression of the gene.

18. EXPRESSIVITY & PENETRANCE
Most of the genes produce identical phenotypes in all the
individuals that are present in the appropriate genotype. For
example, all the seeds having w allele, governing seed shape
in pea, in the homozygous state (ww) have uniformly wrinkle
shape , similarly those seeds have either WW or Ww
genotype will be uniformly round. The ability of a gene to
produce the same phenotype in all individuals carrying it in
the appropriate genotype is known as uniform expressivity. As
opposed to this, .any genes have variable expressivity in that
they produce variable phenotypes in the individuals that have
them in the appropriate genotype. An example of variable
e3xpressivity is furnished by the recessive gene producing
partial deficiency in the cotyledonary leaves of lima beans.
Cotyledonary leaves of seedlings homozygous for this gene
show variable degrees of chlorophyll deficiency-either it is
absent in the entire leaves or variable part. A single gene with

19. EXPRESSIVITY
With variable expressivity produces a number of phenotypes
often producing a quantitative variation in the concerned
characters. Variable expressivity may be due to the following
reasons: Modifying effect of genetic background or modifying
genes, and the effect of various environmental factors on gene
expression. In most cases , both these factors are likely to be
involved. In general, only mutant alleles show variable
expressivity , while the wild type (normal) alleles are buffered
against such variations. Many genetic conditions are identified
as a set of characteristics, occurring together. A group of
recognizable characteristics, which occur due to a common
cause is called a syndrome. Genetic syndromes are quite
variable in their expressivity. Though the mutation of Marfan
syndrome occurs in FBN1, the characteristics of the syndrome
widely vary among people.

20. PENETRANCE
Penetrance is the percentage of individuals with a given
phenotype, who exhibit the phenotype associated with that
particular genotype. In other words, it explains the extent to which
a particular gene or set of genes is expressed in the phenotypes of
individuals carrying it, measured by the proportion of carriers
showing the characteristic phenotype. It is a quantitative
measurement, describing the expression variations on the level of
phenotype. In general, gene express themselves in all individuals
in which they are present in the appropriate genotype; this is
known as complete penetrance. But genes do not produce the
concerned phenotypes in all individuals, which carry them
appropriate genotype; such a situation is known as incomplete
penetrance. For example, the recessive gene producing partial
chlorophyll deficiency in the cotyledonary leaves of lima beans
shows incomplete penetrance as well; it expresses itself only of

21. PENETRANCE
10% individual of the individuals. When a gene is present in the
appropriate genotype, the percent individuals in which it is able
to express itself is a measure of penetrance. Thus chlorophyll
deficiency gene in lima beans has a penetrance of 10%. Almost
all genes showing incomplete penetrance exhibit variable
expressivity as well. Thus, incomplete penetrance is, in fact, an
expression of the variable expressivity in that some individuals
show such a small expression of the gene that the trait
produced by itself has limited chance of detection through
phenotypes expression. Penetrance and expressivity describe
the relationship between a dominance genotype and its
associated phenotype. The expression of BRCA1 gene and
BRCA2 genes develop cancers in some individuals but not in
others as a result of penetrance.

22. Penetrance is the proportion of a particular genotype, expressing
its associated phenotype. It is a quantitative measurement of the
amount of the expression of a particular gene. Expressivity
describes the variations in gene expression of a particular
genotype. It is a qualitative measurement, which correlates with
the extent of gene expression. Therefore, the main difference
between penetrance and expressivity is in their parameters.
The pre- Mendelian genetics was mainly based on the assumption
of alleles that states the expression of phenotype by the single
gene or factor interaction and dominant will always suppressed the
recessive one. No concept of incomplete dominance, lethality, co
dominance and multiple alleles were advocated. But later research
confirmed the above concept and has been strengthen by the
concept of linkage, crossing over and masking effect of gene by
others.
CONCLUSION

23. THANKS FOR YOUR VISIT
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.