3.5 Deviations from Mendel's findings :

Few generalizations were arrived at by Mendel, on the basis of his experiments of garden pea plant- 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 deviations were observed/ identified in the post-Mendelian era, that gave additional information on the patterns of inheritance. These deviations are then described as Neo-Mendelism.

The deviations are :
a. Single trait →Single gene → two alleles showing interactions like codominance and incomplete dominance.
b. Single trait →Single gene more than two alleles (multiple allelism).
c. Single trait → more than one genes (Polygenic inheritance) showing different epistatic interactions or additive effect.
d. Single gene influencing many traits (pleiotropy).

It was observed that the phenotypic expression of a gene can be modified or influenced by the other gene. These gene interactions are of two types.

i. Intragenic interactions :

Occur between the alleles of same gene e.g. 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. e.g. pleiotropy, polygenes, epistasis, supplementary and complementary genes, etc.

a. Incomplete dominance:

In the incomplete dominance, both the alleles (genes) of an allelomorphic pair express themselves partially. One allele (gene) cannot suppress the expression of the other allele (gene) completely. In such case, there is an intermediate expression in the F1 hybrid. A well-known example is the flower colour of Mirabilis jalapa. If a red-flowered (RR) plant is crossed with a white-flowered (rr) plant, then F1 offsprings have pink (Rr) flowers.

b. Co-dominance :

In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in F1 hybrids. Such alleles which are able to express themselves equally independently in hybrids, are called co-dominant alleles. Thus in co-dominance both alleles are expressed.

Classic example of co-dominance is coat colour in cattle. There are two types one with red coat (with red colour hair) and other with white coat (with white hair). When red cattles (RR) are crossed with white cattles (WW), F1 hybrids (RW) are roan. Roans have the mixture of red and white colour hair. Thus both the traits are expressed equally. In F2 generation red (RR), roans (RW) and white (WW) are produced in the ratio 1:2:1. Thus in Co-dominance, the genotypic and phenotypic ratios are 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. Multiple alleles arise by mutations of the wild type of gene. A gene can mutate several times producing a series of alternative expression. Different alleles in a series show dominant- recessive relation or may show co-dominance or incomplete dominance among themselves. Wild type is dominant over all other mutant alleles.

In Drosophila, a large number of multiple alleles are known. e.g. The size of wings from normal wings to vestigial (no) wings, i.e., just stumps, is due to one allele (vg) in homozygous condition. The normal wing is wild type while vestigial wing is recessive type.

Another good example of multiple alleles is A, B, O blood grouping in human beings.

 

d. Pleiotropy :

When a single gene controls two (or more) different traits, it is called pleiotropic gene and the phenomenon is called pleiotropy or pleiotropism. The phenotypic ratio is 1 2 instead of 3:1 because of the death of recessive homozygote. The disease, sickle-cell anaemia, is caused by a gene Hbs. Normal or healthy gene HbA is dominant. The carriers (heterozygotes HbA/Hbs) show signs of mild anaemia as their RBCs become sickle-shaped i.e. half- moon- shaped only under abnormally low O2 concentration.

Representation of Pleiotropy

The homozygotes with recessive gene Hbs however, die of total anaemia. Thus, the gene for sickle- cell anaemia is lethal in homozygous condition and produces sickle cell trait in heterozygous carrier.

Two different expressions are produced by a single gene.

A marriage between two carriers will produce normal, carriers and sickle-cell anaemic children in 1:2:1 ratio. Sickle cell anaemics die leaving carriers and normals in the ratio 1:2. The heterozygotes or carriers can be identified by microscopic examination of blood.