Question

How do incomplete dominance, codominance, polygenic inheritance and pleiotropy, deviate from Mendel's observation? Give an example of each.

Answer 1

Mendel conducted his experiments using several true-breeding pea lines of Pisum sativum where the true breeding lines having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.

Complete answer:
Mendel selected a total of 14 true-breeding pea plant varieties, such as the height of the stem whether tall or dwarf as pairs which were similar except for one character with contrasting traits. It means, Mendel selected 7 characters that are seed shape, seed colour, pod colour in pea plant for carrying out hybridization experiments and to study inheritance.

Mendel crossed pea plants say for example stem height and cross tall and dwarf plants and got 3:1 ratio but some inheritance differs from this ratio as:

Incomplete dominance:
It is the phenomena where none of the two contrasting alleles of a pair is dominant, the expression of a character in ${F_1}$ offspring is an intermediate expression of the two factors. A most popular example is that of flower colour in Snapdragon (dog flower or Antirrhinum majus) where true-breeding species of red-flowered plant (RR) were crossed with white flowered plant (rr). ${F_1}$ offspring (Rr) had pink flowers. Here one allele is incompletely dominant over another so that intermediate phenotype is produced by ${F_1}$ hybrid with respect to the parents. If the ${F_1}$ is made to undergo self-pollination, the plants of ${F_2}$ generation are of three types red (RR), pink (Rr) and white flowered (rr) in the ratio of$1:2:1$. In heterozygous condition (Rr), the phenotypic effect of one allele is more pronounced than that of the other and mixing of both colours (red & white) results in the development of pink colour.

Codominance:
Besides incomplete dominance, certain alleles show codominance. Here in ${F_1}$, hybrid both alleles express themselves equally and there is no mixing of the effect of the both alleles, therefore hybrid progeny (${F_1}$) resembles both parents. ABO blood group is a perfect example of codominance.
Blood group A have antigen A, group B have antigen B, group AB have both antigens while blood group O do not carry any antigen. Thus, a total of six genotypes and four phenotypes are possible.

Polygenic inheritance:
The traits which are controlled by more than one gene are called polygenic traits. The occurrence of melanin pigment in the skin determines the skin colour of an individual. The amount of melanin produced in the individual is determined by three (two also) pairs of genes. These three pairs of genes are present at three different loci and every dominant gene is responsible for the production of a fixed amount of melanin. The effect of all the genes is additive in nature and the amount of melanin synthesized is always proportional to the number of dominant genes.
Studies have shown that as many as six genes may be involved in controlling the skin colour in human beings. So, the ratio is $1:6:15:20:15:6:1\;\;$.

Pleiotropy:
It is basically the ability of a gene to have multiple phenotypic effects because it influences a number of characters simultaneously. And the Mendelian concept of a gene controlling a single character has been deviated in this case. It means in pleiotropy, a single gene product might be able to give rise to more than one effect or control several phenotypes depending on its position. For Example: In phenylketonuria, mutation of a gene that gives description for the enzyme phenylalanine hydroxylase, results in a phenotypic expression characterized by mental retardation and a reduction in hair and skin pigmentation.

Note: Genes generally express themselves individually but many cases are known where two genes of the same allelic pair or genes of two or more different allelic pairs influence one another. This is called gene interaction and exception to Mendelian arises due to this only.