Question

Explain intragenic and intergenic interaction with the help of example.

Answer 1

1. Intragenic interactions:
   These types of interactions occur between the alleles of the same gene. e.g. Incomplete dominance, codominance, and multiple allele series of a gene.
   Incomplete dominance:

1. Incomplete dominance is a deviation of Mendel’s law of dominance.
2. In incomplete dominance, both the alleles (genes) of an allelomorphic pair express themselves partially.
3. One allele (gene) cannot suppress the expression of the other allele (gene) completely.
4. In such a case, there is an intermediate expression in the F₁ hybrid.
5. In Mirabilis jalapa, if a red-flowered (RR) plant is crossed with a white-flowered (RR) plant, then F₁ offsprings have pink (Rr) flowers.

Phenotypic ratio → 1 : 2 : 1 (1 Red : 2 Pink : 1 White)

Genotypic ratio → 1 : 2 : 1 (1 RR : 2 Rr : 1 rr) 

Co-dominance

1. Coat colour in cattle is a classic example of co-dominance.
2. There are two types, one with a red coat (skin with red colour hair) and the other with a white coat (with white hair).
3. When red cattle (RR) is crossed with white cattle (WW), F₁ hybrids (RW) have roan colour. Roans have a mixture of red and white colour hair.
4. Thus, both traits are expressed equally. In F₂ generation (produced by interbreeding of roans), red (RR), roans (RW) and white (WW) are produced in the ratio 1: 2: 1.

Phenotypic ratio → 1 : 2 : 1 (1 Red coat : 2 Roan : 1 White coat)
Genotypic ratio → 1 : 2 : 1 (1 RR : 2 RW : 1 WW)

2. Intergenic interaction:
These types of 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.

1. When a single gene controls two (or more) different traits it is called pleiotropic gene and the phenomenon is called pleiotropy or pleiotropism.
2. The phenotypic ratio is 1: 2 instead of 3: 1 because of the death of recessive homozygote.
3. For example, the disease, sickle cell anaemia is caused by the gene Hb^S. The normal or healthy gene is Hb^A and is dominant.
4. The carriers (heterozygotes – Hb^A /Hb^S) show signs of mild anaemia as their RBCs become sickle-shaped (half-moon shaped) in oxygen deficiency. They are said to have the sickle-cell trait and are normal in normal conditions.
5. The homozygotes with recessive gene Hb^S however, die of fatal anaemia.

A marriage between two carriers will produce normal, carriers and sickle-cell anaemic children in a 1: 2: 1 ratio.