Question

In shorthorn cattle, the coat colours red or white are controlled by a single pair of alleles. A calf which receives the allele for red coat from its mother and the allele for white coat from its father is called a 'roan'. It has an equal number of red and white hairs in its coat.

1. Is this an example of codominance or of incomplete dominance?
2. Give a reason for your answer.
3. With the help of genetic cross explain what will be the consequent phenotype of the calf when,
   
   1. red is dominant over white.
   2. red is incompletely dominant.

Answer 1

a) Codominance

b) Codominance is a condition in which two different alleles for a genetic trait are expressed. Individuals receive one version of a gene, called an allele, from each parent.

c) i) If pure breeding red coated cattles are represented as ‘RR’ and pure breeding white coated as ‘rr’. If Red is dominant over White. A cross between ‘RR’ and ‘rr’ would produce red coated cattles (RR) and white coated cattle (rr) in the ratio of 3 : 1.

Parents:	RR (Red)	×	rr (White)
Gametes:	R		r

 

	R	r
R	RR Red coat	Rr Redcoat
r	Rr Red coat	rr White coat
	F1 generation - 3:1

ii) If the red and white coated cattles produce pink colour on a cross then, they exhibit incomplete dominance in the inheritance of coat colour due to which they produce pink coloured coat upon hybridisation.

If pure breeding red coated cattles are represented as ‘RR’ and pure breeding white coated as ‘rr’, then the pink coated cattles are ‘Rr’.

A cross between ‘RR’ and ‘rr’ would produce pink coated cattles (Rr) and white coated cattle (rr) in the ratio of 1 : 2 : 1.

Parents:	RR (Red)	×	rr (White)
Gametes:	R		W

 

	R		r
R	RR Red coat		Rr Pink coat
r	Rr Pink coat		rr (White)
F0 Generation - 1:2:1