Question

In shorthorn cattle, coat colors are governed by a codominant pair of alleles R and W. The homozygous genotype RR produces red, the other homozygote produces white and the heterozygote produces roan ( a mixture of red and white). The presence of horns is produced by the homozygous recessive genotype pp and the polled condition by its dominant allele P.

If roan cows heterozygous for the horned gene are mated to a horned, roan bull, what phenotype ratio is expected in the offspring?

Answer 1

The phenotypic ratio of the cattle offsprings is as follows:

Red polled = 1/8

Red horned = 1/8

White polled = 1/8

White horned = 1/8

Roan polled = 2/8

Roan horned = 2/8

Step-by-step explanation:

This is a cross involving two genes; one coding for coat color and the other for horn in cattle. The alleles of the first gene (Red, R and White, W) are codominant i.e. will both be expressed in a heterozygous state (Roan, RW). The polled allele (P) is dominant over the horned allele (p) in the second gene.

According to the question, a roan cow that is heterozygous for the horned gene will have genotype, RWPp while a roan bull that is horned will have genotype, RWpp. Hence, in a cross between these two cattles, each parent will undergo meiosis to produce the following possible gamete combinations;

RWPp- RP, Rp, WP, Wp

RWpp- Rp, Rp, Wp, Wp

These gametes used in a punnet square (see attachment) will produce 16 possible offsprings with 6 distinct phenotypes. The phenotypes are:

Red polled (RRPp or RRPP) = 2/16 or 1/8

Red horned (RRpp) = 2/16 or 1/8

White polled (WWPP or WWPp) = 2/16 or 1/8

White horned (WWpp) = 2/16 or 1/8

Roan polled (RWPP or RWPp) = 4/16 or 2/8

Roan horned (RWpp) = 4/16 or 2/8

Answer 2

1/8 Red, polled

1/8 Red, horned

1/4 roan, polled

1/4 roan, horned

1/8 white, horned.

1/8 white, horned.

Step-by-step explanation:

Roan cow with heterozygous horned genotype = RWPp

Roan, horned bull genotype = RWpp

Crossing the two:

RWPp x RWpp

Offspring:

2 RRPp = Red, polled

2 RRpp = Red, horned

4 RWPp = roan, polled

4 RWpp = roan, horned

2 WWPp = white, polled

2 WWpp = white, horned.

Hence, the phenotypic ratio is:

1/8 Red, polled

1/8 Red, horned

1/4 roan, polled

1/4 roan, horned

1/8 white, horned.

1/8 white, horned.