Question

in cattle, the allele for white coat color (w) and the allele for red coat color (r) are codominant. a heterozygote has the roan coat color. two cows with the following genotypes are crossed.

\\(rw \times rr\\)

what is the expected phenotypic ratio of the offspring?

a. 0 red : 2 roan : 2 white
b. 3 red : 0 roan : 1 white
c. 2 red : 2 roan : 0 white
d. 1 red : 2 roan : 1 white

Explanation:

Identify the parental genotypes and alleles

Using the Genotype knowledge point

• Alleles: \(W\) (white), \(R\) (red)
• Cross: \(RW \times RR\)
• Gametes from parent 1 (\(RW\)): \(R\) and \(W\)
• Gametes from parent 2 (\(RR\)): \(R\) only

Determine offspring genotypes

Using the Genotype knowledge point

• Punnett square combinations:
• \(R \times R

ightarrow RR\)

• \(W \times R

ightarrow RW\)

• Genotypic ratio: \(1\ RR : 1\ RW\) (or \(2\ RR : 2\ RW\))

Determine offspring phenotypes

Using the Phenotype knowledge point

• \(RR\) genotype expresses red coat color.
• \(RW\) genotype expresses roan coat color (codominance).
• \(WW\) genotype (not present) expresses white coat color.

Calculate the phenotypic ratio

Using the Phenotype knowledge point

• Red offspring (\(RR\)): 2 out of 4 (or 1/2)
• Roan offspring (\(RW\)): 2 out of 4 (or 1/2)
• White offspring (\(WW\)): 0 out of 4 (or 0)
• Expected phenotypic ratio: 2 red : 2 roan : 0 white

Answer:

• (A) 0 red : 2 roan : 2 white
• (B) 3 red : 0 roan : 1 white
• (C) 2 red : 2 roan : 0 white (Correct answer)
• (D) 1 red : 2 roan : 1 white