In cows, heterozygotes are roan in colour, while homozygotes are either red or white. When crossing a roan and white cow, what proportion of the offspring will be roan in colour?单项选择题
A
a. 0%
B
b. 50%
C
c. 25%
D
d. 75%

Question

In cows, heterozygotes are roan in colour, while homozygotes are either red or white. When crossing a roan and white cow, what proportion of the offspring will be roan in colour?

BlackTom AI · Accepted Answer

Roan in cattle is the heterozygous phenotype produced by codominant alleles: each homozygote shows a pure colour (red or white), while the heterozygote shows both (roan). Represent alleles as R = red, W = white. Given: RR = red, WW = white, RW = roan.

Step 1 — parental genotypes and gametes
- Roan parent must be heterozygous: genotype RW. Its gametes: R or W, each with probability 1/2.
- White parent is homozygous WW. Its gametes: W only (probability 1).

Step 2 — form the Punnett square / calculate offspring genotypes
- Combine gametes:
  - R (from RW, 1/2) × W (from WW, 1) → RW (roan)
  - W (from RW, 1/2) × W (from WW, 1) → WW (white)
- So the possible offspring genotypes are RW and WW, each with probability 1/2 (50%).

Intermediate probability steps:
- Probability offspring is RW = P(roan parent donates R) × P(white parent donates W) = (1/2) × 1 = 1/2 = 50%.
- Probability offspring is WW = (1/2) × 1 = 1/2 = 50%.

Why the correct answer is 50%
Because half the gametes from the roan parent carry the R allele and the white parent always contributes W, half the progeny will be RW (expressing the roan phenotype).

Why the other options are incorrect
- a. 0%: incorrect — the roan parent can contribute an R allele, producing roan offspring.
- c. 25%: incorrect — 25% would arise in a different cross (e.g., two heterozygotes producing 25% RR), not RW × WW.
- d. 75%: incorrect — no combination here yields 75% roan; only two outcomes (50% RW, 50% WW) occur.

Conclusion: The proportion of roan offspring from a roan (RW) × white (WW) cross is 50%, matching the provided answer (b. 50%).

In cows, heterozygotes are roan in colour, while homozygotes are either red or white. When crossing a roan and white cow, what proportion of the offspring will be roan in colour?单项选择题

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