A different walnut rooster was crossed to a rose hen, and all the progeny were walnut. What are the possible genotypes of the parents

The correct answer and explanation is:

Let’s break down the problem carefully:

Question Recap:

• A walnut rooster was crossed to a rose hen.
• All progeny were walnut.
• What are the possible genotypes of the parents?

Step 1: Understand the Traits and Inheritance Pattern

In chickens, comb types are controlled by multiple alleles at a single locus, with dominance relationships:

• Rose comb (R)
• Pea comb (P)
• Walnut comb (W)

Allele dominance hierarchy:

• Walnut (W) is dominant over both rose (R) and pea (P).
• Walnut is actually a combination of rose and pea alleles — i.e., the walnut phenotype results from having one rose allele and one pea allele (genotype R P = walnut comb).

This means:

• Genotype R R = rose comb
• Genotype P P = pea comb
• Genotype R P = walnut comb (because both rose and pea are present)
• Genotypes with homozygous rose or pea will show rose or pea, respectively.
• Walnut comb occurs only in heterozygotes carrying both R and P alleles.

Step 2: Analyze the Parents

• Rooster: walnut comb → genotype R P (heterozygous for rose and pea)
• Hen: rose comb → genotype R R (homozygous rose)

Step 3: Predict Progeny Genotypes

Cross: Rooster (R P) × Hen (R R)

Possible gametes:

• Rooster gametes: R or P
• Hen gametes: R only (since R R)

Possible progeny genotypes:

• R (from rooster) × R (from hen) → R R (rose comb)
• P (from rooster) × R (from hen) → R P (walnut comb)

Resulting offspring:

• 50% R R (rose comb)
• 50% R P (walnut comb)

Step 4: Problem states all progeny were walnut.

The above predicts 50% walnut, 50% rose, so the above genotype for hen can’t be R R (pure rose).

Step 5: Possible alternative genotypes for the hen to yield all walnut offspring

If the hen had pea comb (P P), crossing with walnut (R P) rooster:

Hen: P P
Rooster: R P

Gametes:

• Hen: P only
• Rooster: R or P

Progeny:

• P (hen) × R (rooster) → R P (walnut)
• P (hen) × P (rooster) → P P (pea)

Progeny: 50% walnut, 50% pea → Not all walnut.

Step 6: What about hen walnut (R P) and rooster walnut (R P)?

Cross: R P × R P

Gametes: R or P from both parents

Progeny genotypes:

• R R → rose
• R P → walnut
• P R → walnut
• P P → pea

Progeny ratio: 1 rose : 2 walnut : 1 pea → Not all walnut.

Step 7: What if hen is walnut (R P) and rooster is walnut (R P) but somehow only walnut progeny survive? Unlikely.

Step 8: What if hen is walnut (R P) and rooster is rose (R R)?

Hen: R P
Rooster: R R

Gametes:

• Hen: R or P
• Rooster: R only

Progeny:

• R (hen) × R (rooster) → R R (rose)
• P (hen) × R (rooster) → R P (walnut)

Again, 50% rose, 50% walnut — Not all walnut.

Step 9: What if hen is walnut (R P) and rooster is pea (P P)?

Hen: R P
Rooster: P P

Gametes:

• Hen: R or P
• Rooster: P only

Progeny:

• R (hen) × P (rooster) → R P (walnut)
• P (hen) × P (rooster) → P P (pea)

Again, 50% walnut, 50% pea — Not all walnut.

Step 10: Try hen walnut (R P) and rooster walnut (R P), but progeny selected only walnut.

No, problem says all progeny are walnut, meaning no other phenotypes appear.

Step 11: So what’s left?

If all progeny are walnut, and walnut = R P genotype, then all progeny must be heterozygous R P.

Step 12: Parents must each provide one allele: one R and one P.

• So both parents must be heterozygous R P (walnut) and homozygous for either R or P is not possible to yield all walnut progeny.

Step 13: To get all walnut progeny (R P), both parents must be:

• One parent genotype: R P (walnut)
• Other parent genotype: R P (walnut)

But that produces 1/4 R R (rose), 1/4 P P (pea), and 1/2 R P (walnut) — Not all walnut.

Step 14: How can all progeny be walnut?

If the walnut phenotype is caused by codominance of R and P alleles, the only way all progeny will be walnut is if:

• One parent is R P (walnut)
• The other parent is R P (walnut)
• And somehow only R P offspring survive (selection)

OR

Another explanation:

There is a rare case where the walnut comb is dominant and the alleles work differently:

• Suppose walnut allele is dominant (W), rose is recessive (r).
• Rooster: Ww (walnut)
• Hen: ww (rose)

Crossing Ww × ww yields 50% walnut, 50% rose offspring.

No.

Step 15: Final plausible answer is:

• Rooster genotype: walnut (R P)
• Hen genotype: walnut (R P)

The question might be simplified and assuming all progeny walnut means both parents are heterozygous R P (walnut), and the alleles assort in a way that all progeny show walnut comb.

Summary / Final Answer

• Walnut comb is caused by heterozygosity of the rose and pea alleles (genotype R P).
• The walnut rooster has genotype R P.
• The rose hen has genotype R R.
• Crossing R P × R R produces 50% walnut (R P) and 50% rose (R R) offspring.
• Since all progeny are walnut, the hen genotype cannot be R R.
• For all progeny to be walnut, both parents must be heterozygous R P (walnut) and only R P genotype progeny survive or appear (which is rare but possible by selection).

Explanation (approx. 300 words):

The comb type in chickens is controlled by alleles at a single locus with three common alleles: rose (R), pea (P), and walnut (W). The walnut comb phenotype occurs when a chicken has both the rose and pea alleles — it is heterozygous (R P) and expresses the walnut comb. Both rose and pea alleles are recessive relative to walnut when combined, but neither rose nor pea alone produces walnut.

In the given problem, a walnut rooster is crossed to a rose hen, and all progeny have walnut combs. The walnut rooster genotype is R P, while the rose hen would be R R.

Crossing R P (rooster) × R R (hen) will produce 50% R P (walnut) and 50% R R (rose) progeny. Since all progeny are walnut, the hen’s genotype cannot be R R.

The only way all progeny can be walnut is if both parents have the R P genotype (both are walnut comb). When two R P birds are crossed, the expected progeny genotypes are: 25% R R (rose), 25% P P (pea), and 50% R P (walnut). However, if only the R P genotype progeny survive or are counted (perhaps due to selection), then all observed progeny would have walnut combs.

Thus, the possible genotypes for the parents are:

• Rooster: R P (walnut)
• Hen: R P (walnut)

And all progeny are heterozygous R P (walnut), which matches the observed phenotype.