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Difficulty: MediumGenetics and Heredity

In sheep, the allele for white wool (WW) is dominant over the allele for black wool (ww). If a homozygous dominant ram (WWWW) mates with a heterozygous ewe (WwWw), what percentage of the offspring is expected to display black wool?

  1. A
    25%25\%
  2. B
    50%50\%
  3. 0%0\%Answer
  4. D
    75%75\%

Answer

The expected percentage of offspring displaying black wool is 0%0\%.
The homozygous dominant parent passes a dominant allele (WW) to all offspring. Because a single dominant allele is sufficient to express the dominant trait, all offspring will have white wool (50%50\% WWWW and 50%50\% WwWw), leaving 0%0\% with black wool (wwww).

Step-by-Step Solution

1
Determine the gametes produced by each parent.
The homozygous dominant ram (WWWW) produces only WW gametes. The heterozygous ewe (WwWw) produces 50%50\% WW gametes and 50%50\% ww gametes.
Alleles segregate during meiosis such that each gamete carries one allele for the trait.
2
Determine the offspring genotypes using a Punnett square.
Combining the alleles yields 50%50\% WWWW and 50%50\% WwWw genotypes.
Fertilization randomly pairs one maternal allele with one paternal allele.
3
Determine the phenotypic expression based on dominance.
Both WWWW and WwWw genotypes result in white wool because the presence of the dominant allele WW masks the recessive allele ww.
A recessive phenotype (black wool, wwww) requires two copies of the recessive allele, which cannot occur when one parent contributes only dominant alleles.

Key Concept

Dominant and recessive inheritance in monohybrid crosses
Estimated Time:45s
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Topics

Genetics and Heredity

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