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Difficulty: MediumGenetics and Heredity

In guinea pigs, black fur (BB) is dominant over white fur (bb). If two heterozygous black guinea pigs (BbBb) are crossed, what percentage of their offspring is expected to have black fur?

  1. A
    25%
  2. B
    50%
  3. 75%Answer
  4. D
    100%

Answer

75% of the offspring are expected to have black fur.
Crossing two heterozygous parents (Bb×BbBb \times Bb) produces genotypes in a 1 BB : 2 Bb : 1 bb ratio. Because black fur is dominant, both BBBB (25%) and BbBb (50%) display black fur, resulting in a total expected phenotype of 75% black fur.

Step-by-Step Solution

1
Identify the parental genotypes and set up the monohybrid cross.
The cross is Bb×BbBb \times Bb, where each parent contributes either a BB or bb allele.
Both parents are heterozygous, carrying one dominant allele and one recessive allele.
2
Construct a 2×22 \times 2 Punnett square to determine offspring genotype probabilities.
Genotype distribution is 1 BB1\ BB (25%), 2 Bb2\ Bb (50%), and 1 bb1\ bb (25%).
Combining alleles from both parents yields four equally likely outcomes.
3
Determine which genotypes result in the black fur phenotype.
Both BBBB (25%) and BbBb (50%) produce black fur, totaling 25%+50%=75%25\% + 50\% = 75\%.
The dominant allele (BB) masks the recessive allele (bb), so both homozygous dominant and heterozygous genotypes express the dominant trait.

Key Concept

Monohybrid cross and dominant versus recessive phenotypic ratios
Estimated Time:1m 0s
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Topics

Genetics and Heredity

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