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Difficulty: EasyGenetics and Heredity

In fruit flies, the allele for normal wings (WW) is dominant over the allele for short wings (ww). If two heterozygous (WwWw) flies mate, what percentage of their offspring is expected to have normal wings?

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

Answer

75% of the offspring are expected to have normal wings.
In a cross between two heterozygous parents (Ww×WwWw \times Ww), the resulting genotypic ratio is 1 WW:2 Ww:1 ww1\ WW : 2\ Ww : 1\ ww. Because the allele for normal wings (WW) is dominant, both WWWW (25%) and WwWw (50%) will have the normal-wing phenotype. Adding these together gives an expected total of 75% normal-winged offspring.

Step-by-Step Solution

1
Determine parent gametes and set up a Punnett square for the cross Ww×WwWw \times Ww.
Each parent produces gametes with alleles WW and ww with equal probability (50%50\% each).
Heterozygous organisms produce two types of gametes due to the law of segregation.
2
Combine alleles to find the offspring genotypic ratio.
The genotypes produced are 25% WW25\%\ WW, 50% Ww50\%\ Ww, and 25% ww25\%\ ww.
Crossing the gametes yields 1/4 WW1/4\ WW, 2/4 Ww2/4\ Ww, and 1/4 ww1/4\ ww combinations.
3
Identify all genotypes that express the dominant normal-wing phenotype.
Both WWWW (25%25\%) and WwWw (50%50\%) display normal wings, totaling 25%+50%=75%25\% + 50\% = 75\%.
The dominant allele WW masks the recessive allele ww in heterozygous individuals.

Key Concept

Monohybrid cross phenotypic ratios and dominant allele inheritance
Estimated Time:45s
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Topics

Genetics and Heredity

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