In summer squash, two genes (A and B) interact to form the colour of the fruit. Figure 1 shows a drawing of a summer squash.
       Figure 1
     Summer Squash
Gene A controls whether the fruit has pigmentation or not. The dominant allele of this gene, A, results in no pigmentation, the squash are white. The recessive allele, a, results in pigmentation to the fruit. Gene B controls which pigmentation (yellow or green) the fruit has. The dominant allele, B, causes the fruit to have yellow pigmentation and the recessive allele, b, results in the fruit having green pigmentation. This gene has no effect on squash that have no pigmentation; white squash do not have any yellow or green pigmentation, even if they have the dominant B allele.
Scientists performed 4000 crosses in which white squash, heterozygous for both genes were crossed with yellow squash, heterozygous for gene B. They expected white, yellow and green squash in the offspring in a 4 : 3 : 1 phenotypic ratio.
Complete the genetic diagram to show how this ratio of phenotypes would be produced.
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Parental phenotypes
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White
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Yellow
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Parental genotypes
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Gamete genotypes
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Offspring genotypes
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Offspring phenotypes
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