Final answer:
The white fruit color in summer squash is dominant due to epistasis, where the presence of a dominant W allele determines the white color, regardless of the Y gene alleles. The phenotypic ratio in a cross of heterozygotes shows that most offspring will be white due to this dominance pattern.
Step-by-step explanation:
In summer squash, the white fruit color is dominant over yellow fruit color due to an inheritance pattern known as epistasis. This is when the expression of one gene is masked by a dominant allele at a separate gene. For summer squash, the W gene exhibits dominance such that if there is a dominant W allele present (WW or Ww), the squash will have white fruit regardless of the alleles of the Y gene. Therefore, phenotypic expression is dependent on the interaction of these genes.
When we cross two white heterozygotes (WwYy), the progeny exhibit a phenotypic ratio of 12 white:3 yellow:1 green. This occurs because the presence of at least one dominant W allele (W) suppresses the expression of the Y gene, resulting in white fruit. Squash will only express yellow color (due to YY or Yy) if the W gene is homozygous recessive (ww), and green color is only expressed if both W and Y genes are homozygous recessive (wwyy).