Question

In Drosophila, the X-linked echinus eye phenotype disrupts formation of facets and is recessive to wild-type eye. Autosomal recessive traits vestigial wing and ebony body assort independently of one another. Examine the progeny from the three crosses shown below, and identify the genotype of parents in each cross.
Table showing progeny phenotypes and proportions from three genetic crosses with different parental phenotypes involving echinus, vestigial, and ebony traits.

Table showing progeny phenotypes and proportions from three genetic crosses with different parental phenotypes involving echinus, vestigial, and ebony traits.

Accepted Answer

Step 1: Understand the traits and their inheritance patterns. The echinus eye phenotype is X-linked recessive, meaning males have only one X chromosome and will express the phenotype if they inherit the recessive allele. The vestigial wing and ebony body traits are autosomal recessive and assort independently, so their inheritance follows Mendel's laws of independent assortment. Step 2: Analyze each cross separately by examining the phenotypes of the progeny. For the X-linked echinus trait, determine the sex and eye phenotype ratios to infer the parental genotypes, considering that males inherit their X chromosome from their mother and females inherit one X from each parent. Step 3: For the autosomal recessive traits (vestigial wing and ebony body), use the phenotypic ratios of the offspring to deduce whether the parents are homozygous dominant, heterozygous carriers, or homozygous recessive for each trait. Remember that independent assortment means you can treat each trait separately and then combine the genotypes. Step 4: Use Punnett squares to model each cross. For the X-linked trait, set up a Punnett square with the mother's X chromosomes and the father's X and Y chromosomes. For the autosomal traits, set up separate Punnett squares for each trait and then combine the results to predict the expected phenotypic ratios. Step 5: Compare the expected phenotypic ratios from your Punnett squares with the observed progeny phenotypes in each cross. Adjust the assumed parental genotypes accordingly until the predicted ratios match the observed data, thereby identifying the most likely genotypes of the parents in each cross.

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Key Concepts

X-linked Inheritance

Autosomal Recessive Traits and Independent Assortment

Genotype Determination from Crosses