Four eye-color mutants in Drosophila—apricot, brown, carnation, and purple—are inherited as recessive traits. Red is the dominant wild-type color of fruit-fly eyes. Eight crosses (A through H) are made between parents from pure-breeding lines.

Which of these eye-color mutants are X-linked recessive and which are autosomal recessive? Explain how you distinguish X-linked from autosomal heredity.

Verified step by step guidance

1. Understand the problem: The goal is to determine which of the four eye-color mutants (apricot, brown, carnation, and purple) are X-linked recessive and which are autosomal recessive. X-linked traits are carried on the X chromosome, while autosomal traits are carried on non-sex chromosomes. The inheritance patterns in the crosses will help us distinguish between these two types of inheritance.
2. Step 1: Analyze the inheritance patterns in the eight crosses (A through H). Specifically, look for differences in the phenotypes of male and female offspring. X-linked recessive traits often show distinct patterns, such as males being more frequently affected because they have only one X chromosome.
3. Step 2: For each cross, determine whether the trait is passed from mothers to sons or fathers to daughters. X-linked traits cannot be passed from father to son, as sons inherit their X chromosome from their mother. If a trait is passed from father to son, it is autosomal.
4. Step 3: Examine whether heterozygous females (carriers) show the recessive phenotype. In X-linked recessive traits, heterozygous females typically do not express the trait, while males with the recessive allele do. For autosomal recessive traits, both males and females must inherit two recessive alleles to express the trait.
5. Step 4: Summarize the findings for each mutant (apricot, brown, carnation, and purple) based on the inheritance patterns observed in the crosses. Classify each mutant as either X-linked recessive or autosomal recessive, providing reasoning based on the patterns of inheritance described in the previous steps.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to genes located on the X chromosome. In this mode of inheritance, males (XY) are more likely to express recessive traits because they have only one X chromosome. If a male inherits a recessive allele on his X chromosome, he will exhibit the trait, while females (XX) require two copies of the recessive allele to express the trait. This is crucial for identifying which eye-color mutants are X-linked.

Autosomal Inheritance

Autosomal inheritance involves genes located on the non-sex chromosomes (autosomes). Traits inherited in this manner can be expressed in both males and females equally, as both sexes have two copies of each autosome. Recessive traits will only manifest if an individual inherits two copies of the recessive allele. Understanding this concept helps differentiate between traits that are autosomal recessive and those that are X-linked.

Distinguishing X-linked from Autosomal Traits

To distinguish X-linked traits from autosomal traits, one can analyze the inheritance patterns in offspring. If a trait predominantly affects males and is passed from carrier mothers to affected sons, it is likely X-linked. Conversely, if both sexes are equally affected and the trait can skip generations, it is likely autosomal recessive. This analysis is essential for determining the inheritance patterns of the eye-color mutants in Drosophila.