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. <span>Step 1: Understand the basic genetic concepts involved. In Drosophila, eye color is a trait that can be inherited in different ways. X-linked recessive traits are carried on the X chromosome, while autosomal recessive traits are carried on non-sex chromosomes (autosomes).</span>
2. <span>Step 2: Analyze the inheritance patterns. For X-linked recessive traits, males (XY) are more likely to express the trait because they have only one X chromosome. Females (XX) would need two copies of the recessive allele to express the trait. In contrast, autosomal recessive traits require two copies of the recessive allele in both males and females to be expressed.</span>
3. <span>Step 3: Examine the results of the crosses. Look for patterns where the trait appears more frequently in males than females, which would suggest X-linked recessive inheritance. If the trait appears equally in both sexes, it is likely autosomal recessive.</span>
4. <span>Step 4: Consider the parental genotypes. In pure-breeding lines, parents are homozygous for the trait. If a cross between a mutant and a wild-type results in all wild-type offspring, the mutant trait is likely recessive. If the trait reappears in the F2 generation, it confirms recessive inheritance.</span>
5. <span>Step 5: Use test crosses to confirm. Cross a mutant female with a wild-type male and observe the offspring. If the trait is X-linked recessive, all male offspring will express the trait. If the trait is autosomal recessive, the trait will appear in both sexes in the F2 generation.</span>

Key Concepts

X-linked vs. Autosomal Inheritance

X-linked inheritance refers to traits associated with genes located on the X chromosome, while autosomal inheritance involves genes on non-sex chromosomes. In X-linked recessive traits, males (XY) are more likely to express the trait because they have only one X chromosome, whereas females (XX) can be carriers without showing the trait if they have one dominant allele.

Recessive Traits

Recessive traits require two copies of the recessive allele for expression in autosomal inheritance. In the case of X-linked traits, males only need one copy of the recessive allele on their single X chromosome to express the trait, while females need two copies on both X chromosomes. This difference is crucial for determining the inheritance pattern of the eye-color mutants.

Pure-breeding Lines

Pure-breeding lines consist of organisms that consistently produce offspring with the same phenotype when self-fertilized or crossed with each other. In the context of the Drosophila eye-color mutants, pure-breeding lines help establish the genetic basis of the traits, allowing researchers to determine whether the mutants are X-linked or autosomal by analyzing the offspring's phenotypes in controlled crosses.