Can the Lyon hypothesis be tested in a human female who is homozygous for one allele of the X-linked G6PD gene? Why, or why not?

Verified step by step guidance

1. Step 1: Understand the Lyon hypothesis, which states that in female mammals, one of the two X chromosomes is randomly inactivated in each cell early in development, leading to dosage compensation between males (XY) and females (XX).
2. Step 2: Recognize that the Lyon hypothesis can be tested by observing phenotypic mosaicism in heterozygous females for an X-linked gene, where different cells express different alleles depending on which X chromosome is inactivated.
3. Step 3: Consider the genetic status of the female in question: she is homozygous for one allele of the X-linked G6PD gene, meaning both X chromosomes carry the same allele.
4. Step 4: Analyze the consequence of homozygosity: since both X chromosomes have the same allele, X-inactivation will not produce any phenotypic mosaicism because all cells express the same allele regardless of which X is inactivated.
5. Step 5: Conclude that the Lyon hypothesis cannot be effectively tested in this homozygous female using the G6PD gene, because the lack of allelic variation prevents observation of differential expression due to X-inactivation.

Key Concepts

Lyon Hypothesis (X-Chromosome Inactivation)

The Lyon hypothesis proposes that in female mammals, one of the two X chromosomes is randomly inactivated in each cell early in development, leading to dosage compensation between males and females. This inactivation is stable and results in mosaic expression of X-linked genes in heterozygous females.

X-Linked Genes and Allelic Variation

X-linked genes, like G6PD, are located on the X chromosome. Females have two alleles for these genes, which can be different (heterozygous) or the same (homozygous). The presence of different alleles allows observation of differential expression due to X inactivation.

Testing the Lyon Hypothesis in Homozygous Females

In a female homozygous for one allele of an X-linked gene, both X chromosomes carry the same allele, so X inactivation cannot be detected by phenotypic differences. Therefore, the Lyon hypothesis cannot be directly tested in such individuals because there is no observable mosaicism.