Question

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?

Short answer

Answer: No, the Lyon hypothesis cannot be tested in a homozygous female for one allele of the X-linked G6PD gene, because both X chromosomes carry the same G6PD allele, making it impossible to observe a mosaic pattern of gene expression, which is crucial for testing the Lyon hypothesis.

Step-by-step solution

Understanding the Lyon Hypothesis

The Lyon hypothesis, also known as X-chromosome inactivation, states that in mammalian females, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development. This process ensures that females, who have two X chromosomes, express a similar dosage level of X-linked genes as males, who have only one X chromosome.

Understanding the X-linked G6PD gene

The G6PD gene is located on the X chromosome, and it encodes an enzyme called glucose-6-phosphate dehydrogenase. This enzyme plays a critical role in the pentose phosphate pathway, which provides cells with ribose-5-phosphate, a precursor for nucleotide synthesis, and NADPH, a cofactor involved in various biosynthetic and cellular defense pathways. Mutations in the G6PD gene can lead to G6PD deficiency, which causes various clinical symptoms such as acute hemolytic anemia, chronic non-spherocytic hemolytic anemia, and neonatal jaundice.

Homozygous female for one allele of G6PD

A homozygous female for one allele of the G6PD gene means that both of her X chromosomes carry the same G6PD allele. In this case, even if one of the X chromosomes is inactivated according to the Lyon hypothesis, the other active X chromosome will still express the same G6PD allele.

Testing the Lyon Hypothesis in homozygous females

To test the Lyon hypothesis in a human female, it is essential to observe whether the random inactivation of one of the X chromosomes results in a mosaic pattern of gene expression. This can be done by analyzing the expression patterns of different X-linked genes in various tissues or cell types. However, in the case of a female homozygous for one allele of the G6PD gene, it is impossible to test the Lyon hypothesis because both X chromosomes carry the same G6PD allele. As a result, regardless of which X chromosome is inactivated, the G6PD expression pattern will be identical, making it impossible to observe a mosaic pattern.

Conclusion

In conclusion, the Lyon hypothesis cannot be tested in a human female who is homozygous for one allele of the X-linked G6PD gene. The reason being that both X chromosomes carry the same G6PD allele, making it impossible to observe a mosaic pattern of gene expression, which is crucial for testing the Lyon hypothesis.