In mice, the Sry gene is located on the Y chromosome very close to one of the pseudoautosomal regions that pairs with the X chromosome during male meiosis. Given this information, propose a model to explain the generation of unusual males who have two X chromosomes (with an Sry-containing piece of the Y chromosome attached to one X chromosome).

Verified step by step guidance

1. Understand that the Sry gene is normally located on the Y chromosome and is critical for male sex determination in mice.
2. Recognize that the pseudoautosomal regions (PARs) on the X and Y chromosomes allow pairing and recombination during male meiosis.
3. Propose that an abnormal crossover event occurs between the Y chromosome region containing the Sry gene and the X chromosome within or near the pseudoautosomal region during meiosis.
4. Explain that this crossover transfers the Sry-containing segment from the Y chromosome onto one of the X chromosomes, resulting in an X chromosome carrying Sry.
5. Conclude that fertilization involving this X chromosome with Sry can produce males with two X chromosomes, one of which carries the Sry gene, leading to male development despite the absence of a full Y chromosome.

Key Concepts

Pseudoautosomal Regions and Chromosome Pairing

Pseudoautosomal regions (PARs) are homologous segments found at the ends of the X and Y chromosomes that allow them to pair and recombine during male meiosis. This pairing is essential for proper segregation of sex chromosomes. Because the Sry gene is near a PAR, it can be involved in recombination events that transfer Y chromosome segments to the X chromosome.

Sry Gene and Sex Determination

The Sry gene is the primary sex-determining gene on the Y chromosome that initiates male development by triggering testis formation. Its presence typically leads to male phenotypes, even if the individual has two X chromosomes, as Sry expression overrides the default female developmental pathway.

Chromosomal Translocation and Unusual Karyotypes

Chromosomal translocation involves the transfer of a chromosome segment to a non-homologous chromosome. In this case, a piece of the Y chromosome containing Sry can become attached to an X chromosome during meiosis, producing XX individuals with male characteristics. This explains the presence of males with two X chromosomes but carrying the Sry gene.