Question

In mice, the Sry gene (see Section 5.2) 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).

Short answer

Question: Explain the generation of unusual males with two X chromosomes, where one of them has an Sry-containing piece of the Y chromosome attached. Answer: The generation of unusual males with two X chromosomes occurs due to a rare recombination event during meiosis in the parent. In this event, the Sry gene, which is usually located on the Y chromosome, gets transferred to the X chromosome. As a result, individuals with an X chromosome from the mother and an X' chromosome (X chromosome with Sry gene) from the father would be genotypically XX, but still possess the Sry gene, causing them to develop male characteristics. This leads to unusual males with two X chromosomes, where one of the X chromosomes has an Sry-containing piece of the Y chromosome attached.

Step-by-step solution

Understand Meiosis and Sry Gene Location

Meiosis is the process in which parent diploid cells divide to produce haploid cells, like sperm and eggs, with half the number of chromosomes. During the first phase of meiosis, homologous chromosomes (chromosomes that have the same sequence of genes and structure) pair up and exchange genetic material through recombination. The Sry gene is usually located on the Y chromosome, however, the unusual males will have it on the X chromosome. Since the Sry gene lies close to one of the pseudoautosomal regions where homologous recombination occurs, this detail is essential in our explanation.

Propose a Model of Recombination Involving Sry Gene Transfer

We propose a model that involves a rare recombination event between the X and Y chromosomes at the Sry gene region. During meiosis in the male's parent, a crossing-over event could have occurred between the X and Y chromosomes. This event involves an exchange of genetic material between chromosomes and in this case, the transfer of the Sry-containing piece between the X and the Y chromosome.

Analyze the Result of Recombination

After the recombination event, the outcome would be two types of chromosomes: an X chromosome with the Sry gene (referred to as X') and a Y chromosome without the Sry gene (referred to as Y'). The sperm cells carrying these chromosomes would generate unusual offspring when they fertilize the eggs.

Explain the Generation of Unusual Males

The unusual male offspring would inherit an X chromosome from the mother and an X' chromosome (X chromosome containing the Sry gene) from the father. These individuals would be genotypically XX but still possess the Sry gene, causing them to develop male characteristics. Thus, these males would have two X chromosomes, with one of the X chromosomes having an Sry-containing piece of the Y chromosome attached.

Key concepts

Meiosis

Meiosis is a critical biological process responsible for ensuring genetic diversity. During meiosis, a specialized form of cell division occurs in sexually reproducing organisms, producing gametes – sperm in males and eggs in females. This process halves the number of chromosomes, transitioning from diploid parent cells to haploid cells that only contain a single set of chromosomes. This reduction is crucial for maintaining the species-specific chromosome number across generations.

Meiosis involves two key stages:

• Meiosis I - Homologous chromosomes pair and then separate into two different cells.
• Meiosis II - Sister chromatids (previously duplicated) separate, creating a total of four haploid cells.

A standout feature of meiosis is crossing over, a process where homologous chromosomes exchange segments of genetic material. This genetic shuffling occurs during the first phase of meiosis and plays an important role in producing genetic variation in offspring. Crossing over is significant in the context of the Sry gene because it may lead to unusual genetic combinations, such as the Sry gene transferring from the Y chromosome to the X chromosome in mice.

Pseudoautosomal Regions

Pseudoautosomal regions (PARs) are small regions at the ends of sex chromosomes – X and Y – in mammals where they share homology. Unlike most of the X and Y chromosomes, which have different genes and functions, PARs are identical between the two sex chromosomes. These regions facilitate pairing and recombination during meiosis, similar to how homologous chromosomes behave.

PARs are important for maintaining proper chromosomal alignment and segregation during meiosis. If there is improper recombination or errors in segregation, it can lead to various chromosomal abnormalities. In the context of the Sry gene, which is located close to these pseudoautosomal regions on the Y chromosome, the recombination allows for rare events where genetic material – like the Sry gene – can be transferred to the X chromosome during meiosis.

Chromosomal Recombination

Chromosomal recombination occurs when two chromosomes exchange genetic segments with one another. This natural process is vital during meiosis, as it increases genetic variation by rearranging combinations of alleles in the offspring.

Recombination typically happens between homologous chromosomes. They pair up and exchange equivalent portions of DNA. This exchange happens at points called "chiasmata" and is facilitated by enzymes that break and rejoin the DNA strands.

In the case of the Sry gene, recombination can lead to the Sry gene being transferred from the Y chromosome to the X chromosome. During meiosis in males, if the Sry gene is located near a pseudoautosomal region, it can result in crossing over. This genetic exchange explains why certain males may possess two X chromosomes, yet have male characteristics due to the presence of the Sry gene on one of the X chromosomes. This recombinant event results in gametes contributing to unusual genotypes, which include chromosomal rearrangements like the one described with the transfer of male-determining genes.