Question

In mice, the Sry gene (see Section 7.2 ) 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 how unusual males with two X chromosomes containing an Sry gene can be generated in mice. Answer: Unusual males with two X chromosomes containing an Sry gene can be generated in mice due to a mistake during meiosis. During the recombination process, a crossover event may occur near the pseudoautosomal pairing region between the X and Y chromosomes, causing the Sry gene from the Y chromosome to be transferred to an X chromosome. The resulting offspring will have an XX genotype but will still possess the Sry gene, leading to the development of male characteristics.

Step-by-step solution

Understanding the role of the Sry gene

The Sry gene is critical for sex determination in mammals, including mice. It is located on the Y chromosome and is responsible for triggering the development of male characteristics. If an individual has an X and Y chromosome pair, the Sry gene present on the Y chromosome will determine that the individual is male. However, in some cases, unusual males carrying two X chromosomes can be generated, which requires an explanation.

Discussing the location of the Sry gene on the Y chromosome

The Sry gene is located very close to the pseudoautosomal region of the Y chromosome, which pairs with the X chromosome during male meiosis. This proximity to the pairing region suggests a possible mechanism by which the Sry gene could accidentally be exchanged, leading to an unusual genotype.

Proposing a model for the generation of unusual males

A possible model to explain the generation of unusual males with two X chromosomes involves a mistake during meiosis. Meiosis is the process by which cells divide to form gametes (egg or sperm) containing half the number of chromosomes. Since the Sry gene is located close to the pseudoautosomal region, there is an increased chance of errors during recombination, a process that reorganizes genetic material during meiosis. During the recombination process, near the pairing region between the non-homologous X and Y chromosomes, a crossover event could occur that results in the Sry-containing piece of the Y chromosome being transferred from the Y to the X chromosome. This would yield an X chromosome with the Sry gene (let's call this X*) and a Y chromosome without the Sry gene. If an egg cell containing an X chromosome is fertilized by a sperm cell containing the X* chromosome, the resulting offspring will have an XX genotype, but still possess the Sry gene. Therefore, despite having two X chromosomes, this individual will develop male characteristics due to the presence of the Sry gene on the X* chromosome. This model explains the generation of unusual males who have two X chromosomes with an Sry-containing piece of the Y chromosome attached to one of them.

Key concepts

Genetic Sex Determination

Understanding how sex is determined genetically is fundamental in biology. In mammals, including humans and mice, sex is determined by specific genes on the sex chromosomes. We have two types of sex chromosomes, the X chromosome and the Y chromosome. Typically, females have two XX chromosomes and males have one X and one Y chromosome. The key player in this mechanism is the Sry gene, which is found on the Y chromosome.

When the Sry gene is present, as is usual with the Y chromosome, it initiates a cascade of genetic events that result in the development of male characteristics. Without the Sry gene, which is the default situation in XX individuals, the embryonic gonads develop into ovaries, leading to female characteristics.

Interestingly, sex determination can become complicated when genetic anomalies lead to males having two X chromosomes, one of which carries a fragment of the Y chromosome including the Sry gene. In these cases, the males develop typical male features, despite having a chromosomal arrangement that would usually result in a female. These anomalies highlight the central role of the Sry gene in genetic sex determination.

Pseudoautosomal Region

Sex chromosomes have specialized regions known as pseudoautosomal regions (PARs). These regions are important because they are the only parts of the sex chromosomes that are able to recombine during meiosis, the process of cell division that produces gametes. PARs are present on both the X and Y chromosomes, and during male meiosis, they pair up and undergo recombination, just like autosomal chromosomes do.

The significance of the PAR is underscored when genetic material is exchanged between the sex chromosomes. Since the Sry gene is very close to a PAR, it is prone to being involved in a crossover event with the X chromosome. This can result in a copying error that has a significant impact on the individual’s sex determination. Understanding the role of PARs is crucial in explaining how a gene such as Sry can find its way onto an X chromosome, resulting in the unusual males with two X chromosomes mentioned in the textbook exercise.

Male Meiosis

Understanding Recombination

During male meiosis, the sex chromosomes undergo recombination in the pseudoautosomal regions. Given that recombination is a process involving the exchange of genetic material, it ensures variability in the gametes. However, it also provides an opportunity for uncommon genetic events.

In males, meiosis will typically divide the X and Y chromosomes into separate sperm cells. However, because the Sry gene is very close to the PAR, this orderly segregation can go awry. The proximity increases the likelihood of the Sry gene 'jumping' from the Y chromosome to the X chromosome during the recombination process.

Result of Meiosis in Unusual Males

The outcome of this crossover could produce a sperm cell with an X chromosome that now possesses the Sry gene, leading to the formation of individuals with two X chromosomes, but with male characteristics because of the Sry gene's essential role in sex determination. The process of male meiosis is, therefore, a critical juncture in this unique form of sex determination, where the usual boundaries of chromosomal sex are blurred by a genetic exchange.

Chromosomal Crossover

Role in Genetic Diversity

Chromosomal crossover is a critical event during meiosis that increases genetic diversity. This process allows for the exchange of genetic material between homologous chromosomes and usually contributes to the unique combination of genes in sexually reproducing organisms.

Crossover Near Sry Gene

While most crossovers take place between homologous autosomes, sex chromosomes can also undergo this process in the pseudoautosomal regions. In the context of the Sry gene, crossover near this gene can mistakenly transfer it onto an X chromosome during male meiosis. This leads to the formation of what we refer to as X* chromosome.

Understanding the Impact

The presence of an X* chromosome can lead to individuals developing male characteristics despite not having a Y chromosome because it is the Sry gene that is critical for male sex determination. Therefore, understanding chromosomal crossover is essential not only for grasping how genetic diversity is achieved, but also for recognizing how irregular crossover events can challenge conventional ideas about sex determination and lead to unique genetic conditions.