Within the nucleus of almost every human cell are chromosomes, which carry our genetic information in the form of DNA. A human cell holds 23 pairs of these structures, for a total of 46. Twenty-two of these pairs are autosomes, numbered 1 through 22. The remaining pair consists of sex chromosomes, which determines an individual’s chromosomal sex. The two main combinations of sex chromosomes in humans are XX and XY.
The Process of Sex Determination
The presence of a Y chromosome initiates male development because it contains a specific gene called the SRY gene, which stands for Sex-determining Region Y. This gene functions as a genetic switch that begins a complex sequence of events.
Around the sixth to eighth week of embryonic development, the SRY gene activates. Its activation triggers the undifferentiated gonads to begin developing into testes. This gene’s expression leads to the formation of primary sex cords that will eventually become seminiferous tubules within the testes. The SRY gene’s activity also upregulates another gene, SOX9, which is also involved in the formation of the testes.
Once the testes are formed, specialized cells within them produce and secrete hormones. Leydig cells produce androgens, most notably testosterone, while Sertoli cells produce anti-Müllerian hormone. Testosterone promotes the development of male internal and external reproductive structures, while the anti-Müllerian hormone suppresses female internal structures.
In an embryo with an XX chromosome pair, there is no SRY gene. Without the SRY gene’s signal, the default developmental pathway proceeds. The undifferentiated gonads develop into ovaries instead of testes. In the absence of testosterone and anti-Müllerian hormone, the internal structures develop into a uterus and fallopian tubes, and the external structures develop along a female path.
Genetic Composition of X and Y
The X and Y chromosomes differ significantly in their size and the amount of genetic information they carry. The X chromosome is substantially larger, containing between 900 and 1,400 genes that influence a wide variety of bodily functions. These genes are not solely related to sex characteristics; they play parts in processes like blood clotting, muscle function, and cognitive development.
In contrast, the Y chromosome is one of the smallest human chromosomes, containing only about 70 to 200 protein-coding genes. Many of these genes are highly specialized, including the SRY gene. Other genes on the Y chromosome are primarily involved in functions specific to males, such as spermatogenesis, the process of sperm production.
While most genes on the Y chromosome are unique to it, there are small regions at the tips called pseudoautosomal regions (PARs). The genes in these areas are present on both the X and Y chromosomes, allowing them to pair up and exchange genetic material during the formation of sperm. These shared genes are involved in normal development for all sexes. The vast majority of the Y chromosome, however, does not recombine with the X chromosome.
Inheritance of Sex Chromosomes
The transmission of sex chromosomes from parents to their children follows a clear biological pattern. This process occurs during meiosis, the specialized cell division that produces reproductive cells, or gametes. In females, who have an XX pair, all the eggs they produce will contain a single X chromosome. The mother, therefore, always contributes an X chromosome to her offspring.
The determination of the offspring’s chromosomal sex depends on the sperm cell from the father. Males have an XY chromosome pair, and during sperm production, their sex chromosomes are segregated. Approximately half of the sperm cells will carry an X chromosome, while the other half will carry a Y chromosome.
Fertilization is the moment when the chromosomal sex of the future child is established. If an egg is fertilized by a sperm carrying an X chromosome, the resulting embryo will have an XX pairing and will be biologically female. If the fertilizing sperm carries a Y chromosome, the embryo will have an XY pairing and will be biologically male.
Conditions Involving the XY Pair
Variations in the number or function of sex chromosomes can lead to differences in sexual development. One example is Klinefelter Syndrome, which occurs in individuals who have an extra X chromosome, resulting in an XXY karyotype. This is one of the most common sex chromosome disorders.
Another condition is Swyer Syndrome, which affects individuals with an XY chromosome pair. In these cases, the SRY gene on the Y chromosome is non-functional or absent due to mutation or deletion. Without a functioning SRY gene, the developmental pathway defaults to female, despite the presence of a Y chromosome, resulting in the development of female reproductive structures.
Androgen Insensitivity Syndrome (AIS) highlights the importance of hormonal response. Individuals with AIS have an XY karyotype and their testes produce androgens, but their bodies are unable to respond to these hormones due to mutations in the androgen receptor gene. Depending on the degree of insensitivity, the external physical characteristics can range from typically female to having features of both male and female development.