Chromosomes are thread-like structures found within the nucleus of most cells. Composed of DNA tightly coiled around proteins, they serve as organized packages of genetic information. Humans typically possess 23 pairs of chromosomes, totaling 46. The first 22 pairs are autosomes, carrying genes for general body functions. The remaining pair are the sex chromosomes, also called allosomes, which influence an individual’s biological sex.
Human Sex Determination
In humans, biological sex is typically determined by the combination of sex chromosomes inherited from parents. Females usually have two X chromosomes (XX), while males typically have one X and one Y chromosome (XY). An egg cell always carries an X chromosome from the mother. A sperm cell from the father, however, can carry either an X or a Y chromosome.
The chromosome contributed by the sperm determines the sex of the offspring. If an X-bearing sperm fertilizes the egg, the combination is XX, leading to female development. If a Y-bearing sperm fertilizes the egg, the combination is XY, resulting in male development. The Y chromosome contains the SRY gene (Sex-determining Region Y). This gene initiates the formation of male gonads, specifically the testes, during embryonic development. In the absence of a functional SRY gene, development proceeds towards female characteristics, forming ovaries and other female reproductive structures.
Characteristics of X and Y Chromosomes
Beyond their role in sex determination, the X and Y chromosomes exhibit differences in their physical characteristics and genetic content. The X chromosome is larger than the Y, spanning approximately 155 million base pairs and containing 800 to 1,400 protein-coding genes. In contrast, the Y chromosome is smaller, with about 55 to 59 million base pairs and 50 to 200 genes. Many genes on the X chromosome are involved in various bodily functions, not just sex-related traits.
The differing gene content between the X and Y chromosomes gives rise to unique patterns of inheritance for certain traits, known as X-linked inheritance. Because males have only one X chromosome, they express any gene present on that single X, even if it is a recessive gene. This explains why X-linked recessive conditions, such as red-green color blindness and hemophilia, appear more frequently in males. Females, with two X chromosomes, generally need two copies of a recessive gene (one on each X) to express the condition. If a female carries one copy of an altered gene on one X chromosome and a normal copy on the other, she is typically a carrier and may not show symptoms.
To balance the gene dosage between males and females, a process called X-inactivation occurs in females. Early in embryonic development, one of the two X chromosomes in each cell of a female is randomly inactivated. This inactivated X chromosome condenses into a compact structure called a Barr body. The random nature of this inactivation means some cells inactivate the X from the mother, while others inactivate the X from the father. This cellular mosaicism is visibly apparent in the fur patterns of tortoiseshell and calico cats.
Variations in Sex Chromosome Number
Individuals can be born with a different number of sex chromosomes than the typical XX or XY arrangement. These variations, known as sex chromosome aneuploidies, arise from errors during cell division. Such errors can occur during the formation of egg or sperm cells or in the early stages of embryonic development.
One example is Turner syndrome, where females have only one X chromosome instead of two. This condition often leads to short stature and reduced functioning of the ovaries. Klinefelter syndrome affects males who possess an extra X chromosome. Individuals with Klinefelter syndrome may experience reduced testosterone production, leading to infertility and sometimes learning difficulties.
Another variation is Jacobs syndrome, characterized by males having an extra Y chromosome. Individuals with this condition are often taller than average and may have an increased risk of learning disabilities. Similarly, Triple X syndrome occurs in females who have an extra X chromosome. These individuals may be taller than average and occasionally experience developmental delays or learning difficulties. Many individuals with these chromosomal variations may have mild symptoms or no symptoms at all, leading to a diagnosis only later in life or sometimes never.
Sex Determination in Other Species
While the XY system is prevalent in humans and most mammals, sex determination mechanisms vary across the diversity of life. Other species employ different chromosomal systems, highlighting varied evolutionary paths for sex determination.
One such system is the ZW system, found in birds, some reptiles and insects, such as butterflies and moths. In this system, the female is the heterogametic sex, meaning she has two different sex chromosomes (ZW), while the male is the homogametic sex with two identical chromosomes (ZZ). In contrast to humans, the egg cell determines the sex of the offspring in ZW systems.
An XO system exists in some insects, including grasshoppers and crickets. Here, females have two X chromosomes (XX), but males possess only a single X chromosome (XO), with the “O” indicating the absence of a second sex chromosome. These examples demonstrate that the human XY system is just one of several biological strategies for determining sex.