Autosomes and Sex Chromosomes

The nucleus of a human somatic cell contains 46 chromosomes, inherited in pairs from both parents. The 23rd pair, known as sex chromosomes, determines an individual’s biological sex, with designations X and Y. The Y chromosome, responsible for male traits, is smaller than the X chromosome. The combination of these sex chromosomes (XX for females, XY for males) contributes to the diversity in human gender. 

The inheritance of sex

Whether an individual is male or female is determined by a pair of chromosomes known as sex chromosomes. In females, both sex chromosomes are X chromosomes, and they are of the same size. Conversely, in males, the two sex chromosomes are different sizes—one is an X chromosome, and the other is a smaller Y chromosome. Therefore, female cells have a combination of XX chromosomes, while male cells have a combination of XY chromosomes.

The process of meiosis plays a crucial role in the formation of gametes, which are sex cells. In females, meiosis occurs in the ovary, and each ovum (egg cell) receives one of the X chromosomes. This results in all ova having the same composition of sex chromosomes (XX). In males, meiosis occurs in the testes and leads to 50% of sperm carrying an X chromosome and 50% carrying a Y chromosome.

When fertilization occurs—when sperm fertilizes an egg—the resulting zygote carries a specific combination of sex chromosomes, determining the individual’s sex. 

• If an X sperm fertilizes the ovum, the zygote will be XX, leading to the development of a female. 

• If a Y sperm fertilizes the ovum, the zygote will be XY, leading to the development of a male.