XX Chromosomes

Definition - What does XX Chromosomes mean?

The XX chromosomes are sex chromosomes that determine the female sex and are one of the 23 pairs of chromosomes in a human cell. Fertilization of an egg with a sperm containing an X chromosome gives rise to an embryo with XX chromosomes which further develops to become a female fetus. XX chromosomes play a vital role in the sexual development of a female. Abnormalities of the XX chromosomes can impair the reproductive function leading to infertility.

FertilitySmarts explains XX Chromosomes

A chromosome is a tightly packed DNA strand found inside the nucleus of the cell that carries genetic information in the form of genes. A human cell contains 23 pairs of chromosomes. The last pair of the chromosomes is called the sex chromosomes and its composition determines the sex of the individual. There are two types of sex chromosomes: X and Y. XX chromosomes are seen in females. Males have XY chromosomes.

Prior to normal cell division, also known as mitosis, the chromosomes double in number so that the two cells that result from division each contain 23 pairs of chromosomes. In contrast, during meiosis which takes place during the production of gametes (eggs and sperm cells), each gamete receives only one set of the chromosomes (23). Since females have two copies of X chromosomes, an egg always has an X chromosome. But there can be two types of sperm cells, one containing an X chromosome and one containing a Y chromosome. When an egg is fertilized by a sperm bearing an X chromosome, the embryo develops to be a female fetus with XX chromosomes.

The XX chromosomes play the most important role in sexual differentiation, development of secondary sexual characteristics and the reproductive function in females. Abnormalities in these sex hormones can impair sexual features, sexual activity, hormone levels and production and maturation of eggs causing infertility.

Abnormalities of XX chromosomes can be in the number or in the genes that these chromosomes carry. A common abnormality is the absence of one X chromosome in a female (X_) giving rise to Turner syndrome. Females with this condition usually do not produce eggs and are therefore sterile.

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