Allosome

An allosome (also referred to as a sex chromosome, heterotypical chromosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical pair of mammal allosomes, determine the sex of an individual created in sexual reproduction. Autosomes differ from allosomes because autosomes appear in pairs whose members have the same form but differ from other pairs in a diploid cell, whereas members of an allosome pair may differ from one another and thereby determine sex.

In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes. The first 22 pairs are called autosomes which look exactly the same in both males and females. The 23rd pair of chromosomes is called an allosome. These sex chromosomes usually differ between males and females. females have two copies of the X chromosome, while males have one X chromosome and one Y chromosome. The X chromosome is always present as the 23rd chromosome in the ovum, while either X or Y chromosomes can be present in an individual sperm.

All diploid organisms with allosome-determined sex get half of their allosomes from each of their parents. In mammals, females are XX, they can pass along either of their X’s, and since the males are XY they can pass along either an X or a Y. For a mammal to be considered a female, the individual must receive an X chromosome from both parents, whereas to be considered a male, the individual must receive a X chromosome from their mother and a Y chromosome from their father. It is thus the male’s sperm that determines the sex of each offspring in humans.

Allosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. Sex-linked diseases are passed down through families through one of the X or Y chromosomes. Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes.

...
Wikipedia