Chromosomes X and Y and Sex Determination

In humans, the normal chromosome complement is 46, consisting of 22 pairs of autosomes, chromosomes that are the same in both males and females, and one pair of mostly unmatched sex chromosomes, which determine the sex of an organism and various sex-linked characteristics.

In mammals, the sex chromosomes are called X and Y. Females have paired X chromosomes, one from each parent, while males have an X and a Y chromosome. Males get their single X chromosome only from their mothers and their Y chromosome only from their fathers. In other species, for example, birds and rattlesnakes, the male has two copies of a Z chromosome, while the female has a Z and a W.

Sex Determination

In mammals, females are said to be homogametic because all their gametes contain an X chromosome. Males are called heterogametic because they produce both X bearing gametes and Y-bearing gametes. The sex chromosome in the sperm cell then determines the sex of the zygote. X-bearing sperm produces females and Y-bearing sperm produces males. Interestingly the Y sperm swim just a little faster, so a few more males are initially produced. However, this male advantage is countered later in development by deleterious genes exposed to the X chromosome, so that the birth rate of males and females is about equal.

Chromosomes X and Y

Chromosomes X and Y do not make up a fully homologous pair. The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. However, there is a short region on both the X and Y chromosomes, called the pseudoautosomal region (PAR), that allows the X and Y chromosomes to pair during meiosis for equal distribution of the two chromosomes. Only a few genes are in this region, including the gene for amelogenin, which produces tooth enamel.

Genes on the X chromosome are said to be X-linked or sex-linked. Because only females normally get two X chromosomes, dominance of X-linked genes is only expressed in females. Since males only get one X chromosome, all of their X linked alleles are expressed in the phenotype, regardless of dominance.

In the figure above, the arrows represent sex-linked genes in the X chromosome. In this homologous pairing, all those genes are expressed, because there are no equivalent genes in the Y chromosome. There are also regions on the Y chromosome that are not present on the X. One of these Y-linked genes is the Sry gene, which is male-determining in mammals. So when an individual has an XY chromosome complement (i.e. a male), these sex-linked genes are freely expressed.

X-inactivation

Since males and females have different numbers of X chromosomes, the activity of genes on the X chromosomes has to be regulated to prevent genetic imbalance in males. Monosomy, the possession of only a single copy of a normally paired chromosome, is lethal for other chromosomes. In female mammals, one of the two X chromosomes is largely inactivated in every cell, so that males and females both have a single active X chromosome. This means that females heterozygous for any gene on the X chromosome could be genetic mosaics. There are no visible phenotypes for these X linked genes in humans, but in cats, the presence of the orange gene on the X chromosome produces torties, in which some patches of fur are orange and others are black. In humans mosaicism for the X-linked gene for glucose-6-phosphate dehydrogenase has been detected from cheek cell samples in heterozygous females.

Sex-Linked Characteristics

Find out more about certain types of inheritance that does not follow the Mendelian inheritance patterns in the next tutorial.