The pseudoautosomal region is the portion of both the X and the Y chromosome that is
actually the same.
There is a lot of X chromosome beyond the pseudo-autosomal region, and this large
part of the X chromosome contains many important genes.
The human X chromosome has genes on it that relate to sexual development, but the X
chromosome also has lots of genes that relate to other important processes. For
example in humans an important gene for blood clotting is on the X chromosome. The
gene for color vision is also on the X chromosome.
Outside of the pseudoautosomal region, the human Y chromosome has only a few
genes on it, one gene known as the TDF gene(testis determining factor), is very
important in determining maleness. But the Y lacks most of the genes that are on the X
X-linked genes are genes in the non-pseudoautosomal region of the X chromosome
Y-linked genes are on the non-pseudoautosomal region of the Y chromosome. The
only known Y –linked gene is the TDF gene.
Sex-linked genes is a less precise term that refers to genes on a sex chromosome, either
X or Y; but since the Y chromosome only has one Y specific gene, and the X
chromosome has many X-specific genes, the term sex linked gene often is
used(carelessly) to refer to X-linked genes.
THE IMPACT OF X-LINKAGE ON INHERITANCE
Women, since they have 2 X chromosomes, have two versions of all X-linked genes.
Women can be homozygous or heterozygous for X-linked genes.
Men are neither heterozygous, nor homozygous for most of the genes found on the X
chromosome because they have only one X chromosome. Men are called hemizygous
for all of the X-linked genes.
Hemizygosity is the condition of having only one version of a gene, in an otherwise
Because males are hemizygous for X-linked genes, the pattern of inheritance of X-
linked genes is different from autosomal genes.
The situation is similar in Drosophila – an important model system for genetic studies.
The X chromosome has many important genes on it, and the Y chromosome seems only
to have a gene important to male fertility.
So Drosophila males also are hemizygous.
During meiosis the two X chromosomes pair up, during prophase I, then later during
meiosis, two X chromosomes segregate from each other.
Mammalian females (and female Drosophila) donate one X chromosome to all of her
offspring. Thus all gametes from females have an X chromosome.