Karyotypes are pictures of homologous chromosomes lined up together during Metaphase I of meiosis. The chromosome pictures are then arranged by size and pasted onto a sheet of paper. Karyotype analysis reveals that all normal human eggs carry only one X chromosome. Half of the sperm carry an X and the other half carry a Y.
This animation (Audio - Important) describes karyotyping.
Determination of Gender in Humans
If an X bearing sperm fertilizes an X-bearing egg, a female results. If a Y-bearing sperm fertilizes an X-bearing egg, a male results.
The Y chromosome carries a male-determining gene that is Y-linked. Fewer than two dozen genes have been identified on the Y chromosome. One is the master gene for male sex determination. It is called the SRY gene (Sex-determining region of Y). If SRY is present, testes form. If SRY is absent, ovaries form.
There are also genes on the sex chromosomes that code for nonsexual traits. The X chromosome carries more than 2,300 genes. Most of these genes deal with nonsexual traits. Genes on the X chromosome can be expressed in both males and females.