In general, the gender of an embryo is determined by the presence of either an X or Y chromosome in the sperm which fertilizes the ova. If an X-bearing sperm fertilizes the ova, the embryo is female. If a Y bearing sperm fertilizes the ova, the embryo is male.

Embryos may receive an abnormal number of sex chromosomes and there are genetic mutations which will prevent gonads from developing, regardless of chromosomal gender.
In humans, there are 22 pairs of chromosomes that are equivalent in males and females; these are known as autosomes. It is the 23rd pair of chromosomes, the sex chromosomes, which determine chromosomal gender. It may be interesting to note that there are a number of ways of determining gender in animals. Some animals have nothing but autosomes and gender can be determined by nest temperature (reptiles) or whether or not the egg is fertilized (bees & ants). In many animals (mammals, birds, fruit flies) there is a pair of chromosomes that do not have equal forms in both sexes and are called sex chromosomes. In humans (and fruit flies), males have the normal "X" plus a smaller "Y" while females have 2 "X"s (while in birds and butterflies the opposite is true). Other combinations using multiple X and Y chromosomes are possible. In monotremes, the sexual determination of the platypus is complex: males possess a X1Y1X2Y2X3Y3X4Y4X5Y5 karyotype (Rens, 2004). In echidnas, females are X1X1X2X2 while males are X1X2Y (Solari, 1994).
In humans, males produce 2 types of gametes: one with an X chromosome, one with a Y chromosome. Which of these fertilizes the egg will determine the sex of the offspring. The Y chromosome determines maleness; XXY individuals are male while individuals with only one X chromosome are female.
-- XXY males will show some feminization (posture, breast development; testosterone therapy can improve secondary sexual characteristics but infertility is the rule)
--in Turner Syndrome females (who possess a single X chromosome) ovaries begin developing normally but begin to degenerate in the 15th week of gestation; this degeneration may not be complete: menses may occur in 10-15% and a pregnancy may rarely be possible
-- Human males may also exhibit XYY and XXYY karyotypes while females may also have XXX and XXXX karyotypes.


The ancestral mammalian Y chromosome possessed all the genes located on the X chromosome, most of which have been lost. This loss was not complete by the diversification of mammalian lineages and modern mammal groups differ in the genes which have been lost from their Y chromosomes. Although the X and Y chromosomes have a common therian ancestry, some genes on the X and Y chromosomes in eutherians are autosomal in marsupials (such as ZFY/X, AMELY/X, STS/STSP, and the pseudoautosomal genes). Thus, chromosomal regions were added to the X and Y chromosomes at the base of the eutherian lineage (Pask, 2000). The X-Y homologous gene ubiquitin activating enzyme (UBE1) has been lost from the Y chromosome in the primate lineage. This is predicted in the model that the X and Y chromosomes were derived from autosomes and that selection favors the loss of homologous genes to discourage crossing over between them (Mitchell, 1998). Fish, amphibians, and reptiles possess homologs of Y chromosomal genes (such as ZFY) and SOX genes involved in the determination of gender in mammals (Spotila, 1994) but they are not limited to one gender (Uguz, 2003; Tiersh, 1992).
The development of the reproductive system is under genetic control. Mutations in several genes can cause the absence of gonads in mice, including Lim1, SF-1, WT1, EMX2, and LHX9 (Clarkson, 2002). The genes which are required for the development of gonads include steroidogenic factor 1 (Sf11), Wilms tumor 1 (Wt1), Lim1, Lhx9, and Emx2; mutations in any of genes can cause the absence of gonads in mice. Sf1 is a member of the nuclear hormone receptor family which is required for the development of gonads and Leydig cells in testis. Lim1 and Lhx9 are homeodomain genes. Emx2 is the homolog of a head gap gene in Drosophila. Desert hedgehog and its receptor Patched are expressed in the developing testis but not the developing ovary (Tilmann, 2002). Wnt4 inhibits the formation of Leydig cells in ovaries (Tilmann, 2002). One family of DNA binding proteins which share a DM domain are involved in the development of the male reproductive system in nematodes, insects, and mammals (in mammals Dmrt1) and maintain the seminiferous tubules (Colvin, 2001).