I'm curious what exact information do X and Y chromosomes hold? How does the addition of an X and an X or a X and a Y produce such a complex organism with such variable differences?
In humans, as in other mammals, sex is determined by a pair of sex chromosomes. Females have two copies of the X chromosome (XX), and males a single X and a small, malespecific Y chromosome (XY). The X is a medium-sized chromosome that contains about 1300 genes with a range of functions. The Y is a small chromosome with only about 45 unique protein coding genes. The Y chromosome determines maleness and thus is the basis for phenotypic sex determination in humans. Y chromosome contains a critical gene that controls male sexual development, called the sex determining region Y (SRY). In humans, the absence of a Y chromosome almost always leads to female development; thus, this gene is absent from the X chromosome. At 6 to 8 weeks of development, the SRY gene becomes active in XY embryos. SRY encodes a protein that causes the undifferentiated gonadal tissue of the embryo to form testes. This protein is called the testis-determining factor (TDF).* SRY is the gene responsible for male sex determination In contrast to the Y chromosome, which specializes in male sex and fertility, the X chromosome contains genes with a wide range of functions, such as blood clotting factors, visual pigments and housekeeping enzymes. The X chromosome is generally thought to be conservative, and to change more slowly in evolution than the Y or the autosomes.