In many animals and some plants, the sex of an individual is determined at the time of merging of the male and female sex cells by the presence of sex chromosomes. Mammals are an example of organisms in which the male carries two kinds of sex chromosomes, the X and the Y chromosome, while the female has both sex chromosomes the same, i.e. has two X chromosomes. During meiosis, the two sex chromosomes form a pair and then separate (similar to the other pairs of homologous chromosomes), each to its newly forming sex cell. Thus, two types of gametes are formed in males, one with an X sex chromosome and the other with a Y sex chromosome. Females form only one type of gamete – all with an X chromosome. If, during fertilization, a male gamete bearing an X chromosome merges with a female gamete bearing an X chromosome, then a female embryo is formed. However, if a male gamete bearing a Y chromosome merges with a female gamete bearing an X chromosome, then a male embryo, XY, is formed. The Y chromosome carries genes that are necessary for the formation of male sex organs and the products of these male sex organs subsequently affect the entire development of the embryo in a way such that a male is formed.