According to Mendel's law of segregation, alleles must always separate from each other during the meiotic process with one member of each chromosome pair going into a gamete. With regard to sex chromosomes, the male produces two types of gamete then... one containing an X chromosome and one a Y. The female produces only X containing gametes. However, from time to time, the female's X chromosome bivalent fails to separate properly so that the entire bivalent goes into one of the developing egg cells. Thus it possesses two X chromosomes while the daughter cell possesses none. The problem is diagrammed at the right. Flip to the next card for more information.