The chromosomal basis of sex determination. In the XY type of sex determination, meiosis produces sperm with either an X or a Y chromosome, in a 50:50 ratio (autosomes are not shown). All eggs have one X. Fertilization results in either an XX (female) or XY (male) zygote. The Y chromosome confers maleness. In XO determination, meiosis produces two types of sperm in a 50:50 ratio, either with or without an X chromosome (X or O). All eggs have one X. Fertilization results in either an XX (female) or an XO (male) zygote. The number of copies of X determines the sex.