Usually, sex is determined by an organism's genes. With genetic sex determination, most alleles or genes that influence sexual development are on the same chromosome. That chromosome is then called the sex chromosome. Because genetic sex determination is controlled by a pair of sex chromosomes (or if of one of the chromosomes is there or not), there are usually the same number of male and female offspring. In humans, for instance, sperms carry either an X or a Y chromosome, and they occur in roughly equal numbers.