Genetic Basis of Sex and Sex-linked Traits The modern science of
genetics has provided a scientific explanation about how an
offspring becomes either female or male. Based on the discovery
that among the chromosomes present in the body cells, a special
pair of sex chromosomes exist that bear the genes determining
the sex of the offspring. In the human female, these chromosomes
are identical and are called X chromosomes (indicated by XX).
The male has one X chromosome and one smaller Y chromosome,
which is dominant for maleness. During the process of producing
reproductive cells (see meiosis), each of these chromosomes is
segregated into a different gamete. Thus, when fertilization
occurs, according to Mendelian law, 50% of the offspring will be
XX (female) and 50% XY (male). Deviations from this rule do
occur, but it is generally true. The rule also helps to explain
the inheritance of sex-linked characteristics such as hemophilia
(a blood clotting disorder) and red-green color blindness, since
the X chromosom