This module deals with chromosomes and their role in inheritance, focusing particularly on how the sex of an individual is determined by the complement of chromosomes in its cells.
Objectives
Chromosomes
As described elsewhere (see the module on cell biology), chromosomes are structures made of chromatin, which is a mixture of DNA and a specific family of proteins called histones. During cell division, chromosomes are highly condensed, becoming visible by light microscopy. During interphase, chromosomes are relatively decondensed, and are not visible under the microscope.
Chromosomes are not uniform, however. Some regions of the chromosome consist of chromatin that is always highly condensed, even during interphase. These regions are called heterochromatin ('different' chromatin). The other regions which are uncoiled during interphase, and highly condensed during cell division, are called euchromatin ('good' or 'true' chromatin). Each chromosome of a cell consists of regions of euchromatin interspersed with regions of heterochromatin. These various regions appear appear as light and dark bands when mitotic chromosomes are stained with various dyes. Each chromosome has a different banding pattern, so chromosomes can easily be identified when stained in this way. Photographs of stained mitotic chromosomes for the purposes of chromosome identification (such as that shown at right) are known as karyotypes.
Sex Chromosomes
There is one example where a somatic cell may not have two copies of a particular chromosome under normal circumstances. This involves the sex chromosomes. As we shall see, these chromosomes are involved in determining the gender of an individual. Our consideration of sex chromosomes will focus on the sex chromosomes of Drosophila and humans.
Both fruit flies and humans have two different sex chromosomes: X and Y. Females have two X chromosomes in every somatic cell; males have one X and one Y. The X and Y chromosomes are quite different, so males have two chromosomes that do not appear as a natural pair (this is the situation alluded to above). The Y chromosome is much smaller than the X, and its centromere is closer to one end. There is also little similarity in the DNA sequences found on the two chromosomes. The X and Y do behave as members of a homologous pair, however. They pair up during meiosis, just as other homologues do. They seaparate during meiosis I, producing two types of sperm: those containing an X chromosome (female-producing sperm) and those containing a Y chromosome (male-producing sperm).
Because the sex chromosomes are somewhat unique in their properties, they are classified separately from the other chromosomes. This is indicated by their name: the sex chromosomes. The remaining chromosomes are called autosomes.