Genes are the fundamental units of heredity and encode specific functionalproducts, such as RNA molecules and polypeptides. They are encoded by sequences of bases in DNA molecules and are found at particular positions in chromosomes in the nucleus and also in the relatively small circular DNA molecules in the mitochondria. Only 37 of the approximately 22 000 human genes occur in mitochondria although mutations of these may become clinically significant as described. The genes constitute the blueprint or the set of instructions which affects hereditary characteristics, for example hair and eye color, height and the susceptibility to certain diseases.
When a cell divides the genetic information needs to be replicated accurately so that these instructions pass on to the daughter cells. When changes occur in the base sequence of DNA, either as a result of incorrect replication or from random changes caused by physical or chemical agents, then the instructions become corrupted. This is a mutation, and may eventually lead to disease because the cell is unable to make, for example, a particular enzyme, hormone, transporter or structural protein.
Chromosomes have a complex structure (Figure 15.1). Each is comprised of a single double-stranded DNA molecule associated with numerous proteins.
In general, chromosomes occur in matching or homologous pairs, with each member of a pair containing alleles or different forms of the same gene, which are found at the same loci (singular locus) in each member of the pairs. Normal human somatic (body) cells contain 23 pairs of chromosomes and are said to be diploid (2N). The 46 chromosomes in diploid cells comprise 22 homologous pairs of autosomes (nonsex chromosomes) and one pair of sex chromosomes; XX in females and XY in males (Figure 15.2; see also Figure1.13). Oocytes and spermatozoa have half the diploid number and are said tobe haploid (N); oocytes can only contain an X chromosome but sperm can have an X or a Y chromosome.