ABSTRACT

In a woman, the sex chromosomes are a matching pair of so called X chromosomes while in a man, the pair are an X chromosome and a Y chromosome. The 22 pairs of autosomes are classified according to size and shape and are numbered 1 through 22 with number 1 being the longest. When female eggs and male sperm are formed the chromosomes in each pair separate, leaving 23 chromosomes in these cells rather than 23 pairs. When a female egg is fertilised the chromosomes from the egg and the male sperm form themselves into pairs. When this process operates incorrectly, abnormalities occur (see also, Kumar and Clark, 2005, p. 171).