Thus we find the higher fraction of nervous system and sense organs appear to be over represented on the human X chromosome, where as the genetic representation of body structure as well as basic metabolism is underrepresented. The human disease spectrum consists broadly three classes of diseases. Some solely determined by genes and others solely by pathogens and nutrition based and in between we find a class of diseases that are due to the interplay or interaction between genes and environment. Environmental factors that mainly influence the disease spectrum are the changing life styles, Innumerable mutagenic and carcinogenic molecules in the air we breathe, the water we drink and the food we eat. The genetic disorders are solely determined by genes and chromosomal alterations.
Chromosomes are the structures that carry the genes.
These 24 in number and occur in pairs there are 23 pairs of chromosomes in each of the pair being inherited from the parents. 23 pairs are called autosomes and one pair is called sex chromosomes. The female has 22 pairs of autosomes and 2 X - chromosomes where as the males have 22 pairs of autosomes and one X chromosome and one Y chromosome. The sex is determined by the sex chromosomes. It is the male that determines the sex of the baby and not the female as often believed.