The human deoxyribonucleic acid base pair code in all cells specifies the characteristics of the individual, or their phenotype. In any one human chromosome there are two genes at equivalent loci, each coding for a polypeptide. This code is divided in gametes such that offspring only receive 50% of each parent's genetic constitution, that is, inheritance is particulate. The particle randomly selected is one of the two genes at each loci. The variable selection and segregation of genes into gametes ensures that the diversity of human characteristics is maintained.
From these first principles, diseases can be classified into those resulting from defects in:
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