Golub and Green (1991) describe the methods with
which the immune generates
a diverse response. Lymphocytes recognise pathogens with antigen receptors.
The receptors contain 'heavy' and 'light' protein chains, assembled from
three and two gene segments respectively. The heavy chains contain Variable
(V), Diverse (D), and Joining (J) segments, the light chains contain just V
and J. There are at least six methods in which these genes are used to
express diversity: (Taken from Golub and Green
(1991) except where noted)
1. Genetic (allelic) diversity - different V,D and J genes in different
humans, (Mattila et. al. 1995)
(From: natural responses to uncertainty)
If we consider two parent organisms P1 and P2, with alleles (at a given locus L) (A1,A2) and (A3,A4) respectively.
Through the processes of meiotic division and fertilization, each parent passes one allele to each child. Mendel's first law of genetics (the law of segregation) observes that the children of these parents will consist of equal numbers of four genotypes:
(A1,A3), (A1,A4), (A2,A3), (A2,A4)
If organisms with each of these genotypes survive to breed then the four alleles (A1, A2, A3, A4) will be maintained within the population. However, if not all genotypes survive, alleles may be lost with each new generation.
Example: two children survive each round, with no selection effects, and no mutation. (This can be studied with pen, paper, and a random number generator). Monte Carlo result - from 4 alleles to one allele in 13 generations.
As there is a diversity of V, D, and J alleles in the human population, there must be a mechanism that counter-acts this loss of diversity.