Human cells are diploid, which means they carry two copies of each chromosome. Thus, they carry two copies of each gene, one on each of the paired chromosomes. For a gene such as the adult hemoglobin gene, which can exist in two forms-the "normal" type, , and a variant (mutant) type, *, an individual can have three possible combinations of these genes in the paired chromosomes:

A. + : homozygous (same genes) in the normal type

B. + *: heterozygous (mixed genes)

C. * + *: homozygous in the variant type

Individual A will produce only normal hemoglobin chains. Individual C, will produce only variant hemoglobin chains. Individual B, with genes for both types, will produce both. If the mutation is deleterious, A will be unaffected, but C will be in serious trouble. B, on the other hand, may do fairly well, because he or she will make normal protein chains along with the variant ones.

When two individuals produce offspring, each parent donates to a child one copy of the hemoglobin gene, the selection of which will be random. If both parents are heterozygous for the gene (Figure 7.26), the child has one chance in four of being homozygous normal, one in four of being homozygous for the variant gene, and two in four of being heterozygous. Because most variant hemoglobin genes are rare in the human population, only occasionally do we find an individual homozygous for the variant type.