1. Populations of wild raspberries in Britain are usually polymorphic for the presence or absence of cane hairs, determined by genes H and h, and sometimes for the presence or absence of plant pigment, determined by genes T and t. A high proportion of raspberry cultivars also are heterozygous for these two genes.
2. The aberrant segregation ratios associated with each of these loci can be attributed to their linkage with lethal genes in such a way that the lethals form a balanced system.
3. Since the lethal system confers a selective advantage on the heterozygote, it is probably responsible for the polymorphism and allelic diversity, and also for the relatively high frequency of recessive deleterious genes in this linkage group. But in the case of the H locus, allelic diversity may also be maintained by the tendency of the dominant phenotype to be favoured in some cross-combinations and the recessive in others.
4. The presence of the lethal system helps raspberry populations to maintain heterozygosity for the particular chromosome segment and may help small populations to avoid inbreeding depression.