Principles of Genetics and Breeding

3.1.6.3 Semi-dominant lethal gene

• The locus N (Nigra) in platy is a result of dominant mutation and is widely used by aquarium fish culturists. The mutation results in a markedly blackening of the tail part of the body and of the caudal fin. The pigmentation is due to the accumulation of large melanophores.
• The semi dominant gene Fu producing the black pigmentation of the whole body appears to be an allele of the gene N.
• The homozygotes Fu are non-viable. In the presence of recessive allele n the macromelanophores are completely absent.
• In angel fish, Pterophyllum scalare, there are about 25 colour patterns which are the most important ornamental traits in this species. Golden body colour is determined by a recessive autosomal gene designated by a semi-lethal autosomal gene, BI.