Principles of Genetics and Breeding

4.2.1 Sex linked genes

Qualitative phenotypes may also be controlled by genes located on one of the sex chromosomes and the genes are called sex-linked genes.

• The inheritance of sex-linked phenotype is different from that seen for autosomal phenotype because when a species has sex chromosomes, one sex is homogametic while the other is heterogametic.
• Sex-linked phenotypes have been discovered in relatively few species of fish and most of the information came from the two species guppy and platy.
• The sex-linked inheritance in fish was first described by Johannes Schmidt in 1920 in guppy, Poecilia reticulata.
• Males were small and brightly coloured whilst the females were larger and of grey-green.
• The colour of the males showed strict paternal inheritance.
• To date, all sex-linked phenotypes that are known in fish are controlled by genes that are located on the X and / or Y chromosome.
• No genes restricted to the W or Z chromosomes have been discovered.