Principles of Genetics and Breeding

4.1.6 Polygenic determination of sex

Sex determination in fishes is said to be polyfactorial(polygenic), even in those species in which a sex chromosome mechanism has been demonstrated.

• According to polygenic theory, sex determination in fishes is governed by a large number of sex genes that are located on many chromosomes.
• It is a ratio of male to female factors that ultimately determines the sex into which a given individual will develop.
• The sex chromosomes carry superior sex genes, i.e., male (M) and female (F) determinants with greater potencies than those of autosomes.
• Polygenic determination of sex is of most primitive type. Male and female genes are located in many chromosomes and the determination of sex depends on the balance of these genes. Eg. Sword tail, Xiphophorus helleri, Limia vittata, L. caudo fasciata (F. Poecilidae).
• Each individual gene affecting gonad development is relatively weak in action and therefore a change in environmental conditions and the variation in the genotype may easily be accompanied by changes in the sex ratio.