Principles of Genetics and Breeding

4.1.7 Sex determination mediated by sex chromosomes

• The determination of sex by chromosomes differing in one or a few of the genes is characteristic of many species. For example, salmonidae and cyprinidae.
• The genetic factors affecting the sex (acting apparently via the hormones determining the sex) are, however, present in autosomes as well. Nevertheless, the main part is played by the sex genes located in the sex chromosomes.
• The identification of sex chromosomes in fishes is a difficult task. The sex chromosomes are distinguished from other chromosomes by the presence of sex-chromatin (barr body) and karyotypic difference between two sexes in somatic metaphase counts.
• This type of sex determination is of more advanced type. The sexual diversity also occurs at the level of chromosomes.
• Within this mechanism, the sex chromosomes X and Y or W and Z are different, but the difference merely involve the existence of one or several specific male and female sex genes.
• In Poecilia reticulata, P. variatus, Oryzias sp. Salmo gairdneri and Xiphophorus maculatus distinct sex chromosome has been reported. Sex chromosomes have been marked using the genes responsible for pigmentation.
• In many fishes, female has two X-chromosomes, male has only one. All X-chromosome genes in the female are present in a double dose and in male single dose. This difference in dosage would have an effect on gene expression, leading to unbalance in one of the two sexes.
• Male heterogamety XY was most common.