ABSTRACT
Key Notes https://www.niso.org/standards/z39-96/ns/oasis-exchange/table">
Sex determination
Primary sex determination relates to the formation of ovaries or testes. This can be regulated by genes and/or environmental factors. In alligators sex is determined by the temperature at which eggs are incubated. Many species are hermaphrodite. Secondary sexual characteristics are also under genetic regulation.
Simple genetic sex-determining systems
Unicellular eukaryotes can have sex-determining systems in which two alleles of a single gene determine sex. In Hymenoptera, heterozygosity at one gene determines femaleness.
Sex chromosome systems
Three different sex chromosome systems have been described, XX-XO, XX-XY and ZZ-ZW. The sex which has two identical sex chromosomes is called homogametic and the sex with different sex chromosomes heterogametic.
Sex determination in Drosophila
In this species sex is determined by the ratio of X chromosomes to sets of autosomes. If the ratio is equal or greater than 1.0 the fly is female; if 0.5 or lower the fly is male. Gynandromorphs are flies with male and female sex phenotypes in different parts of their body. This is due to changes in the number of X chromosomes in somatic cells due to nondisjunction.
Sex determination in humans
In humans the study of sex chromosome aneuploids has shown that the presence of a Y chromosome determines maleness irrespective of the presence of different numbers of X chromosomes. A gene, SRY, responsible for inducing maleness has been mapped to the Y chromosome.
Sex determination in plants
Most plants are hermaphrodite, but some examples of XX-XY sex chromosome systems are known in dioecious plants. In Melandrium sex is determined by the ratio of X chromosomes to Y chromosomes.
Secondary sexual characteristics
Mutations in genes such as the testosterone receptor gene can lead XY individuals to develop female secondary sexual characteristics. Transfer of blood cells between male and female twins in cattle can produce freemartins which have defective testes composed of cells containing two X chromosomes.
Evolution of sex chromosomes
Sex chromosomes have evolved on several different occasions. The sex chromosomes evolve from a pair of homologous autosomes. In some species the two sex chromosomes look identical, homomorphic. After divergence, a small region of homology between the two remains for meiotic alignment. It is known as the pseudoautosomal region.
Related topics
Chromosomes
Sex and inheritance
