Sex-determination system

Sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. Occasionally, there are hermaphrodites in species that have one or more variations on this sexual dimorphism. Examples of such variations can be found in some species of reptiles, fish, and invertebrates.

Types of Sex-determination systems

There are a number of specific sex-determination systems. These are differentiated by specific processes, genes, and evolutionary histories.

XY sex-determination

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila), and some plants (Ginkgo). In this system, the sex of an individual is determined by a pair of sex chromosomes. Females typically have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males typically have two distinct sex chromosomes (XY), and are called the heterogametic sex.

ZW sex-determination

The ZW sex-determination system is found in birds, some reptiles, and some insects. In this system, the sex of an individual is determined by a pair of sex chromosomes. Males typically have two of the same kind of sex chromosome (ZZ), and are called the homogametic sex. Females typically have two distinct sex chromosomes (ZW), and are called the heterogametic sex.

Haplodiploidy

Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. In this system, sex is determined by the number of sets of chromosomes an individual receives. An offspring develops into a male if it is haploid (if it has one set of chromosomes) and a female if it is diploid (if it has two sets of chromosomes).

Environmental sex determination

Some species do not have determined sex chromosomes, and they develop sex according to the surrounding environment. The most famous example of this is the crocodile and turtle species, where the temperature of the egg development determines the sex of the offspring.

See also

• Sexual dimorphism
• Gender role
• Sex and gender distinction