ZW sex-determination system
ZW sex-determination system is a method of sex determination in which organisms develop into males or females depending on the presence or absence of certain chromosomes. This system is used by many birds, some reptiles, and some insects. In this system, the sex of an individual is determined by the combination of sex chromosomes they receive from their parents.
Overview
In the ZW sex-determination system, the sex chromosomes are not homologous. This means that they do not pair and exchange genetic material during meiosis. Instead, one of the sex chromosomes, the Z chromosome, is larger and contains more genes, while the other, the W chromosome, is smaller and contains fewer genes.
Females have one Z and one W chromosome (ZW), while males have two Z chromosomes (ZZ). This is the opposite of the XY sex-determination system used by most mammals, where the male has two different kinds of sex chromosomes (XY) and the female has two of the same kind (XX).
Mechanism
The mechanism of the ZW sex-determination system is not fully understood. However, it is believed that the W chromosome carries a sex-determining gene that triggers the development of female characteristics. In the absence of a W chromosome, the individual develops as a male.
Evolution
The ZW sex-determination system is thought to have evolved independently several times. It is believed that the Z and W chromosomes originated from a pair of autosomes that acquired a sex-determining gene. Over time, the W chromosome has lost most of its genes and has become smaller than the Z chromosome.
See also
References
ZW sex-determination system
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Contributors: Prab R. Tumpati, MD
