Sex-determination system .January 13, 2021
A 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 place of one or both sexes. There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization.
The XX/XY sex-determination system is the most familiar, as it is found in humans. The XX/XY system is found in most other mammals, as well as some insects. In this system, most females have two of the same kind of sex chromosome (XX), while most males have two distinct sex chromosomes (XY). The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes (autosomes), and are sometimes called allosomes. In some species, such as humans, organisms remain sex indifferent for a time after they’re created; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.
There are some species, such as the medaka fish, that evolved sex chromosomes separately; their Y chromosome never inverted and can still swap genes with the X. These species’ sex chromosomes are relatively primitive and unspecialized. Because the Y does not have male-specific genes and can interact with the X, XY and YY females can be formed as well as XX males. Non-inverted Y chromosomes with long histories are found in pythons and emus, each system being more than 120 million years old, suggesting that inversions are not necessarily an eventuality.
The ends of the XY chromosomes, highlighted here in green, are all that is left of the original autosomes that can still cross-over with each other.
- Majerus, M. E. N. (2003). Sex wars: genes, bacteria, and biased sex ratios. Princeton University Press. p. 250. ISBN 978-0-691-00981-0. Retrieved 4 November 2011.
All alligators determine the sex of their offspring by the temperature of the nest.
It is unknown how exactly temperature-dependent sex determination evolved. It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season. Environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent amniote was the common ancestor of amniotes with sex chromosomes.
- ^ “Nettie Stevens: A Discoverer of Sex Chromosomes | Learn Science at Scitable”. www.nature.com. Retrieved 7 June 2018.