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The mysterious XY chromosomes: How do they determine our sex?
The XY sex determination system is a sex determination system found in many mammals, including humans, some insects (such as fruit flies), some snakes, some fish (such as guppies), and some plants (such as ginkgo trees). system. In this system, the sex of an individual is usually determined by a pair of sex chromosomes. Normally, females have two identical sex chromosomes (XX), which are called homogametes, while males have two different sex chromosomes (XY), which are called heterogametes.
In humans, the presence of a Y chromosome is responsible for triggering male development; if the Y chromosome is missing, the fetus will undergo female development, although there are some exceptions, such as Swyer syndrome, in which patients have XY chromosomes but present as females, or de People with la Chapelle syndrome have XX chromosomes but present as males. However, these exceptions are relatively rare.
In most species with an XY sex determination system, an organism must have at least one X chromosome in order to survive.
There are several differences between the XY system and the ZW sex determination system of birds and some insects, many reptiles, and various other animals, in which females are heterogametic. In some reptiles and fish, there is also a temperature-dependent sex determination system.
Sex Determination Process
All animals have DNA that encodes their genes on their chromosomes. In humans, most mammals, and some other species, two chromosomes, called the X and Y, are responsible for sex determination. In these species, there are one or more genes on the Y chromosome that determine maleness. Specifically, an X chromosome and a Y chromosome work together to determine the sex of offspring, usually due to genes located on the Y chromosome encoding male traits.
Offspring with two X chromosomes (XX) will develop female characteristics, while offspring with one X and one Y chromosome (XY) will develop male characteristics.
XY system in mammals
In most mammals, sex is determined by the presence of the Y chromosome. This means that individuals with XXY and XYY karyotypes are defined as males, while individuals with X and XXX karyotypes are defined as females. In the 1930s, Alfred Jost determined that the presence of testosterone was necessary for the development of Wolffian ducts in male rabbits.
Sex determination in humans
A single gene (SRY) is present on the Y chromosome and acts as a signal to initiate the development process toward maleness. The presence of the SRY gene begins the process of sex differentiation. Female cells have two X chromosomes and undergo X-inactivation, which means that one of the two X chromosomes is inactivated within the cell and the inactivated X chromosome remains in the cell in the form of a Barr body.
Sex determination in other organismsSome species of turtles, particularly dry and wet turtles, have independently evolved an XY sex determination system. Other species, such as most fruit flies, use the presence of two X chromosomes to determine femaleness; one X chromosome can confer male characteristics, but the presence of Y chromosome genes is required for normal male development.
Sex determination in plants
Angiosperms
Although only a very small number of dioecious angiosperms have an XY sex determination mechanism, due to the diversity of angiosperms, the number of species that actually have XY sex determination is quite considerable, estimated to be around 13,000. Molecular and evolutionary studies have also shown that XY sex determination has evolved independently in 175 unique plant families.
Gymnosperms
Compared to angiosperms, about 65% of gymnosperms are dioecious. Families known to have an XY sex determination system include the Cycadaceae, Ginkgoaceae, and Glycyrrhizae.
Other sex determination systems
While XY sex determination is the most well-known system (because it is the system humans employ), many other alternative systems exist in nature. For example, the ZW system is used in birds and many insects, where females are heterogametic (ZW) and males are homogametic (ZZ). Additionally, many Hymenoptera have a haploid-diploid system, with females being completely diploid (having paired chromosomes) and males being haploid (having only one copy of a chromosome).
Genetic factors
In an interview, researcher Eric Vilain describes the paradigm shift since the discovery of the SRY gene. He said that it has long been believed that SRY activates a cascade of male genes, but in fact, the sex determination pathway may be more complicated.
ConclusionThe initial hypothesis after observing the SRY gene was that it acted as a dominant factor in male development, however, it may exist in some balance with some anti-male genes.
Studying the genetic determinants of sex will not only improve our understanding of sex determination, but may also have broad implications for many biological processes, such as reproduction, aging, and disease. As technology advances, will we be able to truly understand the complex mechanisms of sex determination and use this knowledge to improve human health or the reproduction of other organisms?
