In nature, there are several systems that determine the sexual characteristics that an individual develops. Most organizations that carry out sexual reproduction They have two sexes, in the same hermaphrodite individual or in separate individuals.
In many species, sex is determined by the individual’s genetic background. Female individuals and male individuals have different genes that determine whether they will develop some sexual characteristics or others.
In other organisms, on the contrary, the development of sexual characteristics is determined by environmental factors, for example, temperature, and still others do not have a fixed sex, such as many flowers and fish that can change sex in different cycles throughout their lives.
Genetic sex determination systems in animals are often accompanied by differences in chromosomes, hence these systems are specifically known as chromosomal sex determination systems.
Human beings follow the XX/XY chromosome system, although there are other different chromosome systems, such as ZW, XO and ZO.
The XX/XY sex determination system is one that follows the human species and most mammals. Also found in some insects. The system is characterized by the fact that females have two sex chromosomes of the same type (XX) and males have two different sex chromosomes (X/Y).
The X and Y chromosomes are different in shape and size, unlike autosomal (non-sex) chromosomes which always go in equal pairs.
Within the XX/XY system there are variations in the exact mechanism of sex determination. One mechanism may be that female sex is the default (due to the absence of Y) and that it is a gene on the Y chromosome that initiates development into a male phenotype.
This is the case for the human species and for many mammals. The SRY gene, from English sex determining regionencode for testis determining factor a protein responsible for the germ cells of the embryo’s genital organs to begin forming the testes and producing testosterone.
Many other genes besides SRY are involved, including genes on the X chromosome, although SRY appears to be decisive and one of the first to trigger the process. For example, the absence of the DAX1 gene on the X chromosome causes sterility in male mice, while in humans it causes congenital adrenal hypoplasia.
XY sex determination can also be reversed and that is controlled by the presence of the X chromosome, for example in the fruit fly. Another rare variation has been observed in some fish that have a second type of Y chromosome, called Y’ (y prime), and can form XY’ females and YY’ males (yy prime).
The XX/X0 chromosome system can be thought of as a type of XY system in which the female is XX and the male is X0 (zero indicates absence of another chromosome). This system can be observed in various types of insects, for example in grasshoppers and crickets.
nematode Caenorhabditis elegans the male is X0 and the individual XX is hermaphrodite.
The ZW sex determination system is typical of birds and some reptiles; also occurs in certain types of insects. This system is contrary to the ZW system; ZW individuals are female and ZZ individuals are male.
UV sex chromosomes were observed in some bryophytes and some species of algae that in the gametophyte phase present individuals with separate sexes, one produces male gametes and the other female gametes. The female gametophyte carries the chromosome known as the U and the male gametophyte carries the V chromosome. The sporophyte stage reproduces asexually.
Haplodiploidy is a sex determination system in which males are sterile and are born from unfertilized eggs, while the union of an egg and a sperm always produces a female. Sex is determined by the number of sets of chromosomes the individual receives; the female is diploid, meanwhile he male is haploid.
Haplodiploidy is observed in hymenopteran insects (bees, wasps, ants), thysanoptera, in some beetles and in some hemiptera. Also seen in the mite family Tetranychidae and in rotifers.