Chromosome XX: Influence of this Genetic Structure in the Determination of the Sex of Organisms

Sex determination refers to the hormonal, environmental, and mainly genetic mechanisms that make an organism male or female.

Chromosomal sex determination

It is widely known that people who inherit the sex chromosome X from the mother and Y from the father are genetically male. In contrast, people who inherit X from both parents are genetically female.

Therefore, the sex of an offspring is entirely determined by which of the male’s sperm (one carrying X or Y) fertilizes the egg (which always carries X).

In reality, it is simply the presence of a Y chromosome that makes a person male and its absence that makes a person female.

Some people inherit an XXY combination through misclassification (meiosis) during sperm and egg production, but they are still male (with Klinefelter syndrome). Others inherit only one X and are called XO; they are genetically female (with Turner syndrome ).

Such people are often, but not permanently, sterile. (The YO condition is fatal because the X contains many essential genes for survival.)

The natural law that XX results in a female and XY results in a male is valid not only in humans but in all mammals. However, the opposite is true in birds and most reptiles: XX individuals are male, and XY individuals are female.


In fruit flies (Drosophila), XX is female, XY is male, but Y is inert, and sex is determined by whether there are two X chromosomes or just one. (Therefore, XO is female in humans but male in Drosophila.)

Not all animals have sex chromosomes. In ants, wasps, and bees (Hymenoptera of the insect order), sex is determined by whether or not the egg is fertilized. If not, it remains haploid (n) and produces a male; if fertilized, it becomes diploid (2n) and has a female.

This is also true of some other invertebrates, such as rotifers. In the lashed-tailed lizards of northern Mexico and the southwestern United States, males are nonexistent.

Each egg remains unfertilized and produces a female; however, females have to simulate copulation with each other to induce egg development.

Determination of hormonal sex

Genes are not enough to make a man or a woman. Producing a human male requires not only the XY chromosome pair but also an adequate level of testosterone exposure during fetal development.

If testosterone or the cellular receptors for it are missing, as in androgen insensitivity syndrome (AIS), an XY human can be born with female genitalia. And misidentified as a girl.

Conversely, if an XX fetus is exposed to excess testosterone (from the adrenal glands), the labia may fuse into a scrotal sac. The clitoris may grow to resemble a penis, and the baby may be misidentified as a kid; This is called adrenogenital syndrome (AGS).

The mistaken identity often comes to light only at puberty, when the individual does not develop as he usually would for the erroneously assumed sex. Such late discovery of the child’s genetic sex creates some complex gender identity issues.

Determination of environmental sex

In some fish and reptiles, the sex is determined by the temperature at which the eggs hatch. In lizards and alligators, warm incubation temperatures cause all the eggs to produce males, while temperatures of just 1 or 2 degrees Celsius (34 or 35 degrees Fahrenheit) make females.

The opposite is true for most turtles. Therefore, a sea turtle can have all its daughters if it lays its eggs on a beach site in full sun, but all its children if it puts them in the shade of the vegetation on the dunes.

Conservationists who rescue sea turtle eggs from predators and incubate them in the laboratory quickly learned that they had to vary the incubation temperature if they produced a mixture of sexes.

The sex of an animal is not always fixed for life. Many fish change sex at some point. In some coral reef fish, a male controls a harem of females, and the females have a dominance hierarchy between them.

If the male dies or disappears, the highest-ranking female becomes a male within a few days. His ovaries return, his testicles develop, and he soon produces sperm and takes control of the harem.