Sex inheritance

When talking about the chromosomal theory of inheritance, it was already seen that Drosophila, as in other animals, in addition to autosomes have other chromosomes that are different in males and females. They are the sex chromosomes or heterochromosomes, and they are the ones that determine sex.

One of the two sexes has two homologous sex chromosomes, it is the homogamic sex. The other sex, on the other hand, will be determined by two different, non-homologous sex chromosomes, the sex being heterogametic. The heterogametic sex will be the one that depends on whether the descendants are male or female, since it will provide one of the two chromosomes in its gametes. The homogeneous sex, on the other hand, will always provide the same type of chromosome.

In humans, the Y chromosome is much smaller than the X chromosome, but they have a small homologous part that pairs during meiosis.

Genes on the same chromosome are linked, as they tend to be inherited together. Therefore, genes that are located on the non-homologous part of the X and Y chromosomes are linked to sex.

How is sex inherited?

In meiosis, when the gametes are formed, the sex chromosomes separate and only one of them goes in each gamete. Half of sperm have an X chromosome and the other half a Y chromosome. Instead, all eggs have an X chromosome.

In fertilization, the ovum, with its X chromosome, will unite with the sperm, with the X or Y chromosome, and will give rise to a girl (XX) or a boy (XY), respectively .

This type of chromosome determination is the most common, but in nature there are various forms of sex determination:

Chromosome determination: It is the typical shape, the one we have seen. Sex is determined by two sex chromosomes, other than autosomes. One of the sex chromosomes, the X chromosome is larger, telocentric, and the other, the Y chromosome, telocentric is much smaller. In nature there are several types of chromosome determination:
- XY (Lygaeus) determination system: Sex is determined by two chromosomes. In mammals (such as humans) some fish, the homogametic sex is female, which has two homologous chromosomes (XX). Male sex is heterozygous, determined by two different chromosomes (XY).

Sex inheritance		Male
Sex inheritance		X	Y
Female	X	XX	XY
Female	X	XX	XY

- ZW (Protenor) determination system: For example, in birds or butterflies , the male sex is the homogametic (ZZ), and the female, the heterogametic (ZW).
- X0 determination system (Abraxas): Sex is determined by a single chromosome, as in other species (fish, insects, amphibians, etc.), they do not have a Y chromosome, and sex is determined by the number of X chromosomes, being the male X0 and the female XX.
Phenotypic determination: It is the only case in which sex is not determined directly by genetic information, but by environmental factors. For example, in some crocodiles and alligators, depending on the incubation temperature, they will give rise to males if it is higher than 27 ºC and females if it is lower.
Gene determination: They do not have sex chromosomes, and sex is determined by one or more genes. Example, the devil's pickle (Ecballium elaterium).
Karyotypic determination: sex depends on the chromosomal endowment of the individual, as in bees and ants, in which females are diploid, and males are haploid, by parthenogenesis.

Nettie Stevens, the woman who discovered the X and Y chromosomes

Nettie Stevens was born in Cavendish, Vermont, United States, in 1861. She attended a public school graduating first in her class, being one of the first three women to graduate from the school.

After graduating, although she wanted to pursue research, she had to work as a school teacher and librarian in order to save money and enroll at the University at the age of 35.

Later, studying the Tenebrio molitor beetle, he discovered that its female somatic cells had twenty large chromosomes, ten pairs. In contrast, the male's somatic cells had nine pairs of large chromosomes and one pair with a large and a small chromosome. Thus he came to the conclusion that sperm cells with a small chromosome (Y chromosome) determined male sex, while those with 10 large chromosomes (X chromosome) determined female sex.

Therefore, she was the discoverer that sex is determined by the two types of sperm: those that contain the X chromosome and those that contain the Y chromosome.

Edmun Beecher Wilson, a prestigious biologist, was also researching sex determination, and published results that were in agreement with Stevens. On numerous occasions this discovery was attributed to him, even knowing that he had been aware of the results obtained by Stevens, but as he is a more well-known researcher, she has been attributed a more secondary role.