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Inheritance and sex


Under normal conditions, any human diploid cell contains 23 pairs of homologous chromosomes, that is, 2n = 46.

Of these chromosomes, 44 are autosomes and 2 are the sex chromosomes also known as heterosomes.

Autosomes and heterosomes

Autosomal chromosomes are those related to characteristics common to both sexes, while sexual chromosomes are responsible for the characteristics of each sex. The formation of somatic organs, such as liver, spleen, stomach and others, is due to genes located in autosomes, since these organs exist in both sexes.

The haploid set of autosomes in a cell is represented by the letter A. On the other hand, the formation of reproductive organs, testes and ovaries, characteristic of each sex, is conditioned by genes located on the sex chromosomes and are generally represented by X and Y. The Y chromosome is male only. The X chromosome exists in double-dose women, while in men it is in single dose.


Electron Microscopy of X and Y chromosome. Compare the size difference of each chromosome.

The sex chromosomes

The Y chromosome is shorter and has fewer genes than the X chromosome, as well as containing a shortened portion in which there are male-only genes. Note in the figure below that a part of the X chromosome has no Y alleles, that is, between the two chromosomes there is a nonhomologous region.

Genetic Determination of Sex

The XY System

In some animal species, including humans, the genetic makeup of males is represented by 2AXY and the gametes they produce, AX and THEY; in the female whose genetic constitution is indicated by 2AXX, only gametes are produced AX.

In man the genetic constitution is represented by 44XY and the gametes he produced, 22X and 22Y; on woman 44XX and the gametes, 22X. Individuals that form only one type of gamete, as for the sex chromosomes, are called homogametics. Those that produce two types are called heterogametics. In humans, females are homogametic, while males are heterogametic.