In details

Dose compensation mechanism


In 1949, English researcher Murray Barr discovered that there is a difference between the interphase nuclei of male and female cells: on the periphery of female mammalian cell nuclei there is a mass of chromatin that does not exist in male cells.

This chromatin makes it possible to identify the individual's sex by simply examining the interphase nuclei: it is called sexual chromatin or Barr's corpuscle.

From the 1960s, evidence allowed the English researcher Mary Lyon to hypothesize that each Barr corpuscle lines up an X chromosome that spirals and becomes inactive in the interphasic cell, so that this corpuscle blushes more intensely. that all the other chromosomes, which are active and in the unbound form of chromatin strands.

In Lyon's hypothesis, inactivation strikes at random either of the woman's two X chromosomes, whether from the sperm or the parent's egg. Some authors believe that inactivating a woman's X-chromosome would be a way to match the amount of genes in both sexes. This mechanism is called dose compensation. Since inactivation occurs at random and in a phase of development in which the number of cells is relatively small, half of a woman's cells can be expected to have activated X of paternal origin, while the other half has X of parental origin. maternal origin at work. Therefore, it is said that women are “mosaics” because - as for sex chromosomes, they have two types of cells.


Compare the presence of Barr's corpuscle in male cells (above) with female cells (below).

The determination of nuclear sex (presence of Barr's corpuscle) has been used in Olympic games, when there are doubts about the gender of the individual.

The X0 system

In some species, especially insects, the male does not have the Y chromosome, only the X; the female still carries the sexual chromosome pair X. By the absence of the sexual chromosome Y, we call this system system X0. Females are represented by 2A + XX (homogametics) and males 2A + X0 (heterogametics).

The ZW System

In many birds (including our well-known roosters and chickens), butterflies and some fish, the chromosomal composition of sex is opposite to the one we just studied: homogametic sex is male, while females are heterogametic. Also the symbology used in this case, not to cause confusion with the XY system, is different: the male sex chromosomes are represented by ZZ, while in females the sex chromosomes are represented by ZW.

Bees and Parthenogenesis

In bees, sexual determination differs markedly from what has been studied so far. In these insects, sex does not depend on the presence of sex chromosomes, but on ploidy. Thus, males (drones) are always haploid, while females are diploid. The queen is the only fertile female in the hive, and by meiosis produces hundreds of eggs, many of which will be fertilized. Fertilized eggs give rise to zygotes that develop in females.

If in the larval phase these females receive special nourishment, they will become new queens. Otherwise, they will develop into workers who are sterile.

Unfertilized eggs develop by mitosis in haploid males. This process is called parthenogenesis (from Greek, partheno = virgin, genesis = origin), ie, is considered a process of development of unfertilized eggs in adult haploid individuals.