WHAT ARE HETEROCHROMOSOMES - BIOLOGY - 2025

The heterocromosomas are a chromosome pair are constituted by so- called sex chromosomes, which are different from each other, and autosomes. They are also known as allosomes, idiochromosomes, or heterotypic chromosomes. They determine sex in animals, as well as in plants with chromosomal sex determination systems.

When the chromosomes that define the organisms of a species are arranged in terms of shape, size and other morphological characteristics, we obtain their karyotype.

In diploid organisms each chromosome, particularly the somatic or autosomal chromosomes, has a pair of identical characteristics (homochromosome) - although not necessarily identical in sequence.

The individual carrying the two different types of sex chromosomes is called the heterogametic sex of the species: in the case of humans the heterogametic sex is the man (XY; the women are XX), but in birds it is the females (ZW; males are ZZ).

In other cases, as in some insects, the females are XX and the males X (or XO). In the latter case, as seen in Hymenoptera, males are males only because they are haploid individuals.

For this reason, it would be an extreme case of hemizygosity for X, which forces us to consider this X chromosome alien to the concepts of homo- or heterochromosome. In other animals, environmental conditions determine the sex of the individual.

Differences between sex chromosomes

Sex chromosomes are the heterochromosomes par excellence.

In the case of human beings, as in the rest of mammals, the chromosomes present in male individuals are very different from each other. The Y chromosome is much smaller than the X chromosome - in fact, the Y chromosome is only one-third the size of the X chromosome.

Consequently, the content of genes on the Y chromosome is obviously much lower than on its X "pair": the X chromosome has been estimated to carry no less than 1000 different genes, while the Y chromosome is attributed the ability to code for no more than 200 different genes.

Differences between men and women

This little information, however, establishes big differences between men and women: in fact, the Y chromosome is what makes a man so. The X chromosome, by contrast, makes us all viable humans.

In the fertilization process, upon receiving a Y chromosome, the zygote will give rise to a fetus that will develop testes, and therefore, the individual will have all the sexual characteristics that define the male of the species.

In addition to coding for this testicular development factor, the Y chromosome, among the few genes it possesses, codes for factors that determine male fertility, as well as others that could play an important role in the longevity of the individual.

In other words, to be male or female (or simply to exist) we need at least one X chromosome; But to be a man, we also need a Y chromosome that allows us, among other things, to produce sperm.

In addition to the differences indicated, the regions of homology between both sex chromosomes, unlike what happens with any of the autosomal pairs, is very limited - which indicates that they are not, strictly speaking, homologous.

So much so, that on the X chromosome we can still find vestiges of our past brotherhood with Neanderthals, while on the Y chromosome purifying selection events have removed all traces of them.

The regions of "homology" that determine the contacts necessary to carry out an efficient chromosomal segregation process between the X and Y crosmomas during meiosis are limited to very small subtelomeric portions.

Finally, in females, the X chromosomes actively undergo recombination; in males, the few areas of complementarity between the members of the heterochromatic pair determine that there is basically no recombination - at least as we know it in homologous somatic chromosome pairs, or a XX pair.

Consequently, the DNA repair systems on the Y chromosome are much less efficient than on the X chromosome.

XX / XY sex determination system

In individuals with the XX / XY sex determination system, it is the father who chromosomally defines the sex of the descendants. The mother only produces gametes with X chromosomes, in addition to the haploid set of somatic chromosomes, and is called the homogametic sex of the species.

The father (heterogametic sex) can produce gametes with X chromosomes or gametes with Y chromosomes: the probability of giving rise to individuals of one sex or the other, therefore, is the same and will depend on the sex chromosome carried by the sperm as Every ovum to be fertilized carries only one X chromosome.

It is easy to deduce, therefore, that the Y chromosome is inherited patrilineally: that is, it passes only from parents to children. Just as mitochondria are inherited, men and women, matrilineally from a single ancestral female, all males can trace their Y chromosome to a single male ancestor - but much more recent than the first.

Other uses of the term

Also within the scope of the same genetics, those chromosomes that are rich in heterochromatic regions are called heterochromosomes. Heterochromatin (DNA, in addition to its accompanying proteins) is that portion of the hereditary material (DNA only) that is highly compacted and, therefore, is not expressed.

The most striking and curious case of a highly heterochromatic chromosome is the so-called Barr Body. This is just one of the inactivated X chromosomes from female mammals.

In order to compensate for the gene dosage derived from the presence of two X chromosomes instead of one, as in the case of males of the species, in females, in early stages of development, one of the X chromosomes is silenced, hypermethylated and highly compacted.

In other words, the Barr Body is not only a heterochromosome because it is completely heterochromatic, but also because, morphologically speaking, it is completely different from its non-silenced counterpart (at least as long as the cell does not divide).

References

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2. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Pkiladelphia, PA, USA.
3. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 ^th ed.). New York: WH Freeman, New York, NY, USA.
4. Pertea M., Salzberg, SL (2010) Between a chicken and a grape: estimating the number of human genes. Genome Biology 11: 206.
5. Strachan, T., Read, A. (2010). Human Molecular Genetics. Garland Science. p. 45. ISBN 978-1-136-84407-2.