In many species , the sex of an individual is determined by a chromosomal sex determination. If the same chromosome ( homozygous ; XX) occurs twice in female individuals and two different chromosomes ( hemizygous ; XY) occur in male individuals , these are by definition called X-chromosomes and Y-chromosomes.
The XX / XY system of Geschlechtsdetermination has independently in different groups of animals developed . It occurs in mammals ( Theria , i.e. marsupials and higher mammals (Eutheria) , but not egg-laying mammals ), some insect species and some other animal groups (see sex chromosome ). In some creatures such as birds , however, male individuals have two identical Z chromosomes and females have one W and one Z chromosome.
Theory of the formation of the Y chromosome in mammals
There is no chromosome that is completely homologous to the Y chromosome in the diploid chromosome set. It is only a third the size of the X chromosome and can only recombine with the X chromosome over five percent of its length (in the pseudoautosomal regions close to the telomere ) , although it still has numerous genes of general metabolism that are also found on the X -Chromosome can be found. Many genes on the Y chromosome, however, have no counterpart on the X chromosome. They are mainly genes for sperm production . Based on the knowledge of the base sequence of the human genome by the Human Genome Project is a path of development of this chromosome can be constructed.
The most common theory about the formation of the Y chromosome in mammals is that the two gonosomes X and Y are the result of mutations in a common precursor chromosome (which was diploid analogous to the autosomes ). This contained the genes that are necessary for the formation of both sexes; Exogenous influences such as temperature were responsible for the differentiation between male and female sex . The decisive step was said to have been mutations that introduced genes into one of these chromosomes that can clearly be held responsible for the development of the male sex. At the same time, these mutations must have resulted in the two sex chromosomes differing so much in their sequence that a recombination between them was excluded, so that the new masculinity gene could not be shifted to the unmutated sex chromosome. This “large” mutation is said to have been an inversion on the long arm of the Y chromosome. The result was the SRY gene ( sex determinating region on Y chromosome ), which codes for the testis determinating factor ( TDF ); the inversion prevented the pairing with the previously homologous area on the non-mutated chromosome.
Determining the gender of an individual at random in this way has the effect of the relative equilibrium of the gender distribution, whereas previously populations with a strongly shifted gender ratio were possible.
In the further course of the disease, the Y chromosome repeatedly lost genes that were not associated with the development of the male sex, while on the other hand genes that were important for the fertility of males, for example, gathered more and more on the Y chromosome. However, genes that are homologous to X and Y still exist, which ultimately form the basis of the entire theory of the common precursor chromosome.
With the loss of autosomal genes on the Y chromosome alone, however, there were considerable differences between male and female members of a species in the activity of those genes that were now only present on the X chromosome (women have twice the gene dose and thus theoretically double the gene activity). However, since every woman must be able to pass one of these X chromosomes on to a son again, a solution to the dose problem had to be found that would work equally for both sexes.
On the one hand, many genes of the former common precursor chromosome are found in the theria (i.e. marsupials and higher mammals) on autosomes - male and female individuals each have the same (diploid) gene dose. On the other hand, one of the two X chromosomes in every female cell is inactivated (see Barr bodies ). However, this deactivation apparently does not affect all genes on the X chromosome in question, so that women in some cases have a higher gene activity, which is also popularly popular to explain some gender-specific differences (such as the higher language ability and the more pronounced social behavior of women compared to men ) is used.
Finally, the evolution from the common precursor chromosome to the XY system also has disadvantages for the male individuals of a species. This is because X-linked recessive gene defects, which are usually of little consequence in women due to the accidental inactivation of an X chromosome, cannot be compensated for in the male genotype. An example: A mutation on the X chromosome leads to red-green blindness . As a result of the accidental inactivation of an X chromosome, women have red-green-sensitive and insensitive receptor cells in the retina . Sons of these women ( female carriers ) have a 50 percent risk of inheriting the defective X chromosome from their mother and cannot compensate for this defect. In the case of X-linked recessive diseases , heterozygous mothers always appear as clinically unaffected or only mildly affected carriers.
Changes over time
It is assumed that the Y chromosome was homologous to the X chromosome, i.e. had the same structure and the same genetic locations.
Possibly 350 million years ago, on the longer arm of one X chromosome, the SOX3 gene became the precursor of the sex determining region of Y ( SRY) gene . SRY encodes a signal protein that activates various genes that cause the testes to develop in the embryo . This new gene has probably promoted an individual's ability to develop into a male sex more than was previously possible. In some Sauropsida ("reptiles") the sexual development is influenced by environmental influences, such as the ambient temperature, they do not have an SRY gene. Since the monotremes - and all other mammals - already have this gene, this gene may have originated at the time when the early mammals split off from the reptiles.
320 to 240 million years ago, an inversion occurred in the longer arm of the Y chromosome that affected almost the entire arm. As a result, no more recombinations could take place in this section between the X and Y chromosomes . This led to greater deviations from the homologous gene locations in the X chromosome or even to gene losses ( deletions ). Since the full ability to recombine between the homologous X chromosomes and thus the ability to repair was retained in the female sex, there were no gene losses on the X chromosome.
It is assumed that there were three further inversions 170 to 130, 130 to 80 and 50 to 30 million years ago in the short arm of the Y chromosome, which further restricted the ability to recombine and promoted losses, whereby the Y chromosome was further shortened.
At a time that could not be reconstructed, the SRY gene was moved from the long to the short arm of the Y chromosome.
Some genes of the basic metabolism on the Y chromosome are hardly changed compared to the corresponding gene locations on the X chromosome, even in the inverted sections. If these genes had changed significantly, the survivability of the male gene carriers would have been so reduced that these mutations would have been eradicated by selection.
With the loss of many genes on the Y chromosome, some genes would only have half the activity in the male sex. In the course of evolution, there was a compensation: In order to compensate for the reduced activity due to the lack of second X genes in males, the activity of the corresponding genes on the X chromosome was doubled. However, this would have resulted in excessive gene activity in the female sex. This was compensated by the fact that the genes on one of the two X chromosomes are inactivated in the course of development.
In the fruit fly Drosophila melanogaster , the males double the activity of the genes on the X chromosome that do not have a counterpart on the Y chromosome.
Accumulation of fertility genes
In addition to the loss of genes, an acquisition of the fertility genes in the Y chromosome can also be determined:
- On the one hand, mutations on the Y chromosome created new genes.
- On the other hand, genes from other chromosomes accumulated through translocation on the Y chromosome.
It is not yet clear which evolutionary mechanism led to this accumulation. The fertility genes may have to collect on the Y chromosome, as the females can do without these genes, which are only important for the maturation of the sperm without suffering damage.
Stability of the fertility genes
Actually, the fertility genes that only appear on the Y chromosome should also perish due to the lack of recombination. However, they are present in several copies on one chromosome, which can compensate for individual losses.
Known genes on the Y chromosome
The Y chromosome contains the following genes, among others:
- AMELY amelogenin, Y isoform
- RPS4Y1 40S ribosomal protein S4, Y isoform1
- RPS4Y2 40S ribosomal protein S4, Y isoform2
- AZF1 Azoospermia Factor 1
- BPY2 Testis-specific basic protein Y 2
- DAZ1 Deleted in azoospermia protein 1
- DAZ2 Deleted in azoospermia protein 2
- DDX3Y DEAD-box helicase 3, Y-linked
- PRKY serine / threonine protein kinase PRKY
- RBMY1A1 RNA binding motif protein, Y-linked, family 1, member A1
- SRY sex-determining region of Y
- TSPY Testis-specific Y-encoded protein 1
- USP9Y Ubiquitin specific peptidase 9, Y-linked
- ZFY Zinc finger Y-linked protein
Of the genes on the Y chromosome, two genes are essential in mice, SRY and Eif2s3y . If these two genes are accommodated on other chromosomes, (male) mice without a Y chromosome can be produced.
The Y chromosome is also referred to as the 24th chromosome in bioinformatics if certain databases only allow numbers to be entered.
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- Homo sapiens chromosome Y, GRCh38.p13 Primary Assembly . May 29, 2020 ( nih.gov [accessed July 5, 2020]).
|Evolution tree haplogroups Y-chromosomal DNA (Y-DNA)|
|Adam of the Y chromosome|
|A2'3||A4 = BCDEF|