In the diploid genome of human beings, there are 46 chromosomes, 44 of which are autosomes and two of which are sex chromosomes. The individual inherits one of these chromosomes from each parent.
The human sex chromosomes are called the X chromosome and Y chromosome. Individuals with two X chromosomes (44 + XX) are female. Individuals with one X chromosome and one Y chromosome (44 + XY) are male. (44 + YY individuals do not exist, since the chromosome Y is exclusively inherited from the father.)
The homologous portion of human sex chromosomes is the part which contains genes with alleles in both the Y and X sex chromosomes. The homologous portions are mostly located in the central part of the sex chromosomes, near the centromere.
The heterologous portion of human chromosomes is the part whose genes do not have corresponding alleles in the other sex chromosome. These genes are mostly located in the peripheral regions of the arms of the Y and X chromosomes.
The individual of the male sex is XY and therefore he forms gametes containing either the X chromosome or the Y chromosome in a 1:1 proportion. The individual of the female sex is XX and therefore only forms gametes containing an X chromosome.
It is not only possible for a woman's X chromosome to come from her father; it is certain. Every woman has an X chromosome from her father, while the other X chromosome comes from her mother.
However, in men, the X chromosome always comes from the mother whereas the Y chromosome from the father.
To analyze the X DNA of a mother (assuming no access to her own genetic material), it makes more sense to study the genetic material of her sons, since all the X chromosomes of males come from the mother, whereas daughters have X chromosomes from the mother and from the father. By researching the genetic material of her sons, it is certain that the studied X chromosome is from the mother.
Besides sex genes, sex chromosomes also contain autosomal genes, which codify several proteins related to nonsexual traits.
The inactivation of the X chromosome is a phenomenon that occurs in women. Since women have two X chromosomes, only one of them remains active and functional and mixed with chromatin while the other remains condensed and inactive.
In the same woman, in some cell lineages, the functional X chromosome is the one from the father; in other cell lineages, the functional chromosome is the X chromosome from the mother. This is the feature of a condition known as mosaicism (related to the X chromosome).
Under a microscope, the inactive X chromosome generally appears as a granule in the periphery of the nucleus. This granule is called the Barr body.
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Diseases caused by an abnormal number of sex chromosomes are called sex aneuploidies.
The main sex aneuploidies are: 44 + XXX, or trisomy X (women whose cells have an additional X chromosome); 44 + XXY, or Klinefelter's syndrome (men whose cells have an extra X chromosome); 44 + XYY, or double Y syndrome (men whose cells have an additional Y chromosome); and 44 + X, Turner’s syndrome (women whose cells lack an X chromosome).
Some animals have a sex determination system different from the XY system.
The X0 system is the sex determination system of many insects. In this system, females are XX and males have only one X chromosome (a condition represented by X0).
In birds, in some fish and in lepidopterae insects (butterflies), sex determination is carried out through the ZW system. In this system, females are ZW and males are ZZ.
In another system, called the haploid-diploid sex determination system, one of the sexes is represented by the fertilized diploid individual and the individual of the opposite sex is formed by parthenogenesis, and is haploid (this occurs in bees and other insects).
X-linked traits are phenotypic traits conditioned by genes located in the non-homologous (heterologous) portions of the X chromosome.
For each gene corresponding to an X-linked trait, women always have two alleles, since they have two X chromosomes. Men only have one allele of genes related to X-linked traits, since they have one X chromosome.
Hemophilia is a disease characterized by impaired blood clotting. People affected by it are more prone to internal and external hemorrhages.
Patients with hemophilia A present an alteration in the gene that codifies factor VIII of blood clotting, a gene located in the non-homologous portion of the X chromosome. Patients with hemophilia B present a defect in the gene that codifies clotting factor IX , a gene also located in the non-homologous region of the X chromosome. Therefore, both diseases are X-linked diseases.
Considering the alleles Xh and X, where Xh represents the allele that conditions hemophilia A, in women, the possible genotypes are XX, XXh and XhXh. In men, the possible genotypes are XY and XhY. Concerning the phenotypes, factor VIII is produced in every individual with at least one non-affected X chromosome. Therefore, XX and XXh women and XY men are normal. Only XhXh women and XhY men have the disease.
There are more hemophilic men than hemophilic women because women need to have two affected X chromosomes to develop the disease, whereas, in men, the disease is present when their single X chromosome is affected.
If the mother is not affected by the disease and is a noncarrier of the gene (she does not have an Xh allele), it is impossible for their sons to be hemophilic since the X chromosome of males always comes from the mother. Hemophilic sons are only possible when the mother is hemophilic (homozygous for the hemophilic gene, a very rare situation) or is a carrier of an affected X chromosome (XXh).
X-linked daltonism, also known as color blindness, is a disease in which the affected individual sees the color red as green or confuses these two colours.
Color blindness is passed down through a recessive X-linked inheritance (it is a gene located on the non-homologous portion of the X chromosome).
Color blindness is more frequent in men. since only their single X chromosome needs to be affected for the disease to manifest in them. In women, it is necessary for both X chromosomes to be affected for the disease to appear.
The disease appears due to a defect in the gene that codifies a retinal pigment sensitive to red.
There are many X-linked diseases, such as hemophilia A, hemophilia B and adrenoleukodystrophy, but known Y-linked diseases are few and are very rare.
Holandric genes are genes located on the non-homologous region of the Y chromosome. Holandric genes condition phenotypes that emerge only in men, since individuals of the female sex do not present genes from the non-homologous portion of the Y chromosome (which are existent only in men) in their X chromosomes.
The gene that conditions hypertrichosis pinnae (hair in the ears), a phenotype passed down from fathers to sons through the Y chromosome, was widely known as a holandric gene. Some research findings, however, contradict this hypothesis. Read about the research here: Molecular evidence for absence of Y-linkage of the Hairy Ears trait (this link was a contribution of Ron, a visitor to Biology Questions and Answers). This discussion is a very good example of how science progresses.
Sex-influenced dominance is the phenomenon in which the manifestation of a phenotype of a gene in heterozygosity depends on the sex of the individual. For example, hereditary baldness is a dominant phenotypical form if the individual is male and is a recessive form if the individual is female.
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