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| Sex Chromosomes and
Sex-linked Inheritance Study and Learn Here Sex Determination and Sex-linked Inheritance 1. How is the genetic determination of sex established in humans? In the diploid genome of human beings there are 46 chromosomes, 44 of them are autosomes and two are sex chromosomes. The individual inherits one of these chromosomes from each parent. The human sex chromosomes are called X chromosome and Y chromosome. Individuals having two X chromosomes (44 + XX) are female. Individuals having one X chromosome and one Y chromosome (44 + XY) are male. (Individuals 44 + YY do not exist since the chromosome Y is exclusively from paternal lineage.) Image Diversity: human karyotype sex chromosomes 2. What are the homologous and the heterologous portions of the human sex chromosomes? Homologous portion is that in which there are genes having alleles in both Y and X sex chromosomes. The homologous portions are situated more in the central part of the sex chromosomes, near the centromere. Heterologous portion is that whose genes do not have correspondent alleles in the other sex chromosome. These genes are located more in the peripheral regions of the arms of the Y and X chromosomes. 3. Concerning the sex chromosomes of the XY system which type of gamete do the male and the female individuals respectively produce? The individual of the male sex is XY so 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 thus she forms only gametes containing an X chromosome. 4. Is it possible that an X chromosome of a woman can have come from her father? It is not only possible that an X chromosome of a woman is from her father, it is certain. Every woman has an X chromosome from her father and the other X chromosome from her mother. In men however the X chromosome comes always from his mother and the Y chromosome is always from his father. 5. Is it more indicated for a geneticist desiring to map the X chromosome of the mother of a given family (the researcher does not have access to her DNA, only access to the genetic material of the offspring) to analyze the chromosomes of her daughters or of her sons? To analyze the X DNA of a mother (assuming no access to her own material) it is more indicated to study the genetic material of her sons since all X chromosomes of males come from the mother while the daughters have X chromosomes from the mother and from the father. By researching the material of the sons it is ensured that the studied X chromosome is from the mother. 6. Do the genes of the X and Y chromosomes determine only sex characteristics? Besides sex genes the sex chromosomes have also autosomal genes, genes that codify several proteins related to nonsexual traits. 7. What are the main diseases caused by errors of the number of sex chromosomes in the cells of an individual? Diseases caused by 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); 44 + X, Turner’s syndrome (women whose cells lack an X chromosome). 8. What is the inactivation of the X chromosome? What is a Barr body? 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 mixed to the 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 and in other cell lineages the functional chromosome is the X from the mother characterizing a condition known as mosaicism (related to the X chromosome). Under the microscope the inactive X chromosome is seen as a granule generally in the periphery of the nucleus. This granule is called the Barr body. Image Diversity: Barr body 9. Besides the XY system are there other sex determination systems? 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 the females are XX and the males have only one X chromosome (a conditioned represented by X0). In birds, in some fishes and in lepidopterae (butterflies) insects the sex determination is made by the ZW system; in this system females are ZW and males are ZZ. In another system, 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, being haploid (it occurs in bees and other insects). 10. What are X-linked traits? X-linked traits are phenotypical traits conditioned by genes located in the nonhomologous (heterologous) portions of the X chromosome. 11. How many alleles of genes that condition X-linked traits do female and male individuals respectively present? For each correspondent gene to an X-linked trait women present always two alleles since they have two X chromosomes. Men present only one allele of genes related to X-linked traits since they have one X chromosome. 12. What is the clinical deficiency presented by hemophilic people? What is the genetic cause of that deficiency? Hemophilia is a disease characterized by impaired blood clotting and the affected person is more prone to internal and external hemorrhages. Patients with hemophilia A have alteration in the gene that codifies the factor VIII of blood clotting, a gene located in the nonhomologous portion of the X chromosome. Patients with hemophilia B present a defect of the gene that codifies the factor IX of clotting, a gene also located in the nonhomologous region of the X chromosome. Thus both diseases are X-linked diseases. Image Diversity: hemophilia family tree 13. What are all possibilities of genotypes and phenotypes formed in the combination of alleles responsible for the production of factor VIII? 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 nonaffected X chromosome. So the women XX and X Xh and the men XY are normal. Only women XhXh and men XhY have the disease. 14. Why is it rare to find hemophilic women? There are more hemophilic men than hemophilic women because women need to have two X chromosomes affected to develop the disease while in men the disease manifests when the single X chromosome is affected. 15. Is it possible for any son of a couple formed by a hemophilic man (XhY) and a nonhemophilic noncarrier (XX) woman to be hemophilic? If mothers are not affected by the disease and noncarriers of the gene (do 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 carriers of an affected X chromosome (XXh). 16. What is the clinical manifestation of the disease known as daltonism? The X-linked daltonism is a disease in which the affected individual sees the red color as green or confounds these two colours. 17. What is the type of genetic inheritance of daltonism? Is daltonism more frequent in men or in women? What is the physiological explanation for the daltonism? Daltonism is a recessive X-linked inheritance (gene situated in the nonhomologous portion of the X chromosome). Daltonism is more frequent in men since in them only the single X chromosome needs to be affected for the disease to manifest. In women it is necessary for both X chromosomes to be affected for the disease to come out. The disease appears due to a defect in the gene that codifies a retinal pigment sensitive to red. 18. Are sex-linked diseases associated only to genes of the X chromosome? There are many X-linked diseases, like hemophilia A, hemophilia B and adrenoleukodystrophy, but known Y-linked diseases are few and very rare. 19. What are holandric genes? Holandric genes are genes situated in the nonhomologous region of the Y chromosome. Holandric genes condition phenotypes that emerge only in men since individuals of the female sex do not present in their X chromosomes genes from the nonhomologous portion of the Y chromosome (existent only in men). A widely known holandric gene is the one that conditions hypertrichosis pinnae (hair in the ears), a phenotype inherited from fathers to sons through the Y chromosome. 20. What is sex-influenced dominance? 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 it is a recessive form if the individual is female. |