Usually, cases of sex-related inheritance are classified according to the position occupied by genes in the sex chromosomes. To do so, let's divide them into regions:
THE homologous portion of the X chromosome has genes that correspond to the genes of the homologous portion of the Y chromosome. Therefore, there are allele genes between X and Y in these regions. The genes of the heterologous portion of the X chromosome do not match the genes of the heterologous portion of the Y chromosome. Therefore, there are no allele genes in these regions when an X chromosome pairs with a Y chromosome.
Inheritance linked to sex is that determined by genes located in the heterologous region of the X chromosome. Because women have two X chromosomes, they have two of these regions. Since men, as they have only one X chromosome (since they are XY), have only one of each gene. A recessive gene present on a man's X-chromosome will manifest itself as there is no dominant allele that prevents its expression.
In the human species. The main examples of inheritance linked to sex are as follows.
It is the relative inability to distinguish certain colors, which in their classic form often creates confusion between green and red.
It is a disorder caused by a recessive gene located in the heterologous portion of the X chromosome, the gene. Xdwhile its dominant allele XD determines normal vision.
The genotype woman XDXdAlthough it has a gene for color blindness, it does not manifest the disease because it is a recessive gene. She is called carrier from gene to color blindness. The genotype man XdYdespite having the gene Xd in a single dose, it manifests the disease by the absence of the dominant allele capable of preventing recessive gene expression.
|XDXd||normal woman carrier|
|XD Y||normal man|
|Xd Y||colorblind man|
The man XdY it's neither homozygous or heterozygous: it's recessive hemizigotobecause of the pair of genes it has only one. The genotype man XDY é dominant hemizigoto.
If you can perfectly distinguish the number 74 between the balls in the picture above, then you are not colorblind.