Men differ from women because they have a chromosome X and another Y (unlike the X, the Y has few genes and determines few characteristics), given that women have only X chromosomes. The X and Y chromosomes have few homologous regions, which means that certain characteristics are completely influenced by sex.
is called sex-linked inheritance or X-linked inheritance one in which the genes are located in a non-homologous region of the X chromosome. In this type of inheritance, females have three genotypes, and males have only two.
Imagine, for example, a gene with two alleles (B and b) that are located in the non-homologous region of the X chromosome. A female could have X genotypesBXB, XBXB or XBXB, while the male would only present the X genotypesBY or XBY. As males have only one X chromosome and have only one allele, we say they are hemizygous.
→ Hemophilia
THE hemophilia is one of the prime examples when it comes to sex-related illnesses. This disease, which is recessive and X-linked, compromises the clotting system, which makes small wounds big problems due to excessive blood loss.
The types of hemophilia differ according to which clotting factor is affected. Type A, which is one of the most frequent, is related to a deficiency in factor VIII. Hemophilia is manifested when a man has a recessive gene related to the production of factor VIII. On the other hand, women, when they have a recessive gene, are considered carriers, and when they have two genes, they are hemophiliacs. Calling the H and h alleles, we would have:
Haemophiliacs: XHY, XHXH
Carrier: XHXH
Normal: XHY, XHXH
Women with the disease can pass the gene with problems to their daughters and sons, since the gene linked to the disease is located on the X chromosome and the woman always donates the X chromosome to her heirs. The man, in turn, can pass the defective gene only to his daughter, as he only gives the daughter the X chromosome. As man has XY chromosomes, at the time of fertilization, to generate a boy, he will be responsible for supplying the Y chromosome, which does not have the characteristics linked to the X chromosome.
→ color blindness
O color blindness it is a disease that directly affects color perception. In one of its types, the disease manifests itself in the inability to differentiate red from green. As with hemophilia, it is an x-linked recessive disease.
Considering the D and d alleles to determine color blindness, we have:
Colorblind: XdY, XdXd
Carrier: XDXd
Normal: XDY, XDXD
→ Important conclusions
- Men only inherit genes from their mother's X chromosome;
- Women inherit genes from the X chromosome from the father or mother;
- In cases where the trait is determined by a recessive gene, men are more affected, as they only need one gene to develop the disease.
By Ma. Vanessa dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/biologia/heranca-do-sexo.htm