Some characters only appear in one of the sexes or, if they appear in both, it is much more frequent in one than in the other. These traits are neither primary nor secondary sexual traits, but they are sex-linked traits due to their tendency to manifest in one of them. For example, color blindness or hemophilia , which are almost exclusive in men and very rare in women.
This is explained because the X chromosome is larger than the Y chromosome, so they are not totally homologous. Its genetic content is different. Have:
- Homologous or pairing segments, where the genes with information for the same characters are. Its transmission follows Mendel's laws.
- Non-homologous or differential segments, in which the genes of the chromosomes are located:
- Exclusive to the Y chromosome: hollandric characters, genes that are transmitted exclusively through males.
- Exclusive to the X chromosome: gynandric characters.
Homogenous women, XX, have two alleles for the genes of the differential segments, so they are transmitted according to Mendel's laws.
In men, heterogametic, XY, the genes found in the non-homologous segments or in the segment of X that does not appear in Y will always manifest, even if they are recessive, due to the absence of a complementary locus .
In women (XX), for these recessive alleles to manifest, they must be found on both X chromosomes, which is why it is quite difficult. In contrast, in males, these recessive alleles manifest whether they are on the X chromosome or the Y chromosome.
For problems of inheritance linked to sex we will use this nomenclature:
- In the female genotype, the two alleles of the gene are represented as superscripts or subscripts of the X chromosome, such as XAXA, XAXa or XaXa.
- In the male genotype, the only allele X is represented as subscript or superscript of the single X chromosome having, in addition to the other chromosome Y. For example, XAY or XaY.
- Y-linked inheritance :
- In the female genotype there is no Y chromosome.
- In the male genotype, the single allele is represented as a subscript or superscript of the Y chromosome. Possible genotypes are XYA, XYa.
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