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9.7.2. X-linked inheritance

X-linked inheritance

Regarding the genes located on the X chromosome, women (XX) can be homozygous or heterozygous , while men (they only have one X chromosome) are always hemizygous (XY).

Two of the most characteristic examples are color blindness  and hemophilia, where a recessive gene located on the differential segment of the X chromosome causes the disease. For a woman to manifest the disease, she has to be homozygous recessive, but for a man to have it, he only has to have the gene on his only X chromosome.

The color blindness or color blindness is a hereditary visual defect that causes the affected person has trouble telling red from green.

The hemophilia is a recessive hereditary disease that causes problems in the coagulation of blood and healing of wounds.

Let's look at the case of hemophilia:

Hemophilia is determined by a recessive hallele versus the normal h+.

A carrier woman, if she has children with a normal man, can have hemophiliac or normal sons and normal and carrier daughters:

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

These links contain genetic exercises that have come out in EvAU exams from various universities:

Genetics problems solved:  Color blindness genetics problems  (4th ESO).

Genetic problems solved:  Hemophilia genetic problems  (4th ESO).

Fundamental ideas about X-linked inheritance

Male sex chromosomes are XY, while female sex chromosomes are XX.

If the recessive allele is on the X chromosome, in the case of men it will manifest. In the case of women, it will need to be on both chromosomes (homozygous).

If the allele is dominant, it will manifest in both cases.