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.
Let's look at the case of hemophilia:
Hemophilia is determined by a recessive h- allele 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:
XligadoRecesivo_v1.1.jpg (Imagen JPEG, 307 × 396 píxeles) - Escalado (0 %). (s. f.). Recuperado 9 de enero de 2017, a partir de https://upload.wikimedia.org/wikipedia/commons/a/a7/XligadoRecesivo_v1.1.jpg
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.
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