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3.4.3. Sex-linked inheritance

Sex-linked inheritance

As we have seen, the genes whose loci are on the same chromosome tended to be inherited together, they were the linked genes. If these genes are on a sex chromosome, we will talk about sex-linked inheritance.

Because the sex chromosomes X and Y are different, the genes whose loci are on these chromosomes are transmitted differently in males and females.

The X chromosome, in addition to containing genes responsible for sexual characteristics, also contains genes with information for other characteristics. Females (XX) need to be homozygous  recessive to manifest the phenotype of the recessive trait, but if they are heterozygous, even if they do not manifest this trait, they can transmit it to their offspring. The female is then said to be a carrier for that character. Males (XY), on the other hand, may have a segment of the X chromosome without the corresponding homologue of the Y chromosome, as it is smaller. In this case, it will be enough for the recessive character to appear in that segment of the X chromosome for it to appear. The male cannot be only a carrier, as it happened in the females.

This type of inheritance is known as sex-linked inheritance. Two well-known cases are color blindness and hemophilia, which are transmitted in the same way, except that hemophiliac women are rarely born. In both cases it is due to a recessive allele located on the differential segment of the X chromosome.

In the table below you can see the possible genotypes and phenotypes for color blindness and hemophilia.

D X D = normal vision D Y = normal vision H X H = normal H Y = normal
D X d = normal vision, carrier d Y = color blind H X h = normal carrier h Y = hemophiliac
d X d = color blind   h X h = lethal  

Sex chromosomes, in addition to having genes related to sex, contain other genes for somatic (non-sexual) characters, whose manifestation in the phenotype will depend on the sex of the individual because they are located on the X or Y chromosome.

Hemophilia and color blindness, like other alterations, are due to the presence of a recessive allele linked to the X chromosome. Therefore, men can never be heterozygous, since they will have either the dominant or the recessive allele, being healthy or sick people, respectively. Women, on the other hand, may be homozygous  dominant (normal people), heterozygous (normal, but carriers of the recessive allele causing the disease), and homozygous recessive (color blind or, in the case of hemophilia, is lethal in homozygosity, for what hemophiliacs do not exist).

We will represent the allele D, dominant, as XD , and the recessive allele, Xd , which is what determines these characters.

Are you color blind ? You can do some tests to check it.

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Although color blindness and hemophilia are the two best known disorders when it comes to sex-linked inheritance (to the X chromosome), there are more, such as:

Sex-linked inheritance (X-linked genes)
Demining gene Recessive gene
Normal vision colour blindness
Normal clotting Hemophilia
Normal Duchenne muscular dystrophy
Normal Hydrocephalus
Fragile X syndrome Normal mental development

All of this we are seeing about color blindness is a simplification. There are several types of color blindness, and each has its own characteristics. For example, blue-yellow color blindness is transmitted through non-sexual dominant genes, so both men and women can be affected in the same proportion.

Curiosity: Hemophilia in the royal family

Surely you will not even know the name of your great-grandparents, but if your family is "real" you can even know the diseases that your ancestors had. In this genealogical tree you can see the hemophiliacs and carriers of the royal family, up to Juan Carlos I, father of Felipe VI, the current king of Spain.

Hemofilia en la familia Real

By The original uploader was Caro1409 at German Wikipedia (SVG version of File:Erbgang Bluterkrankheit.jpg) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons