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.
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.
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.