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4.6.2: Pedigrees for X-linked genes

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  • X-linked dominant (XD)

    In X-linked dominant inheritance, the gene responsible for the disease is located on the X-chromosome, and the allele that causes the disease is dominant to the normal allele in females. Because females have twice as many X-chromosomes as males, females tend to be more frequently affected than males in the population. However, not all pedigrees provide sufficient information to distinguish XD and AD. One definitive indication that a trait is inherited as AD, and not XD, is that an affected father passes the disease to a son; this type of transmission is not possible with XD, since males inherit their X chromosome from their mothers.

    Figure \(\PageIndex{1}\)\: Two pedigrees consistent with XD inheritance. (Original-Deyholos_CC:AN)
    Figure \(\PageIndex{2}\)\: Some types of rickets may follow an XD mode of inheritance.

    X-linked recessive (XR)

    Because males have only one X-chromosome, any male that inherits an X-linked recessive disease allele will be affected by it (assuming complete penetrance). Therefore, in XR modes of inheritance, males tend to be affected more frequently than females in a population. This is in contrast to AR and AD, where both sexes tend to be affected equally, and XD, in which females are affected more frequently. Note, however, in the small sample sizes typical of human families, it is usually not possible to accurately determine whether one sex is affected more frequently than others. On the other hand, one feature of a pedigree that can be used to definitively establish that an inheritance pattern is not XR is the presence of an affected daughter from unaffected parents; because she would have had to inherit one X-chromosome from her father, he would also have been affected in XR.

    Figure \(\PageIndex{3}\)\: Some forms of colour blindness are inherited as XR-traits. Colour blindness is diagnosed using tests such as this.
    Ishihara Test. (Wikipedia-unknown-PD)

    Figure \(\PageIndex{4}\)\: A pedigree consistent with XR inheritance. (Original-Deyholos-CC:AN)

    Y-linked Inheritance

    Only males are affected in mammalian Y-linked inheritance. There is only father to son transmission. This is the easiest mode of inheritance to identify, but it is one of the rarest because there are so few genes located on the Y-chromosome. In 1960, the hairy-ear-rim phenotype seen in some Indian families was described as a Y-linked trait (Gates, RR based on 20 pedigrees showing father to son transmission. In addition, the daughters of men with the hairy-ear-rim phenotype were unable to pass on the trait. However, questions remained about the accuracy of this conclusion and in 2004 a genotyping study found a lack of molecular evidence for Y linkage of this trait (Lee et al

    Y-chromosome DNA polymorphisms can be used to follow the male lineage in large families or through ancient ancestral lineages. For example, the Y-chromosome of Mongolian ruler Genghis Khan (1162-1227 CE), and his male relatives, accounts for ~8% of the Y-chromosome lineage of men in Asia (0.5% world wide).

    Thinking about Y linkage

    Aside from the size and small gene number on the Y chromosome, why are Y linked traits rare?

    What functions do you think genes on the Y chromosome perform?

    Mitochondrial Inheritance

    Mutations in Mitochondrial DNA are inherited through the maternal line (from your mother). There are some human diseases associated with mutations in mitochondria genes. These mutations can affect both males and females, but males cannot pass them on as the mitochondria are inherited via the egg, not the sperm. Mitochondrial DNA polymorphisms are also used to investigate evolutionary lineages, both ancient and recent. Because of the relative similarity of sequence mtDNA is also used in species identification in ecology studies.