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Thread: Pedigree X-Linked

  1. #1 Pedigree X-Linked 
    Forum Freshman
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    I can't work out this pedigree for a X-Linked abnormality.


    pedigr3.gif


    I think it would have to be an X-linked dominant abnormaility because the daughter is unaffected.

    The father must have an affected X recessive and the mother an affected domainant X and an unaffected recessive X and so the daughter picks up a unaffected X recessive from the mother and an affected X recessive from the father?




    pedigr32.gif


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  3. #2  
    Forum Professor Zwirko's Avatar
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    The information you provide is a little confusing. Are you trying to draw a pedigree for an X-linked trait and show the genotypes, or are you trying to figure out the genotypes that explain the first pedigree shown? Or something else?

    It can't be an X-linked dominant trait, because all the female offspring of a father with the trait should be affected. Take the cross you suggest: XAXa x XAY, where A is a dominant X-linked trait (note that I've had to change the genotype of the father since your diagram shows he is affected):

    XAXa x XAY = XAXA, XAXa, XAY, XAY

    As can be seen, all female offspring get the A allele and express the trait, yet the pedigree shows and unaffected daughter. That's not possible for the type of cross suggested.

    For an X-linked recessive trait "r" with both parents affected, then all offspring will be affected: XrXr x XrY = XrXr, XrXr, XrY, XrY.

    For an X-linked dominant trait "R" with a heterozygous mother and affected father, then all daughters and half the sons will be affected: XRX x XRY = XRXR, XXR, XRY, XY

    For an X-linked dominant trait "R" with a normal mother and affected father, then all daughters are affected: XX x XRY = XXR, XXR, XY, XY.

    For an X-linked dominant trait "R" with a homozygous mother and affected father, then all offspring will be affected: XRXR x XRY = XRXR, XRXR, XRY, XRY.


    Whatever the question is, the first pedigree doesn't seem to be an example of a sex-linked trait.


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