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Biology Genetics Question

  • 30-05-2011 03:11PM
    #1
    biggaman
    Registered Users, Registered Users 2 Posts: 159 ✭✭


    Just running through a few genetic crosses here, and wasn't able to answer one of them.
    Looked around at a few biology websites but wasn't able to find what i was looking for.
    The question refers to determining the parents from the offspring. I find the normal crosses fine, but I don't know how to do use it in reverse?

    2005 Section C Q 10 C)
    (iii) Red-green colour blindness is a sex (X)-linked condition. Normal red-green vision results from the possession of a dominant allele (C). In each of the following cases give the genotypes of the mother and of the father.

    1. A family in which one daughter is red-green colour blind and one daughter has normal colour vision.

    2. A family in which all the sons are red-green colour blind and all the daughters are carriers (heterozygous). (24)

    Sorry I may sound a bit confusing there but I was just wondering if anyone know where theres a video or something that goes through this kind of cross? I've no problem determining offspring from parents but when asked to do it the other way around I'm getting confused!

    Thanks in advance :)


Comments

  • #2
    crayon1
    Registered Users, Registered Users 2 Posts: 62 ✭✭crayon1


    I would start by writing (C) = normal vision (c) = colour blind.

    1. A family in which one daughter is red-green colour blind and one daughter has normal colour vision.

    So one daughter has to be XXcc (colourblind). The other can be XXCc or XXCC (normal vision).
    So taking each the father has to give a (c) to the first daughter. This makes the father XYc-. Then since the second daughter must have a (c) that she got from the father, then she must be XXCc making the mother XXCc also.


    2. A family in which all the sons are red-green colour blind and all the daughters are carriers (heterozygous).

    The daughters must be XXCc as they are carriers. The sons must be XYc- as they are colourblind.
    The son must get a (c) from the mother (because he gets the Y from the father).
    So the mother must be XXCc as the daughters get a (C) from her. Therefore the mother must be XXcc and father XXC-.


    I hope this helped in some way....it's hard to explain. I would draw out the actual chromosomes as it is much easier to see then rather than the letters above. Sorry if i confused you even more! :cool:


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