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Genetics - Color Blindness Pedigree

  1. Dec 5, 2010 #1
    1. The problem statement, all variables and given/known data
    Color-blindness in humans is a sex-linked recessive trait. A color-blind man marries a woman who has normal vision and they have four children, one girl and three boys. The man has a sister who sees normally and a brother who is also color-blind, but their father (the paternal grandfather of the four children) sees normally. The woman also has a sister and a brother, both of which see normally even though their father (the four children's maternal grandfather) was color-blind.
    A. Draw a pedigree for this situation showing all four grandparents, the two parents and their siblings, and the four children. Include appropriate symbols for sexes and phenotype along with appropriate genotype notations. if the genotype is not known, then put only the possible genotypes based on the information given.
    B. What is the probability that two out of the three boys will be color-blind>?
    C. What is the probability that all four children will be color-blind?
    D. The family just had a set of identical twin boys (hooray!) What are the chances of both twins being color-blind?

    2. Relevant equations

    3. The attempt at a solution
    I've tried considerably to draw the pedigree, but I cannot get it to accurately correspond with the genetics. Without this, I cannot proceed to the other questions. Will someone please help me?
    1. The problem statement, all variables and given/known data
  2. jcsd
  3. Dec 6, 2010 #2


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    You haven't told us enough of what you've done to amount to an attempt at solution. I know it takes time to put figs. up on this site, but you don't really need them, you can write the genotypes in symbols.

    In some ways X-linked inheritance is simpler than other types.

    For all the males, if you know the phenotype you know the genotype, so fill that in first.

    Look at the grandparents and you can tell something about their genotypes from their offsprings' (additional to what you can say directly from grandfathers' phenotypes). So after that you can deduce what all in the second generation either are or might be.

    Remember the male X chromosome is transmitted to daughters.

    The most important result is the genotype of the parents of Q2, 3 and 4. (I understand they had 3 children + now another 2 new). You need this to calculate the probabilities asked which is slightly tricky maybe, but first you need the genotypes so get those.
    Last edited: Dec 6, 2010
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