Biology Forum Genetics Daltonism exercice

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    • #15410

      Hello everybody, well I have problems with this exercice and It goes like this:

      In humans, the ability to see colors depends on three gens (Red, Green, Blue). Gens R and G are on the same sexual chromosome X while B is autosomal. The lost of any of this gens result in Daltonism (result of an homozygous genotype or hemizygous recessive). A daltonic man marries a non daltonic woman and they have 4 girls and 4 boys. All the boys are sick while none of the girls are. What are the most probable genotypes for the parents and all the possible genotypes for the children? (r,g,b, are the mutant recessive forms of the gens)

      So far I have come up with this:
      1)As the all the boys are sick but none of the girls it must be sex related. As we are not involving Y on the problem it must be X recessive related. (but if this is X-recessive related none of the offspring of an affected male should show the phenotype, isnt it?)
      2) As the mother hersefl is not sick she must be heterozygous carrier on the X chromosome (B/b)

      I thought it was autosomal recessive disroder, but if it is, why only the boys are affected? My main roblem is understandig how can I analyse one autsomal gen and one sex gen at the same time.

      Thank you for your help.

    • #106463

      You just need to find which combination would produce this effect.

      1. ALL boys are affected.
      Therefore, BOTH of mother’s "X" chromosomes carry responsible allele.
      2. Mother is NOT affected.
      Therefore, she is heterozygous for "X". There is only one way to reconcile this with the first fact:
      Mother is Gr/gR, thus, heterozygous for both sex linked genes AND carrying a recessive allele on each of two "X" chromosomes.
      3. NONE of the girls are affected.
      Considering mother’s genotype, father’s condition CANNOT be sex linked. Therefore, he is bbGGRR. By the same token, mother CANNOT have a recessive b allele, so she is BBGgrR.

      Lastly, G and R must be linked closely together to allow little or no recombination…

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