What are the genotypes?

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What are the genotypes? Empty What are the genotypes?

Post by wannabnurse on Fri Apr 24, 2009 11:08 pm

Can someone help, i am soo confused....

In humans, color blindness is due to a X-linked rcessive while blue eyes are due to an autosomal recessive. Two brown-eyed persons with normal vision produce a blue-eyed color-blind son. What are the genotypes of the parents?



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What are the genotypes? Empty Re: What are the genotypes?

Post by Alla on Sat Apr 25, 2009 8:57 pm

Letís say that gene A is responsible for eye color, then A is brown and a is blue. Since son has blue eyes Ė recessive phenotype (genotype aa) Ė while both parents have brown eyes, both parents have to be heterozygous for it (Aa).

Letís say that B is color blindness gene, then B is normal vision and b is color blind. Since this gene is X-linked, only females can be heterozygous for it (genotype Bb) while all males will either be normal (genotype BY) or color blind (genotype bY). Considering that father has normal vision, his genotype has to be BY. Son is color blind, so he is bY. This means that son had to inherit color blindness from his mother who has normal vision and, therefore, has to be heterozygous for B (genotype Bb).

Now put everything together and you get:

Mother: AaBb x AaBY: Father (both brown eyes, normal vision)
Son: aabY (blue eyes, color blind).


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