Genetics Problem - 12.5?
In the pedigree shown below, the female, labelled I-2, is a carrier for colour blindness, however
neither male (I-1 or II-1) is colour blind.
Why is the answer 12.5?
neither male (I-1 or II-1) is colour blind.
Why is the answer 12.5?
Reply 1
Original post
by PleaseHelpAsapIn the pedigree shown below, the female, labelled I-2, is a carrier for colour blindness, however
neither male (I-1 or II-1) is colour blind.
Why is the answer 12.5?
neither male (I-1 or II-1) is colour blind.
Why is the answer 12.5?
Reply 2
19
You need to know that the gene for colour-blindness is carried on the X chromosome. The question says that the female I-2 is a "carrier", so she is heterozygous for this [recessive] trait.
That means there is a 50% chance that any offspring of hers will inherit this gene on the X chromosome they get from her.
Since the I-1 male is not colour-blind, he cannot carry the colour-blindness gene and his offspring have zero chance of inheriting it from him.
So the II generation have a 50% chance of being carriers if they are female and the same 50% chance of being colour-blind if they are male.
The question says that the II offspring are mated with a male who does not carry the colour-blind gene, so the III generation will have the same 50% chance of inheriting the gene if their mother is a carrier, and a zero % chance if their mother is not a carrier.
This gives an overall chance of inheriting the colour-blind gene in the III generation as 25%.
However, the question asks for the probability that the III offspring will be colour-blind (not just a carrier). If there is a 50:50 chance of the III-1 offspring being male, the overall chance of III-1 being colour-blind is half of 25% = 12.5%
That means there is a 50% chance that any offspring of hers will inherit this gene on the X chromosome they get from her.
Since the I-1 male is not colour-blind, he cannot carry the colour-blindness gene and his offspring have zero chance of inheriting it from him.
So the II generation have a 50% chance of being carriers if they are female and the same 50% chance of being colour-blind if they are male.
The question says that the II offspring are mated with a male who does not carry the colour-blind gene, so the III generation will have the same 50% chance of inheriting the gene if their mother is a carrier, and a zero % chance if their mother is not a carrier.
This gives an overall chance of inheriting the colour-blind gene in the III generation as 25%.
However, the question asks for the probability that the III offspring will be colour-blind (not just a carrier). If there is a 50:50 chance of the III-1 offspring being male, the overall chance of III-1 being colour-blind is half of 25% = 12.5%
(edited 8 years ago)
Reply 3
I don't get it.. doesn't the 25% already involves the probability of being male? Why divide by 2 again?
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