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?

Reply 1

PleaseHelpAsap

This is the pedigree chart

Original post by PleaseHelpAsap

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?

Reply 2

OxFossil

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%