Very interesting problem!
On first reading, it sounded impossible. Tom has normal vision, and there are no male carriers for x-linked recessive disorders. So the daughter can at best (or worst) be carrier of the disorder.
We are told that the daughter has the disorder, but the daughter can only be a carrier with genotype XBXb.
We are also told that Turner Syndrome, which is a disorder related to the x-chromosome. Half of those affected have one of the x-chromosomes missing (monosomy), and some others have some cells with missing or deformed x-chromosomes (mosaicism). Under these circumstances where the normal chromosome is missing, the X-linked recessive Xb allele will be expressed, hence even a carrier can express the red-green colour-blindness.
From the pedigree chart, we can deduce that the colour-blindness must be inherited from Jill, the mother. Tom with normal vision cannot be a carrier because X-linked recessive disorders do not have male carriers.
The daughter's colour-blindness is derived from two sources,
1. inherited Xb allele from mother Jill
2. Turner syndrome that allowed the single allele to express colour-blindness.
On first reading, it sounded impossible. Tom has normal vision, and there are no male carriers for x-linked recessive disorders. So the daughter can at best (or worst) be carrier of the disorder.
We are told that the daughter has the disorder, but the daughter can only be a carrier with genotype XBXb.
We are also told that Turner Syndrome, which is a disorder related to the x-chromosome. Half of those affected have one of the x-chromosomes missing (monosomy), and some others have some cells with missing or deformed x-chromosomes (mosaicism). Under these circumstances where the normal chromosome is missing, the X-linked recessive Xb allele will be expressed, hence even a carrier can express the red-green colour-blindness.
From the pedigree chart, we can deduce that the colour-blindness must be inherited from Jill, the mother. Tom with normal vision cannot be a carrier because X-linked recessive disorders do not have male carriers.
The daughter's colour-blindness is derived from two sources,
1. inherited Xb allele from mother Jill
2. Turner syndrome that allowed the single allele to express colour-blindness.