Question

Colorblindness is a recessive X-linked disease. Determine the possible genotypes for all the individuals represented in the pedigree.

Answer 1

With pedigrees it’s usually easiest to start with what you know and work backwards and/or forwards.

In this case we’ll use X (wild-type), x (colourblind), and Y as the possible genes for each person. Here, it’s easy to start with affected individuals because we know all their X-chromosomes must be “x”. For men, we also know one of the two genes they pass on must be Y. Therefore, affected women are “xx”, affected men are “xY”, and unaffected men are “XY”. We can start with this:

Kate - ?? // Joe - XY

Curtis - xY // Anne - ?? // Max - xY // Natalie - ??

Devon - ?? // Ken - XY // Kimberly - ?? // Ryan - XY // Donna - xx

We have all the information for Ryan and Donna so easiest to start there, working backwards. The only way for Ryan to have an “X” and Donna to have an “x” is for Natalie to be “Xx”.

Continuing to work backwards along that path, we know Max is “xY”. He could only have gotten his “x” from his mother since his father is unaffected, but his mother is also unaffected. This means she (Kate) must be “Xx”. Now we have:


Kate - Xx // Joe - XY

Curtis - xY // Anne - ?? // Max - xY // Natalie - Xx

Devon - ?? // Ken - XY // Kimberly - ?? // Ryan - XY // Donna - xx

Now we can try working forwards. Anne definitely has “X” from her father Joe, but could have gotten either X allele from her mother Kate. Therefore her genotype could be “XX” or “Xx”, so we need to keep this in mind when considering the possible genotypes of her kids. Unfortunately we can’t narrow down her genotype further since her kids are all unaffected.

Both Devon and Kimberly must have gotten “x” from their affected father Curtis, but since they’re unaffected they must have gotten “X” from their mother. Both their genotypes can only be “Xx”. Again, this doesn’t narrow down Anne’s genotype - she could be “XX” meaning she can only pass on “X”, but she could also be “Xx” and by luck have only passed on “X” to all her children. Anyways, we’re done!

Kate - Xx // Joe - XY

Curtis - xY // Anne - XX or Xx // Max - xY // Natalie - Xx

Devon - Xx // Ken - XY // Kimberly - Xx // Ryan - XY // Donna - xx

So you can see that even though we aren’t given any information initially about genotypes, through logic and rules of Mendelian genetics we can figure out just about everyone’s genotype pretty easily!