My question is in the description.

Question

Most people are familiar with the idea that blood types can be either positive or negative. This trait is

determined by complete dominance, positive blood is dominant over negative blood. A positive person is

referred to as rh+ (++ or +-) while a negative person is referred to as rh- (--). Mary has AB++ blood and

marries Stephen who is A- (his mother had O- blood). Complete the work to show what genotypes and

phenotypes their children might have. Yes, this will have to be a Dihybrid square because you are talking

about two traits.

How would I solve this, when Stephen only has “2 parts” for his genotype, won’t he need four? I’m confused please help!

Answer 1

This is a bit tricky. Biology questions in genetics will do this annoying thing where they leave out the "implied" parts of a genotype. Let's break down Stephen's genotype, looking at "A" and the " - " individually:

Stephen has A blood, and his mother has O blood. In the A-B-O blood system, O is the recessive trait, meaning his mother's genotype is OO. This means Stephen's genotype could only be AO. (an aside, the in the ABO system, alleles A and B are actually co-dominant, but since the allele B is not mentioned, we can ignore this. If this last sentence confused you, ignore everything inside this parenthesis!).

So we've figured out that Stephen is AO. Now let's think about the " - ". Being negative ( - ) is also a recessive trait, meaning that Stephen's full Rh genotype is --.

Together, Stephen's complete blood genotype is AO--.

I hope this helps! If you need more assistance with Punnett squares for dihybrid crosses, please contact me :)