Hi Ertre,
For this question, you have two separate Mendelian traits to consider - two different genes. First is the ABO blood type, and second is the Rh factor (+/-) blood type.
For ABO blood type, the A and B alleles are codominant, and O is recessive. We can use allele designations of IA, IB, and i for A, B, and O, respectively.
Start by figuring out the parents' possible genotype(s) so you know what alleles they can pass on.
- O blood type is easy - a recessive phenotype must come from a homozygous recessive genotype, so Maria must be ii.
- For Cameron, he has a dominant phenotype, B. He could be carrying two dominant alleles, IB IB, aka a homozygous genotype. But there's another option - he might have a heterozygous genotype. What would that option be?
Once you have decided that, set up your Punnett square using all possible alleles that Cameron could be carrying along the top, with Maria's i allele on the side. If any of the offspring boxes contain Carina's genotype (ii, since she has O blood type), then Cameron's paternity is plausible for the ABO blood type.
BUT, we have a second data point to consider! You can do a second Punnett square for the second gene/trait, the Rh factor. The positive phenotype is dominant to the negative phenotype. Once again, determine both parents' possible genotype(s) as well as Carina's possible genotype(s). Set up your Punnett square and determine whether Cameron and Maria could provide the alleles necessary for Carina's blood type.
The cross must work out for BOTH traits in order for Cameron to be a plausible father to Carina. In your answer, be sure to show your Punnett squares but also explain whether/how each one supports or disproves Cameron's paternity.
