Explanation: It was a recessive inheritance pattern. You know that both parents must be carriers as they have an affected child. So the parents are Aa and Aa. From a Punnett square, you would get 1/4 AA, 2/4 Aa and 1/4 aa. However, you know that the person is not aa, as they don’t have the condition. Therefore they can only be AA or Aa. From the likelihoods, you can see that they are twice as likely to be Aa as AA. Therefore they have a 2/3 chance of being Aa = 67%
The next question deals with co-dominance inheritance and the ABO blood groups. The pedigree below shows the known phenotype (i.e. blood group) of the people.
What is the blood group of the person indicated by the arrow?