In humans, there are four blood types: A, B, AB and O. Each parent can give their offspring one of three blood type alleles, IA
(coding for phenotype A), IB (coding for phenotype B), or neither, which is expressed as i (coding for phenotype O). This trait displays codominance. Tina has type O blood and her sister Rosa has type AB blood. What are the blood genotypes their parents must have?
Answer: One parent is IAi and the other parent is IBi
Explanation: As this trait is codominant, the child can inherited IA, IB or i.
Tina has type O, which means she is ii and her sister is AB, so her genotype is IAIB. Now, to have a child with a recessive trait both parents has to carry at least one allele for the recessive, i. Rosa is type AB which means she had to have inherited one allele IA from one of her parents and the other IB from the other parent. Thus, one parent is IAi and the other is IBi.
