SAT Subject Biology Practice Question 398: Answer and Explanation
8. Human ABO blood typing is determined by the proteins present on red blood cells. The gene that codes for these proteins has three alleles: IA, IB, and i. IA codes for protein type A, IB codes for protein type B, and i codes for the absence of protein. IA and IB are codominant and i is recessive to both IA and IB. A woman with blood type A and a man with blood type AB marry and produce children. Which of the following blood types is NOT possible in their children?
A. Type A
B. Type B
C. Type O
D. Type AB
E. All blood types are possible in their children.
Correct Answer: C
C Remember your LEAST/EXCEPT/NOT technique. You should be familiar with the rules for blood typing in humans, but if you aren't, this question gives you the basics. Because the alleles IA and IB are codominant, a man with blood type AB can only have the genotype IAIB. The woman with blood type A could have either IAIA or IAi as her genotype. Here are the Punnett squares:
This is the Punnett square that would result if the genotype of the type A woman was IA IA. Offspring would have either type A or type AB blood.
This is the Punnett square that would result if the genotype of the type A woman was IA i. Offspring would have type A, type B, or type AB blood.
Because type O blood is the recessive phenotype, it requires the homozygous recessive genotype (in this case, ii) in order to be displayed. This is the only blood type not possible in their children. A quick way to solve this question is to recognize that type O requires the homozygous recessive genotype, and for that to happen, each parent must donate a recessive i allele. Because the type AB man does not carry the recessive allele, this is not possible.