Question

mary has type a blood. her son bill has type ab blood. which blood type does bill’s dad have if he is homogenous for blood type?
○ a
○ b
○ ab
○ o

Explanation:

Step1: Recall Blood Type Genetics

Blood type is determined by alleles. Type A can be \( AA \) or \( AO \), type B \( BB \) or \( BO \), type AB \( AB \), type O \( OO \). Mary has type A (so her alleles: \( AA \) or \( AO \)). Bill has AB, so he got A from Mary and B from dad.

Step2: Analyze Dad's Homogenous Condition

Dad is homogenous (homozygous) for blood type. To give a B allele (since Bill has AB, and Mary gave A), dad must be homozygous for B (\( BB \)) or AB? Wait, no—wait, Bill's AB means he has A (from Mary) and B (from dad). If dad is homozygous, his blood type alleles must be the same. For dad to pass B, and be homozygous, his blood type must be BB (type B) or AB? Wait, no—AB is heterozygous for A and B? Wait, no: AB has A and B alleles, so it's heterozygous. Wait, the question says "homogenous" (homozygous) for blood type. So homozygous means two same alleles. So dad must have two B alleles (BB, type B) or two A? No, because Bill has AB. Mary has A (so her allele is A, either AA or AO). Wait, Mary's type A: if she's AA, she can only pass A. Bill has AB, so dad must pass B. Dad is homozygous, so his alleles are the same. So dad must be BB (type B) or AB? Wait, AB is heterozygous (A and B), so not homozygous. Wait, maybe I messed up. Wait, blood type alleles: A and B are codominant, O is recessive. So:

• Type A: \( AA \) (homozygous) or \( AO \) (heterozygous)
• Type B: \( BB \) (homozygous) or \( BO \) (heterozygous)
• Type AB: \( AB \) (heterozygous, codominant)
• Type O: \( OO \) (homozygous)

Bill has AB: so he has A (from Mary) and B (from dad). Mary has type A: her alleles are either \( AA \) or \( AO \). If Mary is \( AA \), she can only pass A. So dad must pass B. Dad is homozygous, so his alleles must be \( BB \) (type B) or \( AB \)? No, \( AB \) is heterozygous (A and B), so not homozygous. Wait, the options include B. Wait, let's re-express:

Mary (type A) → can give A (if \( AA \)) or A/O (if \( AO \)). Bill (AB) → got A from Mary, B from dad. Dad is homozygous: so his blood type is homozygous, meaning his two alleles are the same. So to give B, dad's alleles must be \( BB \) (type B) or \( AB \)? No, \( AB \) is two different alleles (A and B), so not homozygous. Wait, maybe the question's "homogenous" is a typo for "homozygous". So dad is homozygous, so his blood type is either \( AA \), \( BB \), or \( OO \). Since he gave B to Bill, he must have B allele. So only \( BB \) (type B) is homozygous and gives B. Wait, but the options have B as an option. Wait, let's check the options: A, B, AB, O.

Wait, maybe I made a mistake. Let's re-express:

Mary's blood type: A (alleles \( AA \) or \( AO \)). Bill's blood type: AB (alleles \( AB \)). So Bill got A from Mary and B from dad. Dad is homozygous (so his alleles are the same). So dad's alleles must be \( BB \) (type B) because that's the only homozygous type that can pass B. If dad were AB, he's heterozygous (A and B), not homozygous. If dad were AA, he'd pass A, Bill would be AA or AO (not AB). If dad were OO, he'd pass O, Bill would be AO (type A) or OO (type O). So dad must be BB (type B), which is homozygous. So the answer is B.

Answer:

B. B