Consider a marriage in which a man with type O blood marries a woman with type AB blood.?

Question

In humans, the ABO blood type is determined by multiple alleles of a single gene. The 4 blood groups (A, B, AB, or O) are a result of various combinations for the enzyme (I) that attaches the A or B carbohydrate to red blood cells.

Consider a marriage in which a man with type O blood marries a woman with type AB blood.

a) What are the chances that this couple will produce a child with type B blood?

b) What are the chances that this couple will produce a child with type AB blood?
c) What are the chances that this couple will produce a child with type O blood?

d) If this couple were to produce 2 children, what are the chances that both children have type A blood?

Answer 1

Parents are OO x AB (It's conventional to write ii x IAIB, but I'm going to use this shorter notation.)

The Punnett square's boxes will be OA, OB, OA, OB
a. Chance of Type B child = 1/2 or 50%
b. Chance of Type AB child = 0
c. Chance of Type O child = 0.
d. Chance of two Type A children in a row is 1/2 TIMES 1/2 = 1/4 or 25%

Note that in probability "and" means to multiply.
Part d. says that they have two kids and both are Type A.
That means that the first one is Type A (1/2 chance) AND the second is Type A (1/2 chance). 1/2 * 1/2 = 1/4

You can also think of it as a list. With two kids, they could have:
Type A then Type A
Type A then Type B
Type B then Type A
Type B then Type B

Those are all the possible combinations of blood types for two kids from these parents.
Exactly one out of those four possibilities is the one that you want: Type A then Type A. One of four is 1/4.

Answer 2

a.) 25%
b.) 0%
c.) 0%
d.) 25% each, 50% all in all...