Multiple Allele Crosses worksheet with student's handwritten responses.

Let’s solve this step by step.

We are told that in humans, there are four types of blood: A, B, AB, and O.
Blood type is controlled by three alleles: A, B, and O.
- A and B are codominant → if you have both, you get type AB.
- O is recessive → you only get type O if you have two O alleles (OO).

So possible genotypes:
- Type A: AA or AO
- Type B: BB or BO
- Type AB: AB
- Type O: OO

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The problem says:

> In a marriage, there are four types of children: A, B, AB, and O.
> What must be the genotype of each parent?

We need to find two parents whose combination can produce all four blood types in their children.

Let’s think about what alleles each child must inherit:

- Child with type O → must be OO → so both parents must contribute an O allele.
- Child with type AB → must be AB → so one parent must give A, the other must give B.
- Child with type A → could be AA or AO → but since we already know both parents must carry O (to make OO child), then this child is likely AO.
- Child with type B → similarly, likely BO.

So for all four types to appear, each parent must carry two different alleles, and together they must cover A, B, and O.

Try this:

Parent 1: AO → can give A or O
Parent 2: BO → can give B or O

Now let’s do a Punnett square:

| A | O
----------------------
B | AB | BO
----------------------
O | AO | OO

Children:
- AB → type AB ✔
- BO → type B ✔
- AO → type A ✔
- OO → type O ✔

Perfect! All four blood types are possible.

Could any other pair work?

What if Parent 1 = AB and Parent 2 = OO?

Then children: AO, BO → only types A and B → no AB or O? Wait — OO gives O, but AB parent can’t give O → so children would be AO and BO → types A and B only. ✘

What if Parent 1 = AA and Parent 2 = BB? → all children AB → ✘

What if Parent 1 = AO and Parent 2 = AB?

Punnett:

| A | B
----------------------
A | AA | AB
----------------------
O | AO | BO

Children: AA (A), AB (AB), AO (A), BO (B) → missing O → ✘

Only when one parent is AO and the other is BO do we get all four types.

Also note: The worksheet shows a partially filled Punnett square with “AC” and “AD” — that seems like a typo or placeholder. Probably meant to be “A” and “O” for one parent, “B” and “O” for the other. And at the bottom it says “A or / and B” — which matches our conclusion.

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Final Answer:
One parent must have genotype AO and the other parent must have genotype BO.