For hair color, Brown (B) is a dominant trait and Blond (b) is a recessive trait. If one parent has pure blond hair and one parent has hybrid brown hair, complete the Punnett square and questions that follow.

a) What is the probability that their first child will have pure brown hair?

b) What is the probability that their first child will have blond hair?

c) What is the probability that their first child will have brown hair?

One parent is Bb and the other is bb.

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To determine the probabilities, let's first set up the Punnett square for the cross between a parent with pure blond hair (bb) and a parent with hybrid brown hair (Bb):

| B | b |
--------------------------------
B | BB | Bb |
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b | Bb | bb |

a) The probability that their first child will have pure brown hair (BB) is 25% or 1/4. There is only one possible genotype in the Punnett square that corresponds to pure brown hair.

b) The probability that their first child will have blond hair (bb) is also 25% or 1/4. Again, there is only one possible genotype in the Punnett square that corresponds to blond hair.

c) The probability that their first child will have hybrid brown hair (Bb) is 50% or 1/2. This is because there are two possible genotypes in the Punnett square that correspond to brown hair, either BB or Bb.

To answer these questions, we need to use a Punnett square to determine the possible genetic combinations for the hair color of their first child.

Let's represent the parents' hair color alleles as follows:
- Pure blond hair (bb) for one parent
- Hybrid brown hair (Bb) for the other parent

To create the Punnett square, we will place the alleles of one parent (pure blond) on the top of the square and the alleles of the other parent (hybrid brown) on the left side.

The Punnett square for the parents' hair color is as follows:

b b
-----------------
B | Bb Bb
b | bb bb

Now, let's answer the questions based on the Punnett square:

a) The probability that their first child will have pure brown hair (BB) is 0% since neither parent has the BB genotype.

b) The probability that their first child will have blond hair (bb) is 50% since both parents carry the recessive allele b.

c) The probability that their first child will have brown hair (Bb) is 50% since both parents carry the dominant allele B, and there is a 50% chance of passing it on to the child.

By analyzing the Punnett square, we can determine the probabilities for each outcome by counting the number of boxes that represent a particular genotype and dividing it by the total number of boxes.