9 Hands-On Worksheet Punnett Square Worksheet - The Teach Simple Blog - Free Printable

Let's solve this genetics problem step by step based on the information provided.

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Background Information:

- Deafness in dogs is caused by a recessive gene, denoted as "d".
- The dominant allele "D" results in hearing (normal).
- A dog with genotype DD or Dd can hear (since D is dominant).
- A dog with genotype dd is deaf.

The kennel owner has a male dog named Gilbert who can hear. She wants to use him for breeding, but only if he does not carry the deafness gene.

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Question 7: What are the two possible genotypes of Gilbert?

Since Gilbert can hear, he must have at least one D allele.

So, his possible genotypes are:

✔ DD – homozygous dominant (not a carrier)
✔ Dd – heterozygous (carrier of the deafness gene)

> Answer: The two possible genotypes of Gilbert are DD and Dd.

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Question 8: If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Fill in these two Punnett squares to illustrate the crosses for your possible male genotypes (from question 7) with the deaf female dog.

We need to do two crosses:
1. Cross 1: Gilbert (DD) × Deaf female (dd)
2. Cross 2: Gilbert (Dd) × Deaf female (dd)

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#### ✔ Possible Cross 1: DD × dd

Parental genotypes:
- Male: DD
- Female: dd

Gametes:
- Male: D, D
- Female: d, d

Punnett Square:

| | d | d |
|-------|-------|-------|
| D | Dd | Dd |
| D | Dd | Dd |

All offspring = Dd

Now calculate percentages:

- Genotypes:
- DD: 0%
- Dd: 100%
- dd: 0%

- Phenotypes:
- Hearing pups: 100% (because Dd → can hear)
- Deaf pups: 0%

> So:
> - % of possible Genotypes:
> - DD: 0%
> - Dd: 100%
> - dd: 0%
>
> - % of possible Phenotypes:
> - Hearing pups: 100%
> - Deaf pups: 0%

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#### ✔ Possible Cross 2: Dd × dd

Parental genotypes:
- Male: Dd
- Female: dd

Gametes:
- Male: D, d
- Female: d, d

Punnett Square:

| | d | d |
|-------|-------|-------|
| D | Dd | Dd |
| d | dd | dd |

Offspring:
- Dd: 2 out of 4 → 50%
- dd: 2 out of 4 → 50%

Now calculate:

- Genotypes:
- DD: 0%
- Dd: 50%
- dd: 50%

- Phenotypes:
- Hearing pups: Dd → 50%
- Deaf pups: dd → 50%

> So:
> - % of possible Genotypes:
> - DD: 0%
> - Dd: 50%
> - dd: 50%
>
> - % of possible Phenotypes:
> - Hearing pups: 50%
> - Deaf pups: 50%

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Question 9: Explain how could you tell the genotype of Gilbert’s male dog?

To determine whether Gilbert is DD or Dd, we can perform a test cross — breed him to a deaf female (dd).

- If all puppies can hear, then Gilbert is DD (no recessive gene passed).
- If some puppies are deaf (dd), then Gilbert must be Dd (he passed the 'd' allele).

> So:
> - If no deaf puppies → Gilbert is DD
> - If half the puppies are deaf → Gilbert is Dd

This is a classic test cross to reveal the genotype of a dominant phenotype individual.

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✔ Final Answers:

#### 7) Two possible genotypes of Gilbert:
- DD
- Dd

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#### 8) Possible Cross 1: DD × dd

% of possible Genotypes:
- DD: 0%
- Dd: 100%
- dd: 0%

% of possible Phenotypes:
- Hearing pups: 100%
- Deaf pups: 0%

Punnett Square:

| | d | d |
|-------|-------|-------|
| D | Dd | Dd |
| D | Dd | Dd |

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#### Possible Cross 2: Dd × dd

% of possible Genotypes:
- DD: 0%
- Dd: 50%
- dd: 50%

% of possible Phenotypes:
- Hearing pups: 50%
- Deaf pups: 50%

Punnett Square:

| | d | d |
|-------|-------|-------|
| D | Dd | Dd |
| d | dd | dd |

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#### 9) How to tell Gilbert’s genotype?

> Breed Gilbert to a deaf female (dd). Observe the offspring:
> - If all puppies can hear, Gilbert is DD.
> - If some puppies are deaf, Gilbert is Dd.
>
> This is a test cross used to determine the genotype of an organism with a dominant trait.

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