Biology worksheet asking students to identify correct Punnett squares and complete genetic crosses for corn plants.

Problem Analysis and Solution

#### Part B: Punnett Squares

##### Question 12: Examine the following Punnett squares and circle those that are correct.

We need to evaluate each Punnett square to determine if it is correctly filled out based on the alleles provided on the outside of the square.

1. First Punnett Square:
- Alleles on the outside: `D` and `d` (top) and `d` (left).
- Filled in:
```
Dd | dd
Dd | dd
```
- Analysis: This is incorrect. The top row should be `Dd` and `dd`, but the left column should be `Dd` and `dd`. The correct Punnett square should be:
```
Dd | dd
Dd | dd
```
However, the left column is repeated incorrectly. This square is incorrect.

2. Second Punnett Square:
- Alleles on the outside: `D` and `D` (top) and `d` (left).
- Filled in:
```
Dd | DD
Dd | DD
```
- Analysis: This is incorrect. Since both parents have the same allele (`D`), all offspring should be `DD`. The correct Punnett square should be:
```
DD | DD
DD | DD
```
This square is incorrect.

3. Third Punnett Square:
- Alleles on the outside: `A` and `a` (top) and `a` (left).
- Filled in:
```
AA | aa
Aa | Aa
```
- Analysis: This is incorrect. The top row should be `Aa` and `aa`, but the left column should be `Aa` and `aa`. The correct Punnett square should be:
```
Aa | aa
Aa | aa
```
This square is incorrect.

4. Fourth Punnett Square:
- Alleles on the outside: `A` and `a` (top) and `a` (left).
- Filled in:
```
Aa | aa
Aa | aa
```
- Analysis: This is correct. Both the top row and the left column are filled in correctly based on the alleles provided. The correct Punnett square is:
```
Aa | aa
Aa | aa
```
This square is correct.

Correct Answer for Question 12:
- Circle the fourth Punnett square.

---

##### Question 13: What do the letters on the outside of the Punnett square stand for?

The letters on the outside of the Punnett square represent the alleles of the parents involved in the cross. Each parent contributes one allele to the offspring, and these alleles are written along the top and left sides of the Punnett square.

Answer for Question 13:
- The letters on the outside of the Punnett square stand for the alleles of the parents.

---

##### Question 14: What do the letters on the inside of the Punnett square stand for?

The letters on the inside of the Punnett square represent the genotypes of the offspring. Each box in the Punnett square shows the combination of alleles that an offspring would inherit from both parents.

Answer for Question 14:
- The letters on the inside of the Punnett square stand for the genotypes of the offspring.

---

##### Question 15: In corn plants, normal height, N, is dominant to short height, n. Complete these four Punnett squares showing different crosses. Then, shade red all the homozygous dominant offspring, shade green all the heterozygous offspring, and leave all the homozygous recessive offspring unshaded.

We need to complete the Punnett squares for the given crosses and then shade them according to the instructions.

1. First Cross: `n` (left) × `N` and `N` (top)
- Punnett Square:
```
N | N
n | n
```
- Genotypes: `Nn`, `Nn`, `nn`, `nn`
- Shading:
- No homozygous dominant (`NN`), so no red shading.
- Two heterozygous (`Nn`), so shade green.
- Two homozygous recessive (`nn`), so leave unshaded.
- Final Shading:
```
Green | Green
Unshaded | Unshaded
```

2. Second Cross: `N` (left) × `N` and `n` (top)
- Punnett Square:
```
NN | Nn
NN | Nn
```
- Genotypes: `NN`, `Nn`, `NN`, `Nn`
- Shading:
- Two homozygous dominant (`NN`), so shade red.
- Two heterozygous (`Nn`), so shade green.
- No homozygous recessive (`nn`), so no unshaded.
- Final Shading:
```
Red | Green
Red | Green
```

3. Third Cross: `n` (left) × `N` and `n` (top)
- Punnett Square:
```
Nn | nn
Nn | nn
```
- Genotypes: `Nn`, `nn`, `Nn`, `nn`
- Shading:
- No homozygous dominant (`NN`), so no red shading.
- Two heterozygous (`Nn`), so shade green.
- Two homozygous recessive (`nn`), so leave unshaded.
- Final Shading:
```
Green | Unshaded
Green | Unshaded
```

4. Fourth Cross: `n` (left) × `N` and `n` (top)
- Punnett Square:
```
Nn | nn
Nn | nn
```
- Genotypes: `Nn`, `nn`, `Nn`, `nn`
- Shading:
- No homozygous dominant (`NN`), so no red shading.
- Two heterozygous (`Nn`), so shade green.
- Two homozygous recessive (`nn`), so leave unshaded.
- Final Shading:
```
Green | Unshaded
Green | Unshaded
```

---

Final Answers:

1. Question 12: Circle the fourth Punnett square.
2. Question 13: The letters on the outside of the Punnett square stand for the alleles of the parents.
3. Question 14: The letters on the inside of the Punnett square stand for the genotypes of the offspring.
4. Question 15: The completed Punnett squares with shading are as follows:
- First Cross:
```
Green | Green
Unshaded | Unshaded
```
- Second Cross:
```
Red | Green
Red | Green
```
- Third Cross:
```
Green | Unshaded
Green | Unshaded
```
- Fourth Cross:
```
Green | Unshaded
Green | Unshaded
```

Final Answer:
\boxed{4} (for Question 12)