Biology worksheet featuring nine Punnett square problems for practicing monohybrid crosses with dominant and recessive alleles.

Let's solve each of the Punnett squares step by step, determine the genotype ratios and phenotype ratios for each cross, and fill in the worksheet accordingly.

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🔬 Key Concepts:

- A = dominant allele
- a = recessive allele
- AA = homozygous dominant
- Aa = heterozygous
- aa = homozygous recessive
- Phenotype: physical trait (Dominant if at least one A; Recessive only if aa)
- Genotype: genetic makeup

We’ll go through each cross.

---

## ✔ Part 1: Crosses with one homozygous dominant parent

1. AA × AA

```
A A
A | AA | AA
A | AA | AA
```

- Offspring Genotypes:
- AA = 4/4 = 100%
- Aa = 0%
- aa = 0%

- Offspring Phenotypes:
- Dominant = 4/4 = 100%
- Recessive = 0%

✔ Answer:
- Genotype: AA 4, Aa 0, aa 0
- Phenotype: Dominant 4, Recessive 0

---

2. AA × Aa

```
A A
A | AA | AA
a | Aa | Aa
```

- Offspring:
- AA = 2/4 = 50%
- Aa = 2/4 = 50%
- aa = 0%

- All have at least one A → All show dominant phenotype

- Phenotypes:
- Dominant = 4/4 = 100%
- Recessive = 0%

✔ Answer:
- Genotype: AA 2, Aa 2, aa 0
- Phenotype: Dominant 4, Recessive 0

---

3. AA × aa

```
A A
a | Aa | Aa
a | Aa | Aa
```

- Offspring:
- All are Aa → 4/4 = 100%
- AA = 0%, Aa = 4, aa = 0%

- Since A is dominant, all offspring show dominant phenotype

✔ Answer:
- Genotype: AA 0, Aa 4, aa 0
- Phenotype: Dominant 4, Recessive 0

---

## ✔ Part 2: Crosses with one heterozygous parent

4. Aa × AA

This is same as AA × Aa above — just reversed.

```
A a
A | AA | Aa
A | AA | Aa
```

- Offspring:
- AA = 2/4 = 50%
- Aa = 2/4 = 50%
- aa = 0%

- All have at least one A → All dominant phenotype

✔ Answer:
- Genotype: AA 2, Aa 2, aa 0
- Phenotype: Dominant 4, Recessive 0

---

5. Aa × Aa (Classic Mendelian Cross)

```
A a
A | AA | Aa
a | Aa | aa
```

- Offspring:
- AA = 1/4 = 25%
- Aa = 2/4 = 50%
- aa = 1/4 = 25%

- Phenotypes:
- Dominant (AA or Aa) = 3/4 = 75%
- Recessive (aa) = 1/4 = 25%

✔ Answer:
- Genotype: AA 1, Aa 2, aa 1
- Phenotype: Dominant 3, Recessive 1

---

6. Aa × aa

```
A a
a | Aa | aa
a | Aa | aa
```

- Offspring:
- Aa = 2/4 = 50%
- aa = 2/4 = 50%
- AA = 0%

- Phenotypes:
- Dominant (Aa) = 2/4 = 50%
- Recessive (aa) = 2/4 = 50%

✔ Answer:
- Genotype: AA 0, Aa 2, aa 2
- Phenotype: Dominant 2, Recessive 2

---

## ✔ Part 3: Crosses with one homozygous recessive parent

7. aa × AA

Same as AA × aa — just reversed.

```
a a
A | Aa | Aa
A | Aa | Aa
```

- All offspring: Aa
- Genotype: AA 0, Aa 4, aa 0
- Phenotype: All dominant

✔ Answer:
- Genotype: AA 0, Aa 4, aa 0
- Phenotype: Dominant 4, Recessive 0

---

8. aa × Aa

Same as Aa × aa — already done.

```
a a
A | Aa | Aa
a | aa | aa
```

- Offspring:
- Aa = 2/4 = 50%
- aa = 2/4 = 50%

- Phenotype:
- Dominant: 2
- Recessive: 2

✔ Answer:
- Genotype: AA 0, Aa 2, aa 2
- Phenotype: Dominant 2, Recessive 2

---

9. aa × aa

```
a a
a | aa | aa
a | aa | aa
```

- All offspring: aa
- Genotype: AA 0, Aa 0, aa 4
- Phenotype: All recessive

✔ Answer:
- Genotype: AA 0, Aa 0, aa 4
- Phenotype: Dominant 0, Recessive 4

---

## ✔ Final Answer Summary:

Crosses with one homozygous dominant parent:

| Cross | Genotype (AA:Aa:aa) | Phenotype (Dom:Rec) |
|-------------|---------------------|----------------------|
| AA × AA | 4:0:0 | 4:0 |
| AA × Aa | 2:2:0 | 4:0 |
| AA × aa | 0:4:0 | 4:0 |

---

Crosses with one heterozygous parent:

| Cross | Genotype (AA:Aa:aa) | Phenotype (Dom:Rec) |
|-------------|---------------------|----------------------|
| Aa × AA | 2:2:0 | 4:0 |
| Aa × Aa | 1:2:1 | 3:1 |
| Aa × aa | 0:2:2 | 2:2 |

---

Crosses with one homozygous recessive parent:

| Cross | Genotype (AA:Aa:aa) | Phenotype (Dom:Rec) |
|-------------|---------------------|----------------------|
| aa × AA | 0:4:0 | 4:0 |
| aa × Aa | 0:2:2 | 2:2 |
| aa × aa | 0:0:4 | 0:4 |

---

## 📝 Filled Worksheet Answers:

> Note: Fill in numbers in blanks like `AA__`, `Aa__`, etc.

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Crosses with one homozygous dominant parent:

#### AA x AA
- Offspring Genotypes: AA 4, Aa 0, aa 0
- Offspring Phenotypes: Dominant 4, Recessive 0

#### AA x Aa
- Offspring Genotypes: AA 2, Aa 2, aa 0
- Offspring Phenotypes: Dominant 4, Recessive 0

#### AA x aa
- Offspring Genotypes: AA 0, Aa 4, aa 0
- Offspring Phenotypes: Dominant 4, Recessive 0

---

Crosses with one heterozygous parent:

#### Aa x AA
- Offspring Genotypes: AA 2, Aa 2, aa 0
- Offspring Phenotypes: Dominant 4, Recessive 0

#### Aa x Aa
- Offspring Genotypes: AA 1, Aa 2, aa 1
- Offspring Phenotypes: Dominant 3, Recessive 1

#### Aa x aa
- Offspring Genotypes: AA 0, Aa 2, aa 2
- Offspring Phenotypes: Dominant 2, Recessive 2

---

Crosses with one homozygous recessive parent:

#### aa x AA
- Offspring Genotypes: AA 0, Aa 4, aa 0
- Offspring Phenotypes: Dominant 4, Recessive 0

#### aa x Aa
- Offspring Genotypes: AA 0, Aa 2, aa 2
- Offspring Phenotypes: Dominant 2, Recessive 2

#### aa x aa
- Offspring Genotypes: AA 0, Aa 0, aa 4
- Offspring Phenotypes: Dominant 0, Recessive 4

---

✔ All Punnett squares filled correctly!

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