Solved Dihybrid Cross Worksheet 1. Set up a punnett square | Chegg.com - Free Printable

Let's solve each part of this Dihybrid Cross Worksheet step by step.

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Question 1: Set up a Punnett square

Given:
- Dominant allele for tall plants = D
- Recessive allele for dwarf plants = d
- Dominant allele for purple flowers = W
- Recessive allele for white flowers = w

Cross:
Homozygous dominant parent (DDWW) × Homozygous recessive parent (ddww)

#### Step 1: Determine gametes
Each parent produces gametes with one allele for each gene.

- DDWW → Gametes: DW (only one type)
- ddww → Gametes: dw (only one type)

So, all offspring will be DdWw.

Now, set up the Punnett square:

```
DW
-----------
dW | DdWw
dw | DdWw
```

Wait — actually, since both parents are homozygous, we only need a 4x4 square if both parents produce multiple gametes. But here:

- Parent 1 (DDWW): Only DW gametes
- Parent 2 (ddww): Only dw gametes

So the Punnett square is very simple:

| | DW |
|-------|--------|
| dw | DdWw |

But to make it a full 4x4 dihybrid cross format, we write it like this:

| | DW | DW | DW | DW |
|-------|--------|--------|--------|--------|
| dw | DdWw | DdWw | DdWw | DdWw |
| dw | DdWw | DdWw | DdWw | DdWw |
| dw | DdWw | DdWw | DdWw | DdWw |
| dw | DdWw | DdWw | DdWw | DdWw |

But that’s redundant. Since both parents are homozygous, all offspring are DdWw.

So, all 16 boxes (if expanded) will be DdWw.

✔ So, the Punnett square shows:
- All offspring have genotype: DdWw

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Question 2: Using the Punnett square from #1

Since all offspring are DdWw, we can now answer:

#### a) Probability of producing tall plants with purple flowers

- Tall = D_ (D dominant)
- Purple flowers = W_ (W dominant)
- Genotype: DdWw → D and W are dominant → so tall and purple

✔ All offspring are DdWw → tall and purple

→ Probability = 100% or 1

- Possible genotype(s): DdWw

#### b) Probability of producing dwarf plants with white flowers

- Dwarf = dd
- White flowers = ww
- Need ddww

But all offspring are DdWw → not ddww

→ Probability = 0%

- Possible genotype(s): None (or none)

#### c) Probability of producing tall plants with white flowers

- Tall = D_
- White = ww

So need: D_ ww

But all offspring are DdWw → they have W, so not ww

→ Cannot have white flowers

→ Probability = 0%

- Possible genotype(s): None

#### d) Probability of producing dwarf plants with purple flowers

- Dwarf = dd
- Purple = W_

But offspring are DdWw → Dd means not dd → not dwarf

→ Probability = 0%

- Possible genotype(s): None

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✔ Summary for Question 2:

| Part | Phenotype | Probability | Genotype(s) |
|------|-----------|-------------|-------------|
| a | Tall, purple | 100% | DdWw |
| b | Dwarf, white | 0% | None |
| c | Tall, white | 0% | None |
| d | Dwarf, purple | 0% | None |

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Question 3: Set up a Punnett square

Given:
- Black fur = B (dominant), white = b (recessive)
- Rough fur = R (dominant), smooth = r (recessive)

Cross: Heterozygous parent (BbRr) × Heterozygous parent (BbRr)

This is a classic dihybrid cross.

#### Step 1: Gametes

Each parent can produce four types of gametes:
- BR, Br, bR, br

Set up a 4×4 Punnett square:

| | BR | Br | bR | br |
|-------|--------|--------|--------|--------|
| BR | BBRR | BBRr | BbRR | BbRr |
| Br | BBRr | BBrr | BbRr | Bbrr |
| bR | BbRR | BbRr | bbRR | bbRr |
| br | BbRr | Bbrr | bbRr | bbrr |

Now, count the genotypes and phenotypes.

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Question 4: Using the Punnett square from #3

We’ll analyze the probabilities based on the 16 possible combinations.

#### a) Probability of guinea pigs with black, rough fur

- Black = B_ (BB or Bb)
- Rough = R_ (RR or Rr)

From the Punnett square, find all combinations where:
- At least one B (not bb)
- At least one R (not rr)

Count them:

- BBRR → 1
- BBRr → 2
- BbRR → 2
- BbRr → 4

Total = 1 + 2 + 2 + 4 = 9

→ 9/16

Genotypes: BBRR, BBRr, BbRR, BbRr

#### b) Probability of black, smooth fur

- Black = B_
- Smooth = rr

So need: B_ rr

From the square:
- BBrr → 1
- Bbrr → 2

Total = 3 → 3/16

Genotypes: BBrr, Bbrr

#### c) Probability of white, rough fur

- White = bb
- Rough = R_

So: bbR_

From square:
- bbRR → 1
- bbRr → 2

Total = 3 → 3/16

Genotypes: bbRR, bbRr

#### d) Probability of white, smooth fur

- White = bb
- Smooth = rr

So: bbrr

Only one box → 1/16

Genotype: bbrr

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✔ Summary for Question 4:

| Part | Phenotype | Probability | Genotype(s) |
|------|-----------|-------------|-------------|
| a | Black, rough | 9/16 | BBRR, BBRr, BbRR, BbRr |
| b | Black, smooth | 3/16 | BBrr, Bbrr |
| c | White, rough | 3/16 | bbRR, bbRr |
| d | White, smooth | 1/16 | bbrr |

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Final Answers:

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Question 1: Punnett Square

```
Gametes: DW DW DW DW
---------------------
dw DdWw DdWw DdWw DdWw
dw DdWw DdWw DdWw DdWw
dw DdWw DdWw DdWw DdWw
dw DdWw DdWw DdWw DdWw
```

All offspring: DdWw

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Question 2: Probabilities

a) Tall, purple: 100%, genotype: DdWw
b) Dwarf, white: 0%, no genotype
c) Tall, white: 0%, no genotype
d) Dwarf, purple: 0%, no genotype

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Question 3: Punnett Square (BbRr × BbRr)

See above 4×4 table.

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Question 4: Probabilities

a) Black, rough: 9/16, genotypes: BBRR, BBRr, BbRR, BbRr
b) Black, smooth: 3/16, genotypes: BBrr, Bbrr
c) White, rough: 3/16, genotypes: bbRR, bbRr
d) White, smooth: 1/16, genotype: bbrr

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