Dihybrid cross worksheet featuring Punnett squares for genetics problems involving plant height and flower color, and guinea pig fur color and texture.

Let’s solve this step by step.

---

Question 1: Set up a Punnett square for DDWW × ddww

We are crossing:
- Parent 1: DDWW (homozygous dominant for both traits)
- Parent 2: ddww (homozygous recessive for both traits)

Since each parent is homozygous, they can only produce one type of gamete:

- Parent 1 (DDWW) → all gametes = DW
- Parent 2 (ddww) → all gametes = dw

So the Punnett square will be 4x4, but every box will have the same genotype because all gametes from each parent are identical.

Punnett Square:

| | 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 are DdWw

---

Question 2: Using the Punnett square from #1

All offspring are DdWw

Recall:
- D = tall (dominant), d = dwarf (recessive)
- W = purple flowers (dominant), w = white flowers (recessive)

So phenotype of DdWw:
- Tall (because D is present)
- Purple flowers (because W is present)

Now answer each part:

a. Probability of producing tall plants with purple flowers?

→ All 16 boxes are DdWw → all are tall and purple.

Probability = 16/16 = 1 or 100%

Possible genotype(s): DdWw

---

b. Probability of producing dwarf plants with white flowers?

Dwarf = dd, White = ww → need genotype ddww

But in our cross, NO offspring have dd or ww — all are DdWw.

So probability = 0/16 = 0

Possible genotype(s): none

---

c. Probability of producing tall plants with white flowers?

Tall = D_ (at least one D), White = ww

Need genotype like D_ww — e.g., DDww, Ddww

But again, all offspring are DdWw → no ww at all.

So probability = 0

Possible genotype(s): none

---

d. Probability of producing dwarf plants with purple flowers?

Dwarf = dd, Purple = W_

Need genotype ddW_

Again, no offspring have dd — all are Dd.

So probability = 0

Possible genotype(s): none

---

✔ So far, Question 1 and 2 are done.

---

Question 3: Set up Punnett square for BbRr × BbRr

Traits:
- Fur color: B = black (dominant), b = white (recessive)
- Fur texture: R = rough (dominant), r = smooth (recessive)

Both parents are heterozygous: BbRr

Each parent can make 4 types of gametes: BR, Br, bR, br

We’ll set up a 4x4 Punnett square.

Gametes from Parent 1 (top row): BR, Br, bR, br
Gametes from Parent 2 (left column): BR, Br, bR, br

Fill in each box by combining alleles:

Let me write out the full square:

Row labels (Parent 2 gametes): BR, Br, bR, br
Column labels (Parent 1 gametes): BR, Br, bR, br

Box contents:

First row (BR x ...):
- BR x BR → BBRR
- BR x Br → BBRr
- BR x bR → BbRR
- BR x br → BbRr

Second row (Br x ...):
- Br x BR → BBRr
- Br x Br → BBrr
- Br x bR → BbRr
- Br x br → Bbrr

Third row (bR x ...):
- bR x BR → BbRR
- bR x Br → BbRr
- bR x bR → bbRR
- bR x br → bbRr

Fourth row (br x ...):
- br x BR → BbRr
- br x Br → Bbrr
- br x bR → bbRr
- br x br → bbrr

So the full Punnett square (genotypes only):

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

Total = 16 offspring

---

Question 4: Use the Punnett square from #3

We need to find probabilities for phenotypes.

Phenotype rules:
- Black fur: B_ (BB or Bb)
- White fur: bb
- Rough fur: R_ (RR or Rr)
- Smooth fur: rr

---

a. Probability of guinea pigs with black, rough fur?

Black = B_, Rough = R_

Look for genotypes that have at least one B AND at least one R.

Go through each box:

Row 1:
- BBRR → yes
- BBRr → yes
- BbRR → yes
- BbRr → yes → 4

Row 2:
- BBRr → yes
- BBrr → NO (smooth)
- BbRr → yes
- Bbrr → NO → 2 more → total 6

Row 3:
- BbRR → yes
- BbRr → yes
- bbRR → NO (white)
- bbRr → NO → 2 more → total 8

Row 4:
- BbRr → yes
- Bbrr → NO
- bbRr → NO
- bbrr → NO → 1 more → total 9

Wait — let me recount carefully.

List all 16 and mark which are black & rough (B_ and R_):

1. BBRR → yes
2. BBRr → yes
3. BbRR → yes
4. BbRr → yes
5. BBRr → yes
6. BBrr → no (smooth)
7. BbRr → yes
8. Bbrr → no
9. BbRR → yes
10. BbRr → yes
11. bbRR → no (white)
12. bbRr → no
13. BbRr → yes
14. Bbrr → no
15. bbRr → no
16. bbrr → no

Count “yes”: positions 1,2,3,4,5,7,9,10,13 → that’s 9

So 9 out of 16 → probability = 9/16

Possible genotypes: Any combination with B_ and R_ → specifically:

From above: BBRR, BBRr, BbRR, BbRr

(We don’t list duplicates — just unique genotypes that give this phenotype)

So possible genotypes: BBRR, BBRr, BbRR, BbRr

---

b. Probability of black, smooth fur?

Black = B_, Smooth = rr

Look for B_ and rr

Check list:

6. BBrr → yes
8. Bbrr → yes
14. Bbrr → yes

Any others?

Row 2: BBrr (box 6), Bbrr (box 8)
Row 4: Bbrr (box 14)

Is there another? Let's see:

Box 6: BBrr → yes
Box 8: Bbrr → yes
Box 14: Bbrr → yes

That’s 3.

Wait — box 6, 8, 14 → yes, 3 total.

Confirm:

- BBrr → appears once (box 6)
- Bbrr → appears twice (boxes 8 and 14)

Total = 3

Probability = 3/16

Possible genotypes: BBrr, Bbrr

---

c. Probability of white, rough fur?

White = bb, Rough = R_

Look for bb and R_

From list:

11. bbRR → yes
12. bbRr → yes
15. bbRr → yes

That’s 3.

Boxes: 11, 12, 15 → yes, 3

Probability = 3/16

Possible genotypes: bbRR, bbRr

---

d. Probability of white, smooth fur?

White = bb, Smooth = rr

Only one: box 16 → bbrr

Probability = 1/16

Possible genotype(s): bbrr

---

✔ All questions solved.

Final Answer:

1. Punnett Square for DDWW × ddww:

All 16 boxes contain DdWw

2a. Probability of tall, purple: 16/16 = 1
Genotype: DdWw

2b. Probability of dwarf, white: 0
Genotype: none

2c. Probability of tall, white: 0
Genotype: none

2d. Probability of dwarf, purple: 0
Genotype: none

3. Punnett Square for BbRr × BbRr:

As shown above — 4x4 grid with 16 combinations.

4a. Black, rough: 9/16
Genotypes: BBRR, BBRr, BbRR, BbRr

4b. Black, smooth: 3/16
Genotypes: BBrr, Bbrr

4c. White, rough: 3/16
Genotypes: bbRR, bbRr

4d. White, smooth: 1/16
Genotype: bbrr