Dihybrid Crosses Worksheet with Example and Two Practice Problems

Let’s solve problem #1 step by step.

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Problem 1:
A tall green pea plant (TTGg) is crossed with a tall green pea plant (TtGg).

We need to do a dihybrid cross — that means we’re looking at two traits:
- Height: T = tall, t = short
- Color: G = green, g = white

Parent 1: TTGg → can make gametes: TG and Tg
(Why? Because for height, it’s TT → only T alleles. For color, it’s Gg → so G or g. So combinations: TG and Tg)

Parent 2: TtGg → can make gametes: TG, Tg, tG, tg
(Height: T or t; Color: G or g → all combos: TG, Tg, tG, tg)

Now set up the Punnett square.

Top row (gametes from Parent 1): TG, Tg
Left column (gametes from Parent 2): TG, Tg, tG, tg

Wait — actually, since Parent 1 has only 2 types of gametes, and Parent 2 has 4, our grid should be 4 rows × 2 columns? But usually we put the one with more gametes on top or side — doesn’t matter as long as we’re consistent.

Actually, let’s flip it to match standard format: Put Parent 2’s gametes on top (since they have 4), and Parent 1’s on the side (only 2). That makes a 2x4 grid? No — wait, dihybrid cross usually uses 4x4 if both parents are heterozygous for both traits. But here, Parent 1 is homozygous for height (TT), so only 2 gamete types.

So better to do:

Gametes from Parent 1 (TTGg): TG, Tg → 2 types
Gametes from Parent 2 (TtGg): TG, Tg, tG, tg → 4 types

So we’ll make a 2-row (for Parent 1) × 4-column (for Parent 2) grid? Or vice versa? Actually, in the example given in the worksheet, they used 4x4 even when one parent had repeated gametes. Let’s follow their style.

In the example, they wrote:

Parent 1: TTGG → gametes: TG, TG, TG, TG (all same)
Parent 2: ttgg → gametes: tg, tg, tg, tg

Then made 4x4 grid.

So for consistency, let’s list each parent’s possible gametes, repeating them to fill 4 slots if needed.

But actually, for accuracy, we should use the actual unique gametes and their frequencies.

Better approach: List all possible gametes for each parent without forcing 4 unless necessary.

Parent 1: TTGg → produces gametes:
- 50% TG
- 50% Tg

Parent 2: TtGg → produces gametes:
- 25% TG
- 25% Tg
- 25% tG
- 25% tg

To make a Punnett square, we can do 2 rows (for Parent 1’s gametes) and 4 columns (for Parent 2’s gametes). Each cell will represent 1/8 of total offspring (since 2 × 4 = 8 cells).

Let’s build it:

Label rows (Parent 1 gametes): TG, Tg
Label columns (Parent 2 gametes): TG, Tg, tG, tg

Now fill each box:

Row 1 (TG) x Column 1 (TG) → TTGG
Row 1 (TG) x Column 2 (Tg) → TTGg
Row 1 (TG) x Column 3 (tG) → TtGG
Row 1 (TG) x Column 4 (tg) → TtGg

Row 2 (Tg) x Column 1 (TG) → TTGg
Row 2 (Tg) x Column 2 (Tg) → TTgg
Row 2 (Tg) x Column 3 (tG) → TtGg
Row 2 (Tg) x Column 4 (tg) → Ttgg

Now let’s list all 8 offspring genotypes:

1. TTGG
2. TTGg
3. TtGG
4. TtGg
5. TTGg
6. TTgg
7. TtGg
8. Ttgg

Now group by phenotype.

Phenotype depends on:

- Tall: if at least one T → TT or Tt → tall
- Short: only if tt → but in this cross, no tt because Parent 1 is TT, so all offspring get T from Parent 1 → ALL are tall! Wait, is that right?

Parent 1: TT → gives T to every offspring
Parent 2: Tt → gives T or t

So offspring genotypes for height:
From Parent 1: always T
From Parent 2: T or t → so offspring: TT or Tt → both tall → YES, ALL offspring are tall.

For color:

Green: GG or Gg
White: gg

Now look at each offspring:

1. TTGG → Tall, Green
2. TTGg → Tall, Green
3. TtGG → Tall, Green
4. TtGg → Tall, Green
5. TTGg → Tall, Green
6. TTgg → Tall, White
7. TtGg → Tall, Green
8. Ttgg → Tall, White

Count:

Tall/Green: #1,2,3,4,5,7 → that’s 6
Tall/White: #6,8 → that’s 2
Short/Green: none → 0
Short/White: none → 0

Total = 8 offspring

So ratios out of 8:

Tall/Green: 6
Tall/White: 2
Short/Green: 0
Short/White: 0

We can simplify by dividing by 2: 3 : 1 : 0 : 0

But the worksheet asks for numbers like “__ Tall/Green : __ Tall/White : ...”

In the example, they gave absolute numbers based on 16 offspring (4x4 grid). Here we have 8, but maybe we should scale to 16 to match format?

Wait — in the example, they did 4x4 = 16 boxes, even though some gametes were repeated.

For consistency, let’s redo using 4x4 grid.

How?

Parent 1: TTGg → gametes: TG, Tg → but to make 4, repeat each twice: TG, TG, Tg, Tg

Parent 2: TtGg → gametes: TG, Tg, tG, tg → already 4

Now make 4x4 grid:

Columns (Parent 2): TG, Tg, tG, tg
Rows (Parent 1): TG, TG, Tg, Tg

Fill each cell:

Row 1 (TG) x Col1 (TG) → TTGG
Row 1 (TG) x Col2 (Tg) → TTGg
Row 1 (TG) x Col3 (tG) → TtGG
Row 1 (TG) x Col4 (tg) → TtGg

Row 2 (TG) x Col1 (TG) → TTGG
Row 2 (TG) x Col2 (Tg) → TTGg
Row 2 (TG) x Col3 (tG) → TtGG
Row 2 (TG) x Col4 (tg) → TtGg

Row 3 (Tg) x Col1 (TG) → TTGg
Row 3 (Tg) x Col2 (Tg) → TTgg
Row 3 (Tg) x Col3 (tG) → TtGg
Row 3 (Tg) x Col4 (tg) → Ttgg

Row 4 (Tg) x Col1 (TG) → TTGg
Row 4 (Tg) x Col2 (Tg) → TTgg
Row 4 (Tg) x Col3 (tG) → TtGg
Row 4 (Tg) x Col4 (tg) → Ttgg

Now list all 16:

1. TTGG
2. TTGg
3. TtGG
4. TtGg
5. TTGG
6. TTGg
7. TtGG
8. TtGg
9. TTGg
10. TTgg
11. TtGg
12. Ttgg
13. TTGg
14. TTgg
15. TtGg
16. Ttgg

Now count phenotypes:

All are tall (no tt) → so ignore short.

Color:

Green: GG or Gg → not gg
White: gg

Which ones are gg? Look for genotype ending with gg:

#10: TTgg → white
#12: Ttgg → white
#14: TTgg → white
#16: Ttgg → white

That’s 4 white.

The rest are green: 16 - 4 = 12 green.

So:

Tall/Green: 12
Tall/White: 4
Short/Green: 0
Short/White: 0

This matches the ratio 3:1 for green:white among tall plants.

And since all are tall, no short.

So final answer for problem 1:

12 Tall/Green : 4 Tall/White : 0 Short/Green : 0 Short/White

We can write it as:

12 : 4 : 0 : 0

Or simplify? But the example didn’t simplify — they left as 16:0:0:0.

So we’ll leave as 12:4:0:0

But let me double-check counts.

From the 16:

Green: any with G_ (GG or Gg)

List:

1. TTGG → G
2. TTGg → G
3. TtGG → G
4. TtGg → G
5. TTGG → G
6. TTGg → G
7. TtGG → G
8. TtGg → G
9. TTGg → G
10. TTgg → g (white)
11. TtGg → G
12. Ttgg → g (white)
13. TTGg → G
14. TTgg → g (white)
15. TtGg → G
16. Ttgg → g (white)

Green: 1,2,3,4,5,6,7,8,9,11,13,15 → that’s 12
White: 10,12,14,16 → 4

Yes.

All tall: yes, because no tt.

Perfect.

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Final Answer:

12 Tall/Green : 4 Tall/White : 0 Short/Green : 0 Short/White