Worksheet: Dihybrid Crosses - Triton Science - Free Printable

Let’s solve each problem step by step using the 6-step method from the worksheet.

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Problem 1: Man with spotted, non-wooly hair × Woman with non-spotted, wooly hair

We’re told:
- Spotted skin (S) is dominant over non-spotted (s)
- Wooly hair (W) is dominant over non-wooly (w)

Man: heterozygous spotted → Ss; non-wooly → ww
→ Genotype: Ssww

Woman: non-spotted → ss; wooly-haired → but we don’t know if she’s homozygous or heterozygous. Wait — it says “wooly-haired, non-spotted woman” — since wooly is dominant, and no other info, we assume she could be WW or Ww? But in genetics problems like this, if not specified, and they say “wooly-haired”, and you’re crossing to find ratios, usually they mean heterozygous unless stated otherwise? Actually — let’s read again:

“Cross a marriage between a heterozygous spotted, non-wooly man with a wooly-haired, non-spotted woman.”

It doesn’t specify if the woman is homozygous or heterozygous for wooly hair. That’s a problem. But wait — in standard dihybrid cross problems, if they don’t specify, and one trait is dominant phenotype, they often expect you to assume heterozygous if needed to get variation. But actually — let’s think: if she were WW, all offspring would have wooly hair. If she were Ww, then half might not. Since the question asks for genotypic AND phenotypic ratios, it implies there will be variation — so likely she is heterozygous for wooly hair.

But hold on — the problem says “wooly-haired, non-spotted woman”. Non-spotted is recessive → ss. Wooly-haired is dominant → could be WW or Ww. But without more info, we can’t proceed unless we assume. However, looking at typical textbook problems, when they say “a wooly-haired person” in a cross like this, and ask for ratios, they usually mean heterozygous if the other parent is recessive for that trait. Here, the man is ww (non-wooly), so if the woman is Ww, then we’ll see both wooly and non-wooly in kids. If she’s WW, all kids are wooly. Since the problem asks for ratios, probably expects variation → so I’ll assume she is Ww.

Wait — actually, let me double-check the wording: “a wooly-haired, non-spotted woman”. It doesn’t say “heterozygous” for wooly. Hmm. This is ambiguous. But in many such problems, if they want you to do a full dihybrid cross with variation, they imply heterozygosity. Alternatively, maybe she is homozygous? Let’s look at the man: he’s heterozygous spotted (Ss) and non-wooly (ww). The woman is non-spotted (ss) and wooly-haired. To get a dihybrid cross with 4 types of gametes from each parent, both need to be heterozygous for at least one trait. The man is heterozygous for S, homozygous for w. The woman is homozygous for s, and if she’s heterozygous for W, then she produces two types of gametes, man produces two types → 2x2=4 combinations. That makes sense for a dihybrid cross even though neither is heterozygous for both traits — it’s still a dihybrid cross because we’re tracking two genes.

Actually, yes — dihybrid cross just means we’re following two traits, regardless of whether parents are heterozygous for both. So let’s define:

Man: Ss ww → gametes: Sw, sw (since he has S or s, and only w)

Woman: ss W? → if she’s WW, gametes: sW only → then all offspring: Ss Ww or ss Ww → phenotypes: spotted wooly or non-spotted wooly → ratio 1:1 genotypically? Not very interesting.

If she’s Ww, then her gametes: sW, sw

Then cross:

Man gametes: Sw, sw

Woman gametes: sW, sw

Punnett square:

| sW | sw
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Sw | Ss WW? No — wait, man gives S and w, woman gives s and W → offspring: Ss Ww

Wait, let’s write properly.

Man genotype: S s w w → so alleles: for gene 1: S or s; gene 2: w or w → so gametes: S w and s w → so Sw and sw

Woman genotype: s s W w → gametes: s W and s w → so sW and sw

Now Punnett square:

| sW | sw
------------------------
Sw | Ss Ww | Ss ww
sw | ss Ww | ss ww

So offspring genotypes:

- Ss Ww : 1
- Ss ww : 1
- ss Ww : 1
- ss ww : 1

Phenotypes:

- Ss Ww → spotted, wooly
- Ss ww → spotted, non-wooly
- ss Ww → non-spotted, wooly
- ss ww → non-spotted, non-wooly

Each 1 out of 4 → so phenotypic ratio: 1:1:1:1

Genotypic ratio: also 1:1:1:1 for those four genotypes.

But is the woman really Ww? The problem didn’t specify. In many textbooks, if they say “wooly-haired” and don’t specify, and the cross is designed to show variation, they intend heterozygous. Also, if she were WW, then all offspring would have wooly hair, and phenotypic ratio would be 1 spotted wooly : 1 non-spotted wooly — which is possible, but less informative. Given that the problem asks for "genotypic and phenotypic ratios", implying multiple categories, I think we’re meant to assume she is heterozygous for wooly hair.

Alternatively, perhaps I misread. Let me check online or standard interpretation — but since I can’t, I’ll go with the assumption that allows for full variation, as is common in such problems.

So final for Problem 1:

Genotypic ratio: 1 SsWw : 1 Ssww : 1 ssWw : 1 ssww

Phenotypic ratio: 1 spotted wooly : 1 spotted non-wooly : 1 non-spotted wooly : 1 non-spotted non-wooly

But let’s confirm the woman’s genotype. The problem says: “a wooly-haired, non-spotted woman”. In genetics, if a person shows a dominant trait and we don’t know their genotype, and we’re doing a test cross or something, but here it’s not specified. However, in the context of a dihybrid cross worksheet, and since the man is specified as heterozygous for one trait, likely the woman is intended to be heterozygous for the other to make it a proper dihybrid analysis.

I think it’s safe to proceed with woman = ssWw.

So answer for 1:

Genotypic ratio: 1:1:1:1 (SsWw : Ssww : ssWw : ssww)

Phenotypic ratio: 1:1:1:1 (spotted wooly : spotted non-wooly : non-spotted wooly : non-spotted non-wooly)

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Problem 2: Horse coat color and gait

Given:
- Black (B) dominant over chestnut (b)
- Trotting gait (T) dominant over pacing (t)

Cross: homozygous black pacer × homozygous chestnut trotter

Homozygous black pacer: BB tt (black is dominant, so BB; pacer is recessive, so tt)

Homozygous chestnut trotter: bb TT (chestnut is recessive, so bb; trotter is dominant, and homozygous, so TT)

Gametes:

Parent 1 (BBtt): all gametes B t

Parent 2 (bbTT): all gametes b T

Offspring: all Bb Tt

Phenotype: since B is dominant → black; T is dominant → trotting gait

So all F1 generation are black trotters.

Appearance: all black with trotting gait.

No ratios needed since all same.

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Problem 3: Snapdragon flowers — incomplete dominance and height

Given:
- Red color not completely dominant over white → so incomplete dominance: RR = red, Rr = pink, rr = white
- Tall plants dominant over short: T_ = tall, tt = short

Cross: homozygous tall red snapdragon × short white plant

Homozygous tall red: since tall is dominant, homozygous tall = TT; red = RR (since incomplete dominance, red is homozygous)

Short white: short = tt; white = rr (since white is recessive in incomplete dominance)

So parents: TT RR × tt rr

Gametes:

Parent 1: T R

Parent 2: t r

Offspring: all Tt Rr

Phenotype: Tt = tall (since tall dominant); Rr = pink (incomplete dominance)

So all offspring are tall and pink.

Genotypic ratio: all TtRr → 100% TtRr

Phenotypic ratio: all tall pink → 100%

But the question asks for genotypic and phenotypic ratios. Since all are same, ratio is 1 (or 100%) for that one type.

In ratio terms, we can say genotypic ratio: all TtRr

Phenotypic ratio: all tall pink

But typically, we write ratios as numbers. Since only one type, it's 1:0 or just state all are same.

In such cases, we say genotypic ratio: 100% TtRr

Phenotypic ratio: 100% tall pink

Or in ratio form, since only one category, it's fine to say "all" or "1".

But let's see what the problem expects. It says "give genotypic and phenotypic ratios", so for consistency, we can write:

Genotypic ratio: 1 TtRr

Phenotypic ratio: 1 tall pink

Meaning all offspring are that.

Now, to box the answers.

But let me summarize each.

For Problem 1, I assumed woman is heterozygous for wooly hair. Is there another way? Perhaps the problem intends her to be homozygous? Let me think again.

Re-reading: "a wooly-haired, non-spotted woman". In some contexts, if not specified, and it's a dominant trait, they might assume homozygous, but that would make the cross less interesting. However, let's calculate both ways.

Case A: woman is ssWW

Then gametes: sW

Man: Ssww → gametes: Sw, sw

Offspring:

Sw x sW → Ss Ww

sw x sW → ss Ww

So genotypes: 1 SsWw : 1 ssWw

Phenotypes: 1 spotted wooly : 1 non-spotted wooly

Ratio 1:1 for both.

Case B: woman is ssWw → as before, 1:1:1:1

Which one is intended? Looking at the problem, it says "dihybrid crosses", and in dihybrid, often they want the 4x4 or at least variation in both traits. Here, if woman is WW, then no variation in hair type — all wooly. If she is Ww, then variation.

Moreover, in the directions, it says "follow the steps", and step 3 is determine parent's genotypes. For the woman, since it's not specified, perhaps we should note the ambiguity. But in standard educational materials, when they say "a person with dominant trait" in a cross like this, and the other parent is recessive for that trait, they often intend the dominant parent to be heterozygous to show segregation.

Additionally, in Problem 2 and 3, they specify "homozygous", so for Problem 1, since they didn't say homozygous for the woman's wooly hair, and said "heterozygous" for the man's spotted, likely the woman is not specified, so we must assume based on context.

I recall that in many similar problems, for example, "cross a heterozygous tall purple flower with a short white flower" — if purple is dominant, and not specified, they might mean heterozygous, but sometimes homozygous. To resolve, let's look at the expected answer format.

Perhaps I can provide the answer assuming she is heterozygous, as it's more comprehensive.

Another thought: the man is "heterozygous spotted, non-wooly" — so for spotted, he's Ss, for wooly, he's ww (recessive).

The woman is "wooly-haired, non-spotted" — non-spotted is ss (recessive), wooly-haired is dominant, so genotype could be WW or Ww. But since the problem is from a dihybrid cross section, and they want students to practice making Punnett squares with multiple gamete types, likely they intend for both parents to produce two types of gametes, so woman should be Ww.

Yes, I think that's it. So I'll go with that.

So for Problem 1:

Genotypic ratio: 1 SsWw : 1 Ssww : 1 ssWw : 1 ssww

Phenotypic ratio: 1 spotted wooly : 1 spotted non-wooly : 1 non-spotted wooly : 1 non-spotted non-wooly

For Problem 2:

All F1 are black trotters. Genotypic ratio: all BbTt. Phenotypic ratio: all black trotting.

For Problem 3:

All offspring are TtRr, phenotype tall pink. Genotypic ratio: all TtRr. Phenotypic ratio: all tall pink.

Now to write the final answers clearly.

Final Answer:

1. Genotypic ratio: 1 SsWw : 1 Ssww : 1 ssWw : 1 ssww
Phenotypic ratio: 1 spotted wooly : 1 spotted non-wooly : 1 non-spotted wooly : 1 non-spotted non-wooly

2. All F1 offspring are black with trotting gait. Genotypic ratio: 100% BbTt. Phenotypic ratio: 100% black trotter.

3. All offspring are tall with pink flowers. Genotypic ratio: 100% TtRr. Phenotypic ratio: 100% tall pink.

But to be precise with ratios, for 2 and 3, since only one type, we can say the ratio is 1 for that genotype/phenotype.

In genetics, when all are same, we often say "all" or give the ratio as 1.

For consistency, I'll write:

For 2: Genotypic ratio: 1 BbTt
Phenotypic ratio: 1 black trotter

Similarly for 3.

But technically, ratio implies comparison, so if only one, it's fine.

Some write "uniform" or "all".

I think for this level, stating "all" is acceptable, but since the question asks for ratios, I'll use 1.

Let me check standard practice.

In many worksheets, for monohybrid cross of homozygous dominant x recessive, they say genotypic ratio all heterozygous, phenotypic ratio all dominant.

So for reporting, we can say:

Problem 2:
Genotypic ratio: 100% BbTt or simply "all BbTt"
Phenotypic ratio: all black trotters

But to match the format, perhaps:

After careful consideration, here are the answers:

Final Answer:

1. Genotypic ratio: 1 SsWw : 1 Ssww : 1 ssWw : 1 ssww
Phenotypic ratio: 1 spotted, wooly : 1 spotted, non-wooly : 1 non-spotted, wooly : 1 non-spotted, non-wooly

2. Genotypic ratio: 1 BbTt
Phenotypic ratio: 1 black, trotting gait

3. Genotypic ratio: 1 TtRr
Phenotypic ratio: 1 tall, pink

Note: For problems 2 and 3, since all offspring are identical, the ratio is expressed as 1 for the single genotype and phenotype observed.