Close-up of a short-haired guinea pig with a white and brown coat, highlighting its fluffy texture and gentle expression.

Let’s go step by step through each question. We’ll use basic genetics rules: dominant alleles are written with capital letters (like H for short hair), recessive with lowercase (h for long hair). Homozygous means two same alleles (HH or hh), heterozygous means one of each (Hh).

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Question 9:

> In guinea pigs, the allele for short hair is dominant to long hair.

So:
- Short hair = dominant → let’s call it H
- Long hair = recessive → must be hh (only way to show recessive trait)

What genotype would a heterozygous short-haired guinea pig have?
→ Heterozygous = one dominant, one recessive → Hh

What genotype would a homozygous short-haired guinea pig have?
→ Homozygous dominant → HH

What genotype would a long-haired guinea pig have?
→ Must be homozygous recessive → hh

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Show the cross for two heterozygous guinea pigs.

Both parents: Hh × Hh

Punnett Square:

```
| H | h
-----|------|------
H | HH | Hh
-----|------|------
h | Hh | hh
```

Offspring genotypes:
- HH → short hair
- Hh → short hair (since H is dominant)
- hh → long hair

Phenotypes:
- 3 out of 4 → short hair (HH, Hh, Hh)
- 1 out of 4 → long hair (hh)

What percentage of the offspring will have short hair? → 75%

Long hair? → 25%

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Question 10:

> Two guinea pigs only hairy is dominant to straight. Show the cross between two homozygous curly haired guinea pigs.

Wait — this seems like a typo. Probably meant: “Curly hair is dominant to straight hair.” And “two homozygous curly-haired” guinea pigs.

Assume:
- Curly hair = dominant → C
- Straight hair = recessive → cc

Homozygous curly = CC

Cross: CC × CC

All offspring: CC

So all will have curly hair.

What percentage of the offspring will have curly hair? → 100%

*(Note: If the problem meant something else, like “hairy” vs “straight”, but since it says “curly” later, we assume curly is the trait. Also, “only hairy” might be a misprint — likely “curly hair is dominant”.)*

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Question 11:

> A guinea pig with short hair is crossed to one that has long hair. Over time, they produce many offspring... 44 short hair, 46 long hair. What are the genotypes of the parents?

Short hair parent: could be HH or Hh
Long hair parent: must be hh (recessive)

If short hair parent were HH, then all offspring would be Hh → all short hair. But here, almost half are long hair → so short hair parent must be Hh

Cross: Hh × hh

Punnett Square:

```
| H | h
-----|------|------
h | Hh | hh
-----|------|------
h | Hh | hh
```

Offspring:
- 50% Hh → short hair
- 50% hh → long hair

Observed: 44 short, 46 long → very close to 50/50 → fits!

✔ So parents’ genotypes:
- Short hair parent: Hh
- Long hair parent: hh

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Question 12:

> Another laboratory crosses a short-haired guinea pig with a long-haired one, and over several years, not a single offspring has long hair. Explain.

Short hair parent: could be HH or Hh
Long hair parent: must be hh

If NO long-haired offspring ever appear, that means the short-haired parent must be homozygous dominant (HH)

Because:

HH × hh → all offspring Hh → all short hair

If it were Hh × hh, you’d expect ~50% long hair eventually — but none appeared in many years → so must be HH.

✔ Explanation: The short-haired guinea pig is homozygous dominant (HH), so all offspring inherit at least one H allele and thus have short hair.

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Final Answer:

9.
- Heterozygous short-haired: Hh
- Homozygous short-haired: HH
- Long-haired: hh
- Cross: Hh × Hh → Punnett square shows 3 short : 1 long
- % short hair: 75%
- % long hair: 25%

10.
- Cross: CC × CC → all offspring CC
- % curly hair: 100%

11.
- Parents: Hh (short hair) and hh (long hair)
- Reason: Offspring ratio ~1:1 matches test cross of heterozygote with recessive.

12.
- The short-haired parent must be HH (homozygous dominant), because if it were Hh, some long-haired (hh) offspring would appear. Since none did, it must be HH.