Genetics Practice Problems - AP Biology (KEY) by Biologycorner | TPT - Free Printable

Let’s solve this genetics problem step by step.

We’re told that in guinea pigs, the gene for coat color has two alleles:
- B = black (dominant)
- b = white (recessive)

Also, we’re told that when a homozygous black guinea pig is crossed with a homozygous white one, all offspring are black. That makes sense — because black is dominant, so even if they have one B and one b, they’ll look black.

Now, let’s go question by question.

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Question 1:
> A pure breeding black guinea pig is crossed with a pure breeding white guinea pig. What will be the genotype of the F1 generation?

Pure breeding means homozygous.
- Pure black = BB
- Pure white = bb

Cross: BB × bb

All offspring get one allele from each parent → all get B from black parent, b from white parent → all are Bb

✔ Genotype of F1: Bb

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Question 2:
> If two F1 guinea pigs are mated, what phenotypic ratio would you expect in the F2 generation?

F1 are both Bb.

So cross: Bb × Bb

Possible offspring genotypes:
- BB (black)
- Bb (black)
- Bb (black)
- bb (white)

That’s 3 black : 1 white

✔ Phenotypic ratio: 3 black : 1 white

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Question 3:
> In humans, curly hair (C) is dominant over straight hair (c). A woman who is heterozygous for curly hair marries a man with straight hair. What percentage of their children will have curly hair?

Woman: Cc (heterozygous curly)
Man: cc (straight hair — must be homozygous recessive to show straight)

Cross: Cc × cc

Offspring:
- Cc → curly
- cc → straight

Each child has 50% chance of getting C from mom → 50% curly, 50% straight

✔ Percentage with curly hair: 50%

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Question 4:
> In rabbits, short fur (S) is dominant over long fur (s). Two short-furred rabbits produce some long-furred offspring. What must be the genotypes of the parents?

If they produced long-furred offspring (ss), then BOTH parents must have given an s allele.

But both parents have short fur → so they must be S_ (at least one S).

To give an s allele and still have short fur → they must be Ss

✔ Parents’ genotypes: Ss and Ss

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Question 5:
> In dogs, wire hair (W) is dominant over smooth hair (w). A wire-haired dog is crossed with a smooth-haired dog. Half the puppies are wire-haired, half are smooth-haired. What is the genotype of the wire-haired parent?

Smooth-haired dog must be ww (only way to show smooth).

Wire-haired parent could be WW or Ww.

But offspring are 50% wire, 50% smooth → that only happens if wire parent is Ww

Because:
- Ww × ww → 50% Ww (wire), 50% ww (smooth)

If it were WW × ww → all would be Ww → all wire → doesn’t match.

✔ Genotype of wire-haired parent: Ww

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Question 6:
> In pea plants, tall (T) is dominant over short (t). A tall plant is crossed with a short plant. All offspring are tall. What is the genotype of the tall parent?

Short plant = tt

Tall plant = ?

All offspring are tall → so none are tt → meaning the tall parent did NOT give any t allele → so must be TT

Because:
- TT × tt → all Tt → all tall ✔
- If it were Tt × tt → half would be tt (short) → not matching

✔ Genotype of tall parent: TT

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Question 7:
> In mice, gray fur (G) is dominant over white fur (g). A gray mouse is crossed with a white mouse. The litter includes both gray and white pups. What is the genotype of the gray parent?

White mouse = gg

Gray mouse = ?

Since there are white pups (gg), they must have gotten g from both parents → so gray parent must have given a g → so gray parent must be Gg

Because:
- Gg × gg → 50% Gg (gray), 50% gg (white) ✔

If gray parent were GG → all offspring would be Gg → all gray → no white → doesn’t match.

✔ Genotype of gray parent: Gg

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Question 8:
> In fruit flies, red eyes (R) are dominant over white eyes (r). A red-eyed fly is crossed with a white-eyed fly. All offspring have red eyes. What is the genotype of the red-eyed parent?

White-eyed fly = rr

Red-eyed fly = ?

All offspring red → so none are rr → meaning red parent didn’t give any r → so must be RR

Because:
- RR × rr → all Rr → all red ✔
- If Rr × rr → half would be rr (white) → not matching

✔ Genotype of red-eyed parent: RR

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Final Answer:

1. Bb
2. 3 black : 1 white
3. 50%
4. Ss and Ss
5. Ww
6. TT
7. Gg
8. RR