GENETICS PRACTICE 1: BASIC MENDELIAN GENETICS - Quia - Free Printable

Let’s solve each problem step by step.

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Problem 1: Albinism (recessive trait)

We’re told:
- Allele for albinism = a (recessive)
- Allele for normal skin = A (dominant)
- Both parents are heterozygous → that means both are Aa

We make a Punnett square:

```
A a
A AA Aa
a Aa aa
```

Out of 4 possible outcomes:
- AA → normal pigment (homozygous dominant)
- Aa → normal pigment (heterozygous carrier)
- aa → albino (homozygous recessive)

So:
- Normal pigment: AA + Aa + Aa = 3 out of 4 → 75%
- Albino: aa = 1 out of 4 → 25%

Now, part (a):
*If the child is normal, what is the chance it is a carrier?*

“Normal” children are those with genotype AA or Aa. From the Punnett square, there are 3 normal children:
- 1 is AA (not a carrier)
- 2 are Aa (carriers)

So, among normal children, 2 out of 3 are carriers → 2/3 ≈ 67%

✔ This matches the answer shown in the image.

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Problem 2: Purple people eaters — horns

Given:
- One horn (H) is dominant
- No horn (h) is recessive
- Parent 1: heterozygous → Hh
- Parent 2: no horns → must be hh (since no horn is recessive)

Punnett square:

```
H h
h Hh hh
h Hh hh
```

Offspring:
- Hh → has horn (horned) → 2 out of 4 → 50%
- hh → no horn → 2 out of 4 → 50%

Genotypes: 50% Hh, 50% hh
Phenotypes: 50% horned, 50% no horn

✔ Matches the image.

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Problem 3: Eye color in humans

Given:
- Brown eyes (B) dominant
- Blue eyes (b) recessive
- Both parents are heterozygous → Bb

Punnett square:

```
B b
B BB Bb
b Bb bb
```

Offspring genotypes:
- BB → brown eyes → 1 out of 4 → 25%
- Bb → brown eyes → 2 out of 4 → 50%
- bb → blue eyes → 1 out of 4 → 25%

Phenotypes:
- Brown-eyed: BB + Bb = 3 out of 4 → 75%
- Blue-eyed: bb = 1 out of 4 → 25%

The image says:
> 75% chance of brown-eyed
> 25% chance of blue-eyed
> 25% chance of BB
> 50% chance of Bb
> 25% chance of bb

That’s correct!

Note: The way it’s written might look like “25% chance of blue-eyed” and then “25% chance of BB” etc., but actually, the phenotype ratios are 75% brown, 25% blue. The genotype breakdown is separate: 25% BB, 50% Bb, 25% bb.

All answers in the image are accurate.

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Final Answer:
All three problems have been solved correctly as shown in the worksheet.
1. Normal pigment: 75%, Albino: 25%; If normal, chance of being carrier: 2/3 or ~67%
2. Offspring: 50% Hh (horned), 50% hh (no horn)
3. Phenotype: 75% brown-eyed, 25% blue-eyed; Genotype: 25% BB, 50% Bb, 25% bb