Worksheet on dominant and recessive alleles using mouse fur color genetics.

Let’s work through this step by step.

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Experiment 1: White Mice Family

- Father: white
- Mother: white
- All babies: white
- Both parents had parents who were white → meaning their grandparents were also white.

This suggests that white is likely a recessive trait, because if it were dominant, we might see variation — but here, everything is white across generations. Also, since both parents and all offspring are white, and no other color appears, they’re probably homozygous for the white allele.

Let’s assign letters:
- Let’s say W = dominant allele (gray)
- Let’s say w = recessive allele (white)

If white is recessive, then to be white, a mouse must have genotype ww.

So:

1. Phenotype: White → Genotype: ww
2. Phenotype: White → Genotype: ww
3. Phenotype: White → Genotype: ww
4. Phenotype: White → Genotype: ww
5. Phenotype: White → Genotype: ww
6. Phenotype: White → Genotype: ww

7. Is this a “true-breeding” family?
→ Yes. Because when two white mice (ww x ww) breed, they only produce white offspring (ww). No variation. True-breeding means always producing offspring with the same traits as parents.

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Experiment 2: Gray Mice Family

- Father: gray
- Mother: gray
- All babies: gray
- Both parents had parents who were gray → again, consistent over generations.

Since gray appeared in Experiment 3 when crossed with white, and all offspring were gray, that suggests gray is dominant.

So gray mice here are likely homozygous dominant: WW

8. Phenotype: Gray → Genotype: WW
9. Phenotype: Gray → Genotype: WW
10. Phenotype: Gray → Genotype: WW
11. Phenotype: Gray → Genotype: WW
12. Phenotype: Gray → Genotype: WW
13. Phenotype: Gray → Genotype: WW

14. Is this a “true-breeding” family?
→ Yes. WW x WW → all WW → all gray. No variation. True-breeding.

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Experiment 3: Cross between white progeny from Exp 1 and gray progeny from Exp 2

- White parent: ww (from Exp 1)
- Gray parent: WW (from Exp 2)
- All babies: gray

Punnett Square:

W W
+---------
w | Ww Ww
w | Ww Ww

All offspring are Ww → which means they have one dominant (gray) and one recessive (white) allele. Since gray is dominant, they show gray phenotype.

15. Phenotype: White → Genotype: ww
16. Phenotype: Gray → Genotype: WW
17. Phenotype: Gray → Genotype: Ww
18. Phenotype: Gray → Genotype: Ww
19. Phenotype: Gray → Genotype: Ww
20. Phenotype: Gray → Genotype: Ww

21. Which allele is dominant? Gray fur.
Why? Because when you cross white (ww) with gray (WW), all offspring are gray. That means the gray allele masks the white allele — so gray is dominant.

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Question 22: Whisker length experiment

- Two true-breeding mice: one with long whiskers, one with short whiskers.
- All progeny have short whiskers.

True-breeding means homozygous.

Let’s say:
- Long whiskers = LL (if dominant) or ll (if recessive)
- Short whiskers = SS or ss

But all offspring have short whiskers → so short must be dominant.

Because if short were recessive, and you crossed true-breeding long (say LL) with true-breeding short (ll), you’d get all Ll — which would show long if long is dominant. But here, all show short → so short must be dominant.

So:
- Short whiskers = dominant → let’s call it S
- Long whiskers = recessive → s

Parents: SS (short) x ss (long) → all offspring Ss → show short.

Therefore:
→ Dominant allele: short whiskers
→ Recessive allele: long whiskers

How do we know? Because when you cross two purebred parents with different traits, and all offspring show only one trait, that trait is dominant.

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Final Answer:

1. Phenotype: White
2. Genotype: ww
3. Phenotype: White
4. Genotype: ww
5. Phenotype: White
6. Genotype: ww
7. Yes, it is true-breeding because all offspring are white like the parents, showing no variation.
8. Phenotype: Gray
9. Genotype: WW
10. Phenotype: Gray
11. Genotype: WW
12. Phenotype: Gray
13. Genotype: WW
14. Yes, it is true-breeding because all offspring are gray like the parents, showing no variation.
15. Phenotype: White
16. Genotype: ww
17. Phenotype: Gray
18. Genotype: WW
19. Phenotype: Gray
20. Genotype: Ww
21. The allele for gray fur is dominant. When a white mouse (ww) is bred with a gray mouse (WW), all offspring are gray (Ww), meaning the gray allele masks the white allele.
22. Short whiskers are dominant; long whiskers are recessive. When two true-breeding parents (one long, one short) are crossed and all offspring have short whiskers, the short trait must be dominant because it appears in all offspring while the long trait disappears.