Genetics worksheet with fish illustrating dominant and recessive traits and Punnett squares.

Let's go through the problem step by step and solve it thoroughly, especially focusing on Part e (the Punnett squares) and the reasoning behind the correct answer.

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🔍 Problem Overview:

We are analyzing a family of fish in a tank. The trait being studied is the number of back fins, which follows Mendelian inheritance.

- Dominant phenotype: 2 back fins
- Recessive phenotype: 1 back fin

We are given:
- Two adult fish (parents)
- 8 baby fish (offspring):
- 6 with 2 back fins (dominant phenotype)
- 2 with 1 back fin (recessive phenotype)

We need to determine:
- The genotypes of the parents
- Which Punnett square matches the observed offspring ratio

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✔ Answering Each Part

#### a. What is the dominant phenotype?
Answer: 2 back fins
> Because this trait appears more frequently and can be expressed even if only one allele is present.

#### b. What is the recessive phenotype?
Answer: 1 back fin
> This only shows up when no dominant allele is present (i.e., homozygous recessive).

#### c. What are the two possible genotypes of each adult fish?
Answer: FF and Ff
> Since both adults have 2 back fins (dominant phenotype), they could either be:
> - Homozygous dominant (FF) or
> - Heterozygous (Ff)

But we’ll narrow it down based on offspring.

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#### d. How many baby fish have the dominant vs. recessive phenotype?
Answer:
- 6 baby fish have the dominant phenotype (2 back fins)
- 2 baby fish have the recessive phenotype (1 back fin)

This gives us a 3:1 ratio (6:2 = 3:1), which is classic for a heterozygous cross.

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#### e. Complete both Punnett Squares below, one each for all possible genotypes of the upper adult fish.

There are four Punnett squares shown:

Let’s analyze each one:

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🔹 Punnett Square 1: FF × FF

| | F | F |
|-------|-------|-------|
| F | FF | FF |
| F | FF | FF |

→ All offspring: FF → All have 2 back fins
→ No recessive offspring → ✘ Not possible, because we see 2 with 1 back fin.

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🔹 Punnett Square 2: FF × Ff

| | F | F |
|-------|-------|-------|
| F | FF | FF |
| f | Ff | Ff |

→ Offspring:
- 50% FF (2 fins)
- 50% Ff (2 fins)
→ All have 2 fins → No 1-finned babies → ✘ Not possible

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🔹 Punnett Square 3: ff × ff

| | f | f |
|-------|-------|-------|
| f | ff | ff |
| f | ff | ff |

→ All offspring: ff → All have 1 back fin → ✘ Not possible, since 6 have 2 fins.

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🔹 Punnett Square 4: Ff × Ff ← THIS IS THE CORRECT ONE

| | F | f |
|-------|-------|-------|
| F | FF | Ff |
| f | Ff | ff |

→ Offspring:
- FF: 1/4 → 2 fins
- Ff: 2/4 → 2 fins
- ff: 1/4 → 1 fin

So expected ratio:
- 3/4 (75%) have 2 back fins
- 1/4 (25%) have 1 back fin

Which matches our data:
- 6 out of 8 = 75% with 2 fins
- 2 out of 8 = 25% with 1 fin

✔ Perfect match!

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✔ So, the correct Punnett square is the last one (Ff × Ff).

You were right to circle it!

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f. Circle the Punnett Square that is correct for the fish shown here.

➡️ Circle the last Punnett Square (Ff × Ff)

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g. What are the genotypes of the parents?

Answer: Ff and Ff

> Both parents must be heterozygous (Ff) to produce:
> - 75% with 2 fins (FF or Ff)
> - 25% with 1 fin (ff)

If either parent were FF, you couldn’t get any ff offspring.

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h. Why are the other Punnett squares not possible for this family?

Answer:

The only possible Punnett square is the last one (Ff × Ff), since there is a 75% probability to produce 2-back-fin offspring and a 25% probability to produce 1-back-fin offspring.

The other squares cannot be considered because:
- FF × FF produces only FF → no recessive phenotype → ✘
- FF × Ff produces only Ff or FF → all have 2 fins → ✘
- ff × ff produces only ff → all have 1 fin → ✘

Only Ff × Ff produces both phenotypes in the correct 3:1 ratio.

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🎯 Final Summary:

| Question | Answer |
|--------|--------|
| a. Dominant phenotype | 2 back fins |
| b. Recessive phenotype | 1 back fin |
| c. Possible genotypes | FF and Ff |
| d. Offspring | 6 dominant, 2 recessive |
| e. Correct Punnett Square | Ff × Ff (last one) |
| f. Circle | Last square |
| g. Parent genotypes | Ff and Ff |
| h. Reason | Only Ff × Ff gives 3:1 ratio; others don't allow for recessive offspring |

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✔ Conclusion: You did everything correctly — especially identifying the correct Punnett square and explaining why the others don’t work. Great job!