Biology worksheet illustrating Mendelian genetics using fish back fins, complete with Punnett square exercises and genotype determination.

You're absolutely right to focus on the circled Punnett square — it’s the key to solving this genetics problem correctly. Let’s walk through the entire solution step-by-step, correcting any misunderstandings and explaining why the circled square is the only one that fits the observed offspring.

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## 🐟 Problem Summary:

We have a family of fish with two types of back fins:
- 2 back fins (dominant phenotype)
- 1 back fin (recessive phenotype)

There are 2 adult fish (parents) and 8 baby fish (offspring).
From the image:
- 6 babies have 2 back fins
- 2 babies have 1 back fin

This gives us a 3:1 phenotypic ratio — a classic Mendelian ratio for a heterozygous cross.

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## ✔ Step-by-Step Solution:

a. What is the dominant phenotype?

> Answer: 2 back fins

✔ *Explanation:* The dominant trait is the one that appears more frequently in the offspring when both parents show the same dominant phenotype. Also, since some offspring show the “1 back fin” trait while both parents have “2 back fins,” the “2 back fins” must be dominant — because recessive traits can only appear if both parents carry the recessive allele.

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b. What is the recessive phenotype?

> Answer: 1 back fin

✔ *Explanation:* This trait only appears when an individual has two copies of the recessive allele (ff). Since it appears in some offspring even though both parents have 2 fins, it must be recessive.

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c. What are the two possible genotypes of each adult fish?

> Answer: FF and Ff

✔ *Explanation:* Since both adults show the dominant phenotype (2 back fins), they could be either homozygous dominant (FF) or heterozygous (Ff). We’ll figure out which one they actually are using the offspring ratios.

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d. How many baby fish have the dominant phenotype? How many have the recessive phenotype?

> Answer: 6 have dominant phenotype (2 back fins), 2 have recessive phenotype (1 back fin)

✔ *Confirmed by counting the fish in the image.*

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

The question asks you to complete Punnett squares assuming the upper adult fish could be either FF or Ff, and then test against the lower adult fish (which we assume also has 2 fins → so genotype FF or Ff).

But here’s the catch: we need to find which combination produces 6 dominant : 2 recessive offspring (i.e., 3:1 ratio).

Let’s evaluate all possibilities:

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#### ➤ Punnett Square 1: FF x FF
```
| F | F
----|-----|-----
F | FF | FF
F | FF | FF
```
→ All offspring = FF → ALL 8 have 2 back fins → ✘ Does NOT match observed 6:2 ratio.

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#### ➤ Punnett Square 2: FF x Ff
```
| F | F
----|-----|-----
F | FF | FF
f | Ff | Ff
```
→ All offspring = FF or Ff → ALL 8 have 2 back fins → ✘ Still no recessive phenotype.

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#### ➤ Punnett Square 3: Ff x Ff ← THIS IS THE CIRCLED ONE!
```
| F | f
----|-----|-----
F | FF | Ff
f | Ff | ff
```
→ Genotype ratio: 1 FF : 2 Ff : 1 ff
→ Phenotype ratio: 3 dominant (FF + Ff) : 1 recessive (ff)

✔ So out of 8 offspring:
- 6 should have 2 back fins (dominant)
- 2 should have 1 back fin (recessive)

✔️ Perfect match to the observed data!

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#### ➤ Punnett Square 4: Ff x ff (if lower parent were recessive — but it’s not!)
```
| F | f
----|-----|-----
f | Ff | ff
f | Ff | ff
```
→ 50% dominant, 50% recessive → 4:4 ratio → ✘ Doesn’t match 6:2.

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

> Circle the Ff x Ff square (the circled one in your image)

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## ✔ g. What are the genotypes of the parents?

> Answer: Both parents are Ff

✔ *Explanation:* Only when both parents are heterozygous (Ff) do you get the 3:1 phenotypic ratio (6 dominant : 2 recessive) in the offspring.

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## ✔ h. Why are the other Punnett Squares not possible for this family?

> Answer:
The only Punnett square that produces a 3:1 phenotypic ratio (6 with 2 back fins, 2 with 1 back fin) is the Ff x Ff cross.
All other combinations (FF x FF, FF x Ff, Ff x ff) produce either:
- 100% dominant offspring (no 1-fin fish), OR
- 50% recessive (which would mean 4 babies with 1 fin — not 2).

Since we observe exactly 2 babies with 1 back fin, only the Ff x Ff cross matches the data.

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## 🔚 Final Answer Summary:

a. Dominant phenotype: 2 back fins
b. Recessive phenotype: 1 back fin
c. Possible genotypes of each adult: FF and Ff
d. Baby fish: 6 dominant, 2 recessive
e. Correct Punnett Square: Ff x Ff (the circled one)
f. Circle the Ff x Ff square
g. Genotypes of parents: Ff and Ff
h. Other squares don’t produce the 3:1 ratio seen in offspring.

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✔ You’ve got it right — the circled Punnett square (Ff x Ff) is the only correct one based on the offspring data!

Let me know if you want to visualize the fish genotypes or draw the full family tree! 🐠