Basic Genetics Practice Problems - Free Printable

Let’s go step by step through each question. We’ll use the Punnett squares shown and match them to the results described.

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Question 1:
He crossed a red flowered R plant with a white flowered r plant.
Results: 126 red, 122 white → almost 50% red, 50% white.

That means half the offspring are Rr (red) and half are rr (white).
Look at the Punnett squares:

- Square #4 shows: Rr, rr, Rr, rr → that’s 2 red, 2 white → 50/50 split. ✔

So, Punnett square #4 best fits.

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Question 2:
Red flowered plant × white flowered plant → 307 red, 0 white → ALL red.

That means all offspring must have at least one R allele → so they’re all Rr or RR.

But since one parent is white (rr), it can only give “r” alleles. So for all kids to be red, the red parent must be RR (so gives only R), and then all kids are Rr → red.

Which square shows that? Look at square #1:
Parents: R and R on top, r and r on side → all offspring are Rr → all red. ✔

Wait — but in square #1, the parents are labeled as R and R on top? Actually, looking again:

In square #1: Top row has R and R, left column has r and r → so cross is RR × rr → all Rr → all red. Yes!

So, Punnett square #1 best fits.

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Question 3:
Red × Red → 306 red, 110 white → roughly 3:1 ratio.

Classic Mendelian ratio for heterozygous cross: Rr × Rr → 3 red : 1 white.

Check square #3: Parents are R and r on top, R and r on side → that’s Rr × Rr.

Offspring: RR, Rr, Rr, rr → 3 red, 1 white → perfect match. ✔

So, Punnett square #3 best fits.

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Question 4:
Red × Red → 300 red, 0 white → ALL red.

So both parents must be homozygous dominant? Or maybe one is RR and other is Rr? But if any parent contributes an r, and the other also might, you could get rr.

To guarantee NO white flowers, neither parent can contribute an r unless the other always gives R.

Best case: both parents are RR → all kids RR → all red.

Or one is RR and other is Rr → still all kids get at least one R → all red.

But look at the squares:

Square #2: Parents are R and R on top, R and r on side → so RR × Rr.

Offspring: RR, RR, Rr, Rr → all red. ✔

Yes! That matches.

So, Punnett square #2 best fits.

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Now Part 2: Mink coat color.

B = brown (dominant), b = silverblue (recessive)

Cross: homozygous brown mink × silverblue mink.

Homozygous brown = BB
Silverblue = bb (since it’s recessive, must be homozygous)

Cross: BB × bb

All offspring get B from first parent, b from second → all are Bb.

Since B is dominant, all will show brown coat.

The problem says: “There were 9 offspring... All 9 would be ______.”

Answer: brown

And the Punnett square shown confirms: all boxes say Bb → all brown.

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Final Answers:

1. Punnett square #4
2. Punnett square #1
3. Punnett square #3
4. Punnett square #2
Part 2: All 9 offspring would be brown

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Final Answer:
1. 4
2. 1
3. 3
4. 2
Part 2: brown