OSMIA LEARNING · GENETICS · ANSWER KEY

Punnett Square Practice — Answer Key

Topic: Monohybrid inheritance Grade band: 7–10 NGSS*: MS-LS3-2, HS-LS3-3

Teacher copy. Worked solutions; accept reasonable student wording on open-response items. Learning target labels show the focus of each section.

Part A — Vocabulary & genotypes Vocabulary · Determine genotypes

1. Black (B) dominant, brown (b) recessive.

a. Homozygous black = BB | b. Heterozygous black = Bb | c. Brown = bb

2. White-flower pea plant genotype.

Must be pp. White is recessive, so the only way to show the recessive phenotype is to carry two recessive alleles (homozygous recessive). (Determine genotypes)

3. Definitions.

Genotype: the alleles an organism carries (e.g., Bb). Phenotype: the observable trait that results (e.g., black fur). (Vocabulary)

Part B — Build the square Build the square · Predict ratios

4. Bb × Bb.

	B	b
B	BB	Bb
b	Bb	bb

Genotype ratio = 1 BB : 2 Bb : 1 bb | Phenotype ratio (black : brown) = 3 : 1. (Build the square, Predict ratios)

5. Bb × bb.

	b	b
B	Bb	Bb
b	bb	bb

Percent brown = 50% (2 of 4 are bb). (Build the square, Predict ratios)

* NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it. NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press.

Part C — Predict & reason Predict ratios · Interpret & reason

6. PP × pp.

	p	p
P	Pp	Pp
P	Pp	Pp

All offspring are Pp (heterozygous), phenotype purple. (Predict ratios)

7. Four offspring, none brown — did the breeder err?

No. The 3 : 1 ratio is a probability, not a guarantee. With only 4 offspring, getting zero brown is a normal chance outcome (each baby independently has a 1/4 chance of being brown). Larger numbers trend closer to the predicted ratio. (Interpret & reason)

8. Test cross for an unknown black rabbit.

Cross the black rabbit with a brown (bb) rabbit. If any offspring are brown (bb), the black parent must be Bb (it passed a recessive allele). If all offspring are black, it is likely BB — though Bb cannot be fully ruled out with a small number of offspring. (Interpret & reason)

9. Why can two Bb parents produce a brown offspring?

Each Bb parent can pass on the recessive b allele. When both parents pass b, the offspring is bb (brown). This occurs in 1 of 4 boxes, so ~25% of offspring are expected to be brown even though both parents are black. (Interpret & reason)