Inheritance, Probability & Punnett Squares — Practice · Grade 7 Science

Chapter: Genes, Cell Division & Heredity

What this practice covers

By the end of Inheritance, Probability & Punnett Squares you should be able to:

8 questions, written answer, multiple choice and numeric, each with a worked explanation.

Vocabulary used: genotype, heterozygous, homozygous, dominance, probability.

Problem 1

What can an Aa parent contribute at the studied region?
Answer and explanation

Either A or a, one version per gamete in the stipulated model.

Problem 2

Why are Aa and aA not separate phenotype categories by letter order?
Answer and explanation

They contain the same two allele versions; the written order does not create a different genotype category in this model.

Problem 3

Does dominance make an Aa parent transmit A more than half the time in this model?
Answer and explanation

No. Dominance concerns phenotype expression, while the supplied gamete probabilities remain one half A and one half a.

Problem 4

Must four offspring contain exactly one aa individual?
Answer and explanation

No. One quarter is a probability for each event, not a fixed allocation in every group of four.

Problem 5

Illustrative cross: Aa×aa; A is completely dominant and gametes have equal modeled probabilities. List possible genotypes, calculate recessive phenotype probability, and explain why four offspring need not split exactly two and two.
Answer and explanation

The expected genotype probabilities are 50% Aa and 50% aa; the recessive phenotype probability is 50%. Four independent offspring need not contain exactly two of each. These predictions depend on the fictional single-region model and supplied dominance rule; they are not general predictions for complex human traits.

Aa contributes A or a; aa contributes a.

Combinations are Aa and aa with equal probabilities.

Apply the stipulated phenotype rule and separate probability from count.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

In an Aa×Aa model, a sample of four has no aa offspring. Does that alone prove the probability model wrong?
Answer and explanation

No. Small independent samples can differ from expected ratios. Examine more evidence and the assumptions before choosing between sampling variation and a different model.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 7

Which genotype is heterozygous at the fictional region?
  1. AA
  2. aa
  3. Aa
  4. A alone as a full diploid genotype
Answer and explanation

Aa

Aa contains two different versions at the studied region.

Problem 8

In the stipulated Aa×Aa complete-dominance model, what percent of offspring events are expected to be aa?
Answer and explanation

25 %

One of four equally probable combinations is aa: 1/4×100=25%.

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