Mendelian Genetics — Practice · Grade 10 Science

Chapter: DNA, Inheritance & Gene Expression

What this practice covers

By the end of Mendelian Genetics you should be able to:

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

Vocabulary used: allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, Punnett square.

Problem 1

An organism that is homozygous for a trait has
  1. no alleles for that trait
  2. two identical alleles (for example BB or bb)
  3. two different alleles (Bb)
  4. only one allele in total
Answer and explanation

two identical alleles (for example BB or bb)

Homozygous means the two alleles match (BB or bb); heterozygous (Bb) means they differ.

Problem 2

If B (brown) is dominant over b (white), an organism with genotype Bb will have which phenotype?
  1. A blend of brown and white
  2. It cannot be determined
  3. Brown
  4. White
Answer and explanation

Brown

One dominant B is enough to produce the brown phenotype, so Bb is brown.

Problem 3

In a cross between two Bb parents (B dominant), what percent of the offspring are expected to show the recessive phenotype?
Answer and explanation

25 %

Bb x Bb gives BB, Bb, Bb, bb — 1 of the 4 boxes (25%) is bb, the recessive phenotype.

Problem 4

In a hypothetical complete-dominance model, an organism with dominant phenotype could be BB or Bb. Explain a model test cross and the limit of a small sample.
Answer and explanation

Cross with bb under the model. Any bb offspring implies the tested parent supplied b. All dominant offspring in a small sample do not prove BB because Bb can produce such a sample by chance.

Problem 5

Two Tt parents (T dominant). The probability that two of their offspring in a row are both tt equals 1 out of what number?
Answer and explanation

16 (1 out of this many)

Each tt has probability 1/4; multiplying independent events, 1/4 x 1/4 = 1/16, so 1 out of 16.

Problem 6

In a hypothetical complete-dominance model, cross Aa with Aa. Find genotype probabilities. Explain the reasoning in your own words.
Answer and explanation

¼ AA, ½ Aa and ¼ aa; each offspring is a new probability event.

Each parent contributes A or a with probability one half.

The four equally likely combinations are AA, Aa, aA and aa.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

For Aa×Aa, find expected recessive aa probability. Show the reasoning, not just the conclusion.
Hints

Probabilities describe many outcomes, not a guaranteed pattern in every small family.

Answer and explanation

1/4=25%, combining a from each parent.

Problem 8

Can a dominant phenotype alone distinguish AA from Aa in this model?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Both produce the dominant phenotype; more genotype or family evidence is required.

Problem 9

An expected ratio does not dictate each finite familyAn expected ratio does not dictate each finite familyRecessive-category count / 80 offspringTrial family A15Trial family B25Trial family C20Trial family D17Expected mean under p=1/4: 20; actual samples can vary.
Original simulated family counts, not real breeding data or personal reproductive predictions.
Interpret the supplied visual. After three dominant-category outcomes, is a recessive-category fourth outcome guaranteed?
Answer and explanation

No. Each modeled independent outcome retains its probability; earlier outcomes do not force a balancing result.

A probability describes a model of possible outcomes, not a promised number in every small sample. For a recessive-category probability of 1/4 and 80 independent modeled offspring, the expected count is 20. Actual samples vary through chance, and unexpected counts require evaluation of sample size, model conditions and measurement rather than immediate rejection or certainty. Independent births in the model do not acquire a memory that forces the next child to balance earlier outcomes.

No. Each modeled independent outcome retains its probability; earlier outcomes do not force a balancing result.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

Under complete dominance, Aa×aa produces independent offspring with equal genotype probabilities. Find the expected aa count among 60 and the probability that the first three are all Aa.
Answer and explanation

Expected aa count is 60×1/2=30. The probability of three Aa outcomes is (1/2)³=1/8. Neither expectation forces a specific finite family outcome.

Expected aa count is 60×1/2=30. The probability of three Aa outcomes is (1/2)³=1/8. Neither expectation forces a specific finite family outcome.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

OpenSciEd chromosome symbols for Rob, the extra-muscular offspring in the cattle case; two copies have matching orange and blue symbols but different green allele-category shapes.
OpenSciEd’s selected chromosome-symbol model for Rob; shapes represent allele categories, not actual chromosome outlines or base sequences. Chromosome-symbol pair for Rob · OpenSciEd · 8.5 Genetics, Lesson 6 chromosome model, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Compare the two homolog-symbol copies at their orange, green and blue marked positions. Which categories match and differ? Explain what a drawn category difference does not establish about nucleotide sequence, dominance or the cause of a trait.
Answer and explanation

The orange and blue categories match while the green categories differ. They represent selected allele versions in a supplied model. A shape difference is not a nucleotide sequence, a measured chromosome deformity or proof of a dominance/trait relationship; those require the stated mapping and relevant functional/context evidence.

Compare corresponding marked positions on both homolog symbols.

Keep allele-category symbols separate from physical chromosome shape.

Request sequence, phenotype-model and functional/environment evidence before broader conclusions.

Response criteria

  • Correctly compare matching orange/blue and differing green categories.
  • Identify the drawing as an allele-category model.
  • State the limits for sequence, dominance or causal trait claims.

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