Inheritance Beyond One-Gene Models — Practice · Grade 10 Science

Chapter: DNA, Inheritance & Gene Expression

What this practice covers

By the end of Inheritance Beyond One-Gene Models you should be able to:

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

Vocabulary used: incomplete dominance, codominance, polygenic trait.

Problem 1

In RW × RW, what percent are expected to be RW?
Answer and explanation

50 %

Two of four equally likely combinations are RW.

Problem 2

“Dominant” means…
  1. expressed in the heterozygote under the stated model
  2. always more common
  3. always healthier
  4. more genes than recessive
Answer and explanation

expressed in the heterozygote under the stated model

Dominance describes the genotype–phenotype relationship.

Problem 3

Why is eye color a poor example of a single-gene classroom cross?
Answer and explanation

Several genes contribute to pigment and observed color; a two-allele model omits important biology.

Problem 4

A hypothetical flower has RR red, WW white and RW pink. Cross RW with RW. Explain the reasoning in your own words.
Answer and explanation

1:2:1 phenotype ratio under the stated incomplete-dominance model.

Each parent supplies R or W.

The combinations RR, RW, WR and WW give ¼ red, ½ pink and ¼ white.

Response criteria

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

Problem 5

A heterozygote distinctly expresses both allele products. Which pattern fits? Show the reasoning, not just the conclusion.
Hints

Do not extend one trait's pattern to all traits.

Answer and explanation

Codominance, rather than an intermediate blended phenotype.

Problem 6

Why can height differ among people with similar inherited variants?
Hints

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

Answer and explanation

Many genes and environmental factors such as nutrition influence a complex trait.

Problem 7

Sex-linked inheritance requires a specified chromosome and locus modelSex-linked inheritance requires a specified chromosome and locus modelXᴬYXᴬXᴬXᴬXᴬYXᵃXᵃXᴬXᵃYSelected X-linked locus; Y has no corresponding allele hereThe model concerns a locus, not every characteristic of sex or gender.
Original constrained X-linked inheritance model, not a diagnostic pedigree or a complete account of human sex development.
Interpret the supplied visual. Why is an autosomal AA/Aa/aa model not automatically applicable to the single-X condition here?
Answer and explanation

The relevant allele-copy arrangement differs; the locus and chromosome-system assumptions determine the genotype possibilities.

A sex-linked locus lies on a sex chromosome. In a selected X-linked model with no corresponding allele on Y, a single X-linked copy can have a different expression pattern from a two-copy genotype. The square follows the stated gamete contributions and trait relationship. It does not imply all traits are X-linked, every organism uses the same chromosome system, or sex/gender characteristics reduce to one allele. Linkage and copy count must be stated rather than borrowed from an autosomal complete-dominance example.

The relevant allele-copy arrangement differs; the locus and chromosome-system assumptions determine the genotype possibilities.

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 8

A heterozygote has an intermediate phenotype under an incomplete-dominance model. Predict genotype and phenotype probabilities from Rr×Rr and explain what happens to the alleles.
Answer and explanation

Genotypes are 1/4 RR, 1/2 Rr, 1/4 rr, giving three phenotype categories in the same ratio under this model. The alleles remain distinct and segregate; the intermediate appearance does not erase them.

Genotypes are 1/4 RR, 1/2 Rr, 1/4 rr, giving three phenotype categories in the same ratio under this model. The alleles remain distinct and segregate; the intermediate appearance does not erase them.

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.

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