Genes, Cell Division & Heredity — Problem set, Grade 7

Five focused units, a three-session investigation, chapter review, and chapter quiz.

Review the key ideas

DNA, Genes & Chromosomes

DNA is a molecule carrying genetic information in its sequence. A gene is a region of genetic material contributing to functional products and biological processes; a chromosome is a larger organized structure containing DNA. Genes are not separate tiny organisms, and one chromosome contains many regions. In a typical eukaryotic cell, most chromosomes are in the nucleus.

The Cell Cycle & Mitosis

Cells grow, use resources, copy genetic material, and sometimes divide. In a typical eukaryotic cell cycle, DNA is copied before the nucleus divides through mitosis. Division of the cell contents produces daughter cells. Mitosis helps growth, repair, and some asexual processes; it is not the gamete-producing process called meiosis.

Meiosis, Gametes & Genetic Variation

In the diploid sexual life cycle studied here, body cells have chromosome pairs and gametes carry one member of each pair. Meiosis follows DNA replication and includes two divisions that produce reduced chromosome sets. Fertilization combines two gamete sets, restoring the paired arrangement in the resulting zygote.

Inheritance, Probability & Punnett Squares

A Punnett square models possible allele combinations from parental gametes under stated assumptions. In the fictional one-region diploid model, each offspring receives one allele from each parent. The square lists possibilities and probabilities, not a schedule guaranteeing particular offspring in a particular order.

Inherited Traits, Environment & Mutation

Observed traits can reflect inherited genetic information, environmental conditions, and interactions between them. A seedling’s height can depend on inherited growth patterns and also light, water, and other resources. “Inherited” and “environmentally influenced” are not always mutually exclusive labels for an entire complex trait.

Problem 1

Illustrative draft places a chromosome inside one gene and says every body cell with the same DNA performs the same task. Repair the nested relationship and explain cell specialization without inventing separate inherited genomes for every cell type.
Answer and explanation

Gene regions are parts of chromosome DNA, not containers for whole chromosomes. Cells with broadly similar genetic information can use different portions and develop different structures and roles. The model does not show every gene, product, or interaction, so a shared genome does not imply identical activity in all cells.

Place gene regions along chromosome DNA.

Connect information use with functional products.

Distinguish shared information from identical activity.

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 2

Is a gene normally a whole chromosome?
Answer and explanation

No. A gene is a region of genetic material; a chromosome contains many regions and other DNA.

Problem 3

Illustrative cell starts with six chromosomes. DNA is copied, corresponding copies separate normally, and the cell forms two daughters. State each daughter’s chromosome count and distinguish DNA copying from permanent doubling of the chromosome set.
Answer and explanation

Each daughter receives six chromosomes in the stipulated normal model. Replication supplied DNA copies so both receive the set; it does not leave each daughter with twelve chromosomes. The result depends on normal distribution and does not describe every real division error or specialized cell.

Begin with the six-chromosome set.

Represent each as copied before separation.

Distribute one corresponding copy to each daughter.

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 4

Why is copying needed before the modeled division?
Answer and explanation

It provides corresponding genetic information for the daughter nuclei rather than merely splitting the original information into incomplete portions.

Problem 5

Illustrative species has eight chromosomes in a diploid body cell, arranged in four pairs. Normal gametes form and two fuse. Find gamete and zygote counts and explain how different gametes can carry different combinations.
Answer and explanation

Each modeled gamete has four chromosomes; fusion yields a zygote with eight. Different pair distributions and exchanges can produce varied combinations despite equal counts. Equal chromosome count does not imply identical allele versions. The conclusion assumes the stipulated normal diploid life cycle, not every organism’s reproductive arrangement.

Divide the paired set into one per pair: four.

Combine two gamete sets: four plus four equals eight.

Connect distributions with varied inherited combinations.

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

Why is “any random half of the chromosomes” inadequate?
Answer and explanation

A functional gamete in this simplified normal model needs one representative of each type, not an arbitrary subset missing some types.

Problem 7

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 8

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 9

Illustrative identical-genotype seedlings: one group receives adequate light and averages 8 cm; another receives low light and averages 5 cm. No sequence change is recorded. Explain the supported environmental comparison and correct a claim that every shorter plant must have acquired a heritable mutation.
Answer and explanation

The supplied record supports an environmental growth comparison, with a 3 cm mean difference under the stated light conditions. It does not document a sequence change or heritable mutation. Actual causal interpretation still depends on the fairness of the comparison and variation. A trait difference alone cannot establish mutation or transmission to offspring.

Calculate the observed 3 cm mean difference.

Use the stated equal-genotype information.

Separate environmental response from undocumented mutation and transmission.

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 10

Does a height difference prove a genetic difference?
Answer and explanation

No. Resources and other environmental conditions can also affect height.

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