Meiosis & Sexual Reproduction — Practice · Grade 10 Science

Chapter: Cell Division, Reproduction & Development

What this practice covers

By the end of Meiosis & Sexual Reproduction you should be able to:

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

Vocabulary used: meiosis, gamete, diploid, haploid, homologous chromosomes, crossing over, fertilization, ploidy, nondisjunction.

Problem 1

The main job of meiosis is to produce
  1. two identical body cells for growth
  2. new proteins from mRNA
  3. an exact copy of a single chromosome
  4. four haploid gametes (sex cells) with half the chromosome number
Answer and explanation

four haploid gametes (sex cells) with half the chromosome number

Meiosis makes four haploid gametes, each with half the parent cell's chromosome number.

Problem 2

A human body cell has 46 chromosomes. How many chromosomes are in a normal human egg or sperm cell?
Answer and explanation

23 chromosomes

Gametes are haploid, carrying half of 46, which is 23.

Problem 3

Crossing over increases genetic variation because it
  1. exchanges matching segments between homologous chromosomes, creating new allele combinations
  2. copies a chromosome exactly
  3. destroys half of a cell's genes
  4. turns a body cell into a gamete without any division
Answer and explanation

exchanges matching segments between homologous chromosomes, creating new allele combinations

In crossing over, homologous chromosomes swap segments, producing chromosomes with new mixes of alleles.

Problem 4

Explain why meiosis (followed by fertilization) keeps a species' chromosome number constant from one generation to the next.
Answer and explanation

Meiosis halves the chromosome number when it makes gametes, so each gamete carries only one set. When two gametes join at fertilization, the two halves add back up to the full number, keeping it constant instead of doubling each generation.

Problem 5

An organism has 5 pairs of homologous chromosomes. From independent assortment alone, how many genetically different gametes are possible?
Answer and explanation

32 gametes

Each of the 5 pairs assorts independently in 2 ways: 2^5 = 32.

Problem 6

A species has diploid number 16. What is the normal gamete number and zygote number after fertilization? Explain the reasoning in your own words.
Answer and explanation

Gametes have 8 chromosomes; the zygote has 16.

The gamete has one set, half of 16: 8.

Two normal gametes combine: 8+8=16.

Response criteria

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

Problem 7

For 2n=14, give n and the count after fusion of two normal gametes. Show the reasoning, not just the conclusion.
Hints

Halving refers to homologous sets, not arbitrary removal of any chromosomes.

Answer and explanation

n=7; fertilization restores 14.

Problem 8

How can crossing over generate new combinations without making new alleles?
Hints

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

Answer and explanation

It exchanges segments between homologous chromosomes, rearranging existing variants.

Problem 9

Compare set number, DNA copies and final products separatelyCompare set number, DNA copies and final products separatelyChromosome counts in a supplied 2n = 4 modelStarting diploid nucleus4After DNA copying4After meiosis I /nucleus2After meiosis II /nucleus2DNA-copy counts differ: 4 → 8 → 4 → 2 per nucleus.
Original chromosome-count account for a two-pair model, not human numbers or all possible errors.
Interpret the supplied visual. Why would “meiosis halves each chromosome” be inaccurate?
Answer and explanation

It reduces chromosome-set number by segregation; chromosomes remain whole structures, with possible exchanged regions.

In the supplied 2n=4 model, DNA replication gives eight chromatids within four replicated chromosomes. After meiosis I each nucleus has two replicated chromosomes and four chromatids; after meiosis II each normally has two unreplicated chromosomes. Typical mitosis instead preserves the starting ploidy in daughter nuclei. This distinguishes division number, chromosome number and DNA content. The standard model predicts variation but does not guarantee that every meiotic product must be different at every locus; sequence, pairing and chance constrain the actual combinations.

It reduces chromosome-set number by segregation; chromosomes remain whole structures, with possible exchanged regions.

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

A diploid model has 2n=6. Give chromosome and chromatid counts after S phase, after meiosis I per nucleus and after meiosis II per nucleus.
Answer and explanation

After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.

After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.

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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