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1.6.2Inquiry Process

Chromosome Number — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 16 of 160

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Part 1 of 5Chromosome Numbers

1.6.2 Chromosome Number

Members of a species share the same chromosome number, so it helps distinguish species.

Diploid number
  • The number of chromosomes (2n) in body cells, found in homologous pairs, one set from each parent.
Haploid number
  • The number of chromosomes (n) in a gamete, containing one chromosome from each homologous pair.
Homologous pair
  • Two chromosomes of the same size and gene order, one maternal and one paternal.
Autosomes
  • All the chromosomes that are not sex chromosomes and carry genes for general body features.
Sex chromosomes
  • Chromosomes that determine sex, such as X and Y in humans, with XX female and XY male.

Part 2 of 5Examples of Diploid Numbers

Examples of Diploid Numbers
Human
  • Body cells have 46 chromosomes, which are 23 homologous pairs, so gametes carry 23.
Chimpanzee
  • Has 48 chromosomes; humans differ because two ancestral chromosomes fused to make human chromosome 2.
Fruit fly
  • Drosophila has only 8 chromosomes, which is why it was a favourite for early genetic studies.
Pea plant
  • Pisum sativum has 14 chromosomes, the species used by Mendel in his inheritance experiments.
Organism2n
Parascaris univalens (roundworm)2
Drosophila melanogaster8
Pisum sativum14
Homo sapiens46
Pan troglodytes48
Canis familiaris78

Part 3 of 5What Chromosome Number Does and Does Not Show

1
Each species has a characteristic number, so a different number suggests a different species.
2
Different species can share the same number, so number alone cannot prove species identity.
3
Chromosome number does not indicate complexity, since a dog has more chromosomes than a human.
4
Mistakes at meiosis can change the number, as in trisomy 21, giving 47 chromosomes in humans.
5
Chromosome number must stay constant through mitosis so each daughter cell receives a complete set.

Part 4 of 5Worked Example

Question

  • A cell from the roundworm Parascaris univalens has a diploid number of 2.
  • A cell from the fruit fly Drosophila melanogaster has 2n = 8, and a human body cell has 2n = 46.
  • Give the haploid number of each and the number of homologous pairs in the fly.

  1. The haploid number n is half the diploid number 2n.
  2. Roundworm
    • 2 ÷ 2 = 1.
    • Fruit fly: 8 ÷ 2 = 4.
    • Human: 46 ÷ 2 = 23.
  3. The number of homologous pairs equals n, so the fly has 4 pairs.

Answer Haploid numbers are 1, 4 and 23 respectively, and the fly has 4 homologous pairs.

Part 5 of 5Exam Checklist & Tips

1.6.2 – Key Points for the Exam

  • Diploid (2n) Having two complete sets of chromosomes.
  • Haploid (n) Having one set of chromosomes.
  • Homologous chromosomes Chromosomes with the same genes in the same order.
  • Autosome A non-sex chromosome.

Common Mistakes

  • Saying a larger chromosome number means a more complex organism.
  • Giving the haploid number as the number of chromosomes in a body cell.
  • Counting a pair of homologous chromosomes as one chromosome.
  • Forgetting that gametes are haploid.

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