K325 / 2027

Lesson 2 of 12 / Reproduction

Gametes and chromosome sets

What do haploid, diploid and homologous mean?

I can define sexual reproduction and explain chromosome sets before and after fertilisation.

  • I can define sexual reproduction.Syllabus

    Syllabus K325, Topic 13(d). Define sexual reproduction.

  • I can explain ploidy and why reduction precedes fertilisation.Syllabus

    Syllabus K325, Topic 13(e). Explain ploidy and why reduction precedes fertilisation.

  • I can explain homologous chromosomes.Syllabus

    Syllabus K325, Topic 13(f). Explain homologous chromosomes.

About 5 min

Make a guess. You are not marked.

Human body cells have 46 chromosomes. If sperm and egg cells also had 46, what would the baby have?

  1. 23, because only half of each is used.
  2. 46, because the extra ones are thrown away.
  3. 92, double the normal number.
Show the answer

92, double the normal number.

That is why gametes are haploid. With 23 each, fertilisation restores 46 instead of doubling it.

The key idea

Haploid gametes carry one chromosome set each; their nuclei fuse to form a diploid zygote.

Two haploid gamete nuclei each contain two complete chromosomesHaploid: n = 2Haploid: n = 2One complete set in each nucleus
Each gamete contains one long and one short chromosome. Haploid means one set, not chromosomes cut into fragments.

A model species with four diploid chromosomes. In humans, the usual numbers are 23 + 23 = 46.

Explanation

Sexual reproduction involves fusion of the nuclei of a male and a female gamete to form a zygote. Offspring are genetically dissimilar because they receive a new combination of inherited information.

A diploid nucleus has two sets of chromosomes, one inherited from each parent. Its chromosomes can be arranged in homologous pairs. Members of a pair have corresponding genes at the same positions, although the versions of those genes can differ.

A haploid nucleus has one set: one chromosome from each homologous pair. Human body-cell nuclei usually have 46 chromosomes, while sperm and egg nuclei have 23. A haploid set is not half a chromosome; it contains complete chromosomes.

During fertilisation, 23 chromosomes from each human gamete combine to give the diploid 46 in the zygote. A reduction division before fertilisation prevents chromosome number doubling in each generation.

Homologous does not mean genetically identical. The two members of a pair may carry different alleles. Also distinguish homologous chromosomes from the matching copied parts of one duplicated chromosome.

Step by step
  1. 1

    Count sets, not drawing arms

    A diploid cell has two sets even when the artist draws chromosomes as rods rather than X shapes.

  2. 2

    Match homologous pairs

    Look for corresponding genes and positions, not necessarily identical alleles.

  3. 3

    Restore the diploid number

    Add the two haploid contributions at fertilisation.

Worked example

An imaginary species

A species has a diploid number of 8. Predict its gamete and zygote chromosome numbers.

One way to explain it

Each gamete is haploid and has 4 chromosomes. Fusion of two gamete nuclei restores 8 in the diploid zygote.

Why this answer works
  • The diploid number is halved before fertilisation.
  • Fertilisation combines complete haploid sets: 4 + 4 = 8.

Watch out for this

A student says: "A homologous pair always contains two identical versions of every gene." What is wrong with this?

Show the answer

The chromosomes carry the same genes at corresponding positions, but their alleles can differ.

Check your understanding

Why must chromosome number be reduced before gamete nuclei fuse?

  1. To restore the usual diploid number at fertilisation instead of doubling it each generation.
  2. To cut every chromosome physically in half.
  3. So that each offspring receives its genetic information from one parent, making it identical to that parent.
Show the answer

To restore the usual diploid number at fertilisation instead of doubling it each generation.

Haploid + haploid produces a diploid zygote.

Quick recall

Card 1 of 3. Answer in your head, then check.

What do haploid, diploid and homologous mean?

You can return to this lesson any time.