Lesson 2 of 12 / Reproduction
Gametes and chromosome sets
What do haploid, diploid and homologous mean?
In this lesson: Define sexual reproduction and explain chromosome sets before and after fertilisation.
About 5 min
The key ideaHaploid gametes carry one chromosome set each; their nuclei fuse to form a diploid zygote.
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
Count sets, not drawing arms
A diploid cell has two sets even when the artist draws chromosomes as rods rather than X shapes.
- 2
Match homologous pairs
Look for corresponding genes and positions, not necessarily identical alleles.
- 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.
Is this true? "A homologous pair always contains two identical versions of every gene."
The chromosomes carry the same genes at corresponding positions, but their alleles can differ.