Half and half makes a whole
Fertilisation brings the original chromosome number back.
Two halves
A human body cell has 46 chromosomes. Sperm and egg cells are gametes.
If a sperm and an egg each had 46 chromosomes, what would a fertilised egg have?
What this lesson covers
The idea
Gametes carry half the chromosome number of the parent cell and create variation among offspring; when gametes from two individuals combine in fertilisation, the original chromosome number is restored.
Two halves
A human body cell has 46 chromosomes. Sperm and egg cells are gametes.
If a sperm and an egg each had 46 chromosomes, what would a fertilised egg have?
- 92, which is too many. So gametes must carry only half the number
- 46
- 23
Follow the chromosomes
Press + to step from body cells to the zygote. Try the three numbers.
Half and half
Gametes carry half the number of chromosomes of the parent cell, and they create variation among offspring. When gametes from two individuals combine in fertilisation, the original chromosome number is restored.
Children resemble their parents but are not exactly the same, because each gamete carries a different half of the parent’s chromosomes.
- Cell | Human | Pea plant
- Body cell | 46 | 14
- Gamete | 23 | 7
- Zygote (sperm + egg) | 46 | 14
Notes
Gametes carry half the chromosomes of the parent cell. In fertilisation two gametes from two individuals combine and the original chromosome number is restored.
Check yourself
A pea plant body cell has 14 chromosomes. How many chromosomes does each of its gametes have?
Answer: 7 chromosomes
14 ÷ 2 = 7 chromosomes in each gamete.
A sperm has 23 chromosomes and an egg has 23. How many chromosomes does the fertilised egg (zygote) have?
Answer: 46 chromosomes
23 + 23 = 46: the original number is restored.
Why must gametes carry only half the chromosome number?
Gametes create what among offspring?
- So that fertilisation restores the original number — correct. Yes.
- So that the offspring has fewer chromosomes than its parents. It has the same number as its parents.
- So that gametes are bigger. It is about the chromosome number.
- Variation — correct. Yes. Children resemble their parents but are not exactly the same.
- Identical copies of one parent. That is the result of mitosis.
- No difference at all. Gametes create variation.