Making gametes

The idea

Gametogenesis is the formation of gametes by meiosis in the testes and ovaries; halving the chromosomes (46 to 23 in humans) gives the zygote the same chromosome number as the parents.

Keeping the number steady

A child has the same number of chromosomes as each parent, even though it gets chromosomes from both.

How do you think that works?

• Each gamete carries only half the number
• Each gamete carries double the number
• The extra chromosomes are thrown away after birth

Follow the number

Press Next step.

Half plus half

The cells made 23 and 23, and these added to 46 again. So the zygote has the parents' number.

Gametogenesis is the formation of gametes by meiosis in the testes and ovaries. Halving the chromosomes (46 to 23 in humans) gives the zygote the same chromosome number as the parents.

Notes

Gametogenesis: meiosis in the testes and ovaries halves the chromosomes (46 to 23), so the zygote has the same number as the parents.

Check yourself

Gametogenesis is the formation of…

Where does gametogenesis take place in humans?

A human sperm has 23 chromosomes and an egg has 23. How many chromosomes does the zygote have?

Answer: 46

23 + 23 = 46, the same number as in the parents.

Why is it important that gametes have half the chromosomes?

• fruits. A fruit forms from the ovary of a flower.
• hormones. The testes and ovaries also make hormones, but gametogenesis is the formation of gametes.
• gametes — correct. Yes!
• In the testes and the ovaries — correct. Yes!
• In the uterus. The uterus is where a foetus develops.
• In the vagina. The vagina is a passage joined to the uterus by the cervix. Gametes form in the testes and ovaries.
• So the zygote has half the number of its parents. The gametes have half, but together they give back the full number.
• The zygote then has the same number as the parents — correct. Yes!
• So the sperm and egg do not combine. Halving lets them combine to make a zygote with the full number.