← All chapters

Genetics · Chapter 3: Sexual reproduction and meiosis

Year 11 Science · About 15 minutes with activities · Turn your sound on

Chapter 3 · Sexual reproduction
Genetically identical
Genetically different
Egg23 chromosomes
Sperm23 chromosomes
Zygote46 chromosomes, in 23 pairs
Random: any sperm could fertilise the egg
Somatic (body) cell2 sets of chromosomes
Gamete (sex cell)1 set: half the number
Without halving: 46 → 92 → 184 → … ✕offspring would not be viable
Homologous pairbefore crossing over
Recombinant chromatidsnew combinations of alleles
Possibility 1
Possibility 2
Gametes: AB and ab
Gametes: Ab and aB
A new disease arrives…

Chapter 3 recap

  • Asexual reproduction: one parent, no gametes, genetically identical offspring, fast. Sexual reproduction: two parents, gametes and fertilisation, genetically different offspring, slower.
  • Meiosis is a reduction division: it makes genetically unique gametes with half the number of chromosomes (one set), so fertilisation restores two sets.
  • Crossing over (homologous pairs exchange sections of DNA) and independent assortment (random arrangement of pairs) make new combinations of alleles. Random fertilisation mixes them again.
  • Next: Chapter 4 · Population genetics and natural selection

Why don’t you have 92 chromosomes?

Chapter 3 · About 15 minutes · sound on

0:00
In Chrome this uses Google’s voice; other browsers use the recorded New Zealand voice. Your choice here is remembered.