Sexual and asexual reproductionSpec D3.1.1, D3.1.2
In short
Asexual reproduction produces genetically identical offspring from one parent, while sexual reproduction produces offspring with new gene combinations by meiosis and fusion of gametes. Asexual reproduction suits individuals already adapted to an existing environment. Sexual reproduction creates the variation needed for adaptation to a changed environment, because meiosis breaks up parental allele combinations and fertilization makes new ones.
In asexual reproduction one parent produces offspring by mitosis (or a similar division). The offspring are genetically identical to the parent and to each other, apart from rare mutations.
In sexual reproduction two parents each produce haploid gametes. The gametes fuse, so each offspring has a new combination of genes that is different from both parents and from its siblings.
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Number of parents | One | Usually two (or one hermaphrodite producing both gametes) |
| Genetic outcome | Offspring genetically identical to the parent | Offspring with new gene combinations, so genetic variation |
| When it is an advantage | Parent is already adapted to an existing, stable environment, so its successful combination of alleles is kept | Environment changes: variation gives some offspring traits suited to the new conditions, so the population can adapt |
| Other points | Fast; no need to find a mate; every individual can produce offspring | Slower; needs gametes from two parents to meet |
Meiosis and fusion of gametes in the sexual life cycle
A sexual life cycle alternates between a diploid and a haploid stage. Meiosis halves the chromosome number to produce haploid gametes, and fusion of gametes (also called fertilization) restores the diploid number in the zygote.
- Meiosis breaks up parental combinations of alleles: crossing over and random orientation of bivalents mean each gamete carries a unique mix of the parent's maternal and paternal alleles.
- Fusion of gametes produces new combinations: a gamete from one parent joins a gamete from another, so the zygote combines two different sets of alleles.
- Without meiosis the chromosome number would double every generation.
When asked for an advantage, tie it to the environment: asexual reproduction for an existing environment the parent is adapted to, sexual reproduction for a changed environment.
Written and checked against the IB Biology SL specification · Updated October 2026