Asexual reproductionSpec 3.1BTriple only
In asexual reproduction there is only one parent. The offspring are genetically identical to the parent, so there is no variation in the population.
| Point | Why it matters | |
|---|---|---|
| Advantage | No need to find a mate | Saves time and energy. Useful when individuals are far apart. |
| Advantage | Rapid reproductive cycle | Many offspring are made quickly, so the population can grow fast. |
| Disadvantage | No variation in the population | All the individuals have the same weaknesses. A new disease or a change in the environment could kill them all. |
The key idea is the trade-off. Asexual reproduction is quick and needs no partner, but the lack of variation makes the population vulnerable if conditions change.
Sexual reproductionSpec 3.2BTriple only
In sexual reproduction two parents each produce gametes. The gametes fuse at fertilisation, so the offspring inherit a mixture of alleles from both parents. This produces variation in the population.
| Point | Why it matters | |
|---|---|---|
| Advantage | Variation in the population | If the environment changes or a disease appears, some individuals are likely to have characteristics that let them survive and reproduce. |
| Disadvantage | Requirement to find a mate | This takes time and energy, and may not always be possible. |
MeiosisSpec 3.3
Meiotic cell division (meiosis) is the type of cell division that makes gametes. In humans it happens in the reproductive organs.
- One parent cell divides to produce four daughter cells.
- Each daughter cell has half the number of chromosomes of the parent cell.
- The daughter cells are genetically different from each other and from the parent cell.
- The daughter cells are haploid gametes, with one set of chromosomes.
A human body cell has 46 chromosomes. Meiosis makes sperm and egg cells with 23 chromosomes each. At fertilisation the two gametes fuse and the zygote has 46 chromosomes again, with half from each parent.
| Body cell (diploid) | Gamete (haploid) | |
|---|---|---|
| Chromosomes in a human cell | 46 | 23 |
| Genetically identical to the others? | Yes, if made by mitosis | No, each gamete is different |
| Made by | Mitosis | Meiosis |
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Give one advantage and one disadvantage of asexual reproduction.
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Advantage: no need to find a mate (or rapid reproductive cycle). Disadvantage: no variation in the population.
Give one advantage and one disadvantage of sexual reproduction.
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Advantage: variation in the population. Disadvantage: the requirement to find a mate.
How many daughter cells does one meiotic division produce?
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Four.
How many chromosomes are in a human gamete?
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23, which is half the number in a body cell (46).