ASEXUAL REPRODUCTION involves only ONE parent organism — no gametes (sex cells) are involved.
New offspring are produced by MITOSIS — cell division that creates genetically IDENTICAL copies of the parent.
Offspring from asexual reproduction are called CLONES — they are genetically identical to the parent and to each other.
ADVANTAGES:
Fast — large numbers of offspring can be produced very quickly.
No mate needed — energy efficient.
All offspring are well-adapted if the parent is well-adapted to its environment.
Useful in stable environments where the parent's traits are advantageous.
DISADVANTAGES:
No genetic variation — all offspring identical.
If the environment changes or a new disease emerges, ALL offspring are equally vulnerable.
Cannot adapt to new conditions.
EXAMPLES: Bacteria (binary fission), plants via runners (strawberries), bulbs (daffodils), tubers (potatoes), budding (hydra), some fungi.
Sexual Reproduction
SEXUAL REPRODUCTION involves TWO parent organisms — each contributes a GAMETE (sex cell).
Gametes are produced by MEIOSIS — a special type of cell division that HALVES the chromosome number.
FERTILISATION: a male gamete and female gamete FUSE to form a ZYGOTE (fertilised egg), restoring the full chromosome number.
Because DNA from TWO different individuals is combined, offspring are genetically DIFFERENT from both parents and from each other.
ADVANTAGES:
Creates genetic VARIATION — offspring differ from parents and from each other.
Variation means some offspring may be better adapted if the environment changes.
Drives EVOLUTION — natural selection can act on the variation produced.
DISADVANTAGES:
Requires two parents — time and energy needed to find a mate.
Slower — fewer offspring produced.
Some offspring may inherit unfavourable gene combinations.
Meiosis — Producing Gametes
MEIOSIS is the type of cell division used to produce gametes (sperm, eggs, pollen).
Starting cell: a body cell with 46 chromosomes (23 pairs).
In meiosis:
The chromosome number is HALVED — each gamete receives 23 chromosomes (one from each pair).
FOUR daughter cells are produced.
Each daughter cell is GENETICALLY DIFFERENT from the others.
WHY halve the chromosome number? When two gametes fuse at fertilisation, 23 + 23 = 46 — restoring the correct number. If gametes had 46 chromosomes each, the offspring would have 92 — doubling every generation.
Genetic variation in meiosis arises because:
Chromosomes are shuffled before dividing (independent assortment).
Sections of chromosomes can be exchanged between homologous pairs (crossing over).
This means every gamete has a unique combination of alleles.
⚠️ Common Mistake
MITOSIS produces IDENTICAL cells (used for growth and repair). MEIOSIS produces GAMETES with HALF the chromosome number — and they are genetically different from each other. Students often confuse these two. Remember: MEiosis → gamEtes. Mitosis → most body cells.