Sexual Reproduction in HumansCambridge IGCSE Biology: Revision notes
Section 1
What are gametes and how do they differ from other cells?
Gametes are specialised sex cells produced by the reproductive system. In humans, the male gamete is the sperm cell and the female gamete is the ovum (egg cell).
A key feature of gametes is that they are haploid – they contain only half the number of chromosomes found in a normal body cell. In humans, a haploid gamete contains 23 chromosomes, whereas a normal body cell (somatic cell) contains 46 chromosomes (23 pairs).
This reduction in chromosome number is crucial because it allows two gametes to fuse and restore the full diploid number in the offspring.
Examiners expect you to state that gametes are haploid with 23 chromosomes AND that body cells are diploid with 46 chromosomes. Use precise chromosome numbers in your answers.
Think of gametes like recipe cards with half the ingredients – two cards together make a complete recipe (the new organism).
Section 2
What is fertilisation and what happens when gametes fuse?
Fertilisation is the process in which the nucleus of a male gamete (sperm) fuses with the nucleus of a female gamete (ovum) to form a single cell called a zygote.
During fertilisation:
- The sperm cell meets and penetrates the ovum
- The haploid nucleus of the sperm (23 chromosomes) fuses with the haploid nucleus of the ovum (23 chromosomes)
- This fusion produces a zygote nucleus containing 46 chromosomes (diploid)
- The zygote is now a genetically unique cell with genetic material from both parents
The zygote then undergoes mitosis and cell division to develop into an embryo and eventually a fully developed organism.
Examiners want to see that you understand fertilisation produces a diploid zygote from two haploid gametes. State the chromosome numbers (23 + 23 = 46) explicitly.
Students often confuse fertilisation with implantation or pregnancy. Fertilisation is ONLY the fusion of nuclei – it happens in the fallopian tube, not in the uterus.
Section 3
How does sexual reproduction lead to genetic variation in offspring?
Sexual reproduction produces genetically different offspring because:
- Each parent contributes different combinations of alleles (versions of genes) through their gametes
- The fusion of two different gametes means offspring inherit a unique mix of genetic material from both parents
- Even though offspring are related to their parents, they are not identical to either parent or to siblings (except identical twins)
- This genetic variation within a population means different individuals have different traits, such as height, eye colour, or disease resistance
In contrast, asexual reproduction (producing offspring from one parent) creates genetically identical offspring called clones, with no genetic variation.
Genetic variation is an important advantage of sexual reproduction because it increases the likelihood that some individuals in a population possess traits that help them survive in changing environments.
Two parents with different eye colours (one brown-eyed, one blue-eyed) produce offspring with various eye colours due to different combinations of alleles inherited. Each child is genetically unique because they inherit a different mix of the parents' alleles.
Section 4
What are the advantages of sexual reproduction to wild populations?
In wild populations, sexual reproduction provides several critical advantages:
| Advantage | Explanation |
|---|---|
| Genetic variation | Offspring are genetically different, so population has diverse traits |
| Adaptation to environment change | If environment changes, genetic diversity means some individuals may have traits for survival |
| Disease resistance | Different individuals have varied immune responses; some may resist new diseases |
| Evolutionary potential | Genetic variation is the raw material for natural selection to act upon |
| Inbreeding avoidance | Genetic diversity prevents harmful effects of mating between close relatives |
These advantages mean a sexually reproducing population is more likely to survive environmental pressures and long-term changes.
Examiners expect you to link genetic variation to survival advantages. Use phrases like 'some individuals may have traits that allow survival' or 'greater chance of population survival if environment changes'.
Section 5
What are the disadvantages of sexual reproduction to wild populations?
Although sexual reproduction has many advantages, it also has disadvantages in wild populations:
- Time and energy costs: Finding a mate and producing gametes requires significant time, energy, and resources
- Slower reproduction rate: Sexual reproduction is generally slower than asexual reproduction; fewer offspring can be produced in the same timeframe
- Unpredictable offspring: Genetic variation means offspring are unpredictable – some may inherit unfavourable traits that reduce survival
- Dependency on finding mates: In sparse populations, individuals may struggle to find breeding partners
- Genetic incompatibility: If populations are isolated, genetic differences may eventually prevent successful breeding
These disadvantages mean sexual reproduction may be less efficient for rapid population growth compared to asexual alternatives.
Students often say sexual reproduction is 'bad' because it has disadvantages. Remember: the disadvantages are outweighed by the long-term survival advantage of genetic variation. Both advantages AND disadvantages should be discussed.
Section 6
How does sexual reproduction affect crop production?
In crop production, sexual reproduction creates both advantages and disadvantages:
Advantages:
- Genetic variation in crops provides different traits (yield, drought resistance, disease resistance)
- Selective breeding can be used – farmers select plants with desirable traits and breed them together
- New varieties can be developed to suit changing conditions or market demands
- Disease resistance genes can be combined; if disease emerges, some plants may resist it
Disadvantages:
- Unpredictable offspring: Traits are variable; not all seedlings will have desired characteristics
- Time-consuming: Breeding programmes take multiple generations to develop new varieties
- Maintenance of variety: Without careful management, desirable traits may be lost in subsequent generations
- Lower short-term yield: Variable offspring may not all be high-yielding, reducing overall productivity
- Farmer complexity: Farmers must carefully select seeds each year or purchase new seed stock
For these reasons, many commercial crops are now produced using asexual reproduction methods (cuttings, tissue culture) to ensure consistent, predictable yields.
For crop production questions, examiners want you to recognise the trade-off: genetic variation in sexual reproduction allows flexibility and adaptation (advantage), but produces unpredictable, variable crops (disadvantage). Mention why asexual reproduction (clones) is preferred for consistent commercial production.
A farmer breeding wheat for drought resistance selects plants that survived dry conditions and breeds them together. Some offspring will inherit drought-resistant genes, but others may not – this unpredictability is a disadvantage, yet the genetic variation allows adaptation to climate change (advantage).
Must Know
- Gametes are haploid (23 chromosomes); they fuse during fertilisation to form a diploid zygote (46 chromosomes)
- Fertilisation is the fusion of the nuclei of two gametes (one sperm, one ovum), producing a genetically unique zygote
- Sexual reproduction produces genetically different offspring because each gamete carries a different combination of alleles from each parent
- Advantages to wild populations: genetic variation allows adaptation to environmental change, provides disease resistance, and is raw material for natural selection; increases survival probability
- Disadvantages to wild populations: requires time and energy to find mates, slower reproduction rate, offspring traits are unpredictable, difficult in sparse populations
- For crop production: sexual reproduction allows selective breeding and development of new varieties (advantage), but produces unpredictable, variable crops and is time-consuming (disadvantage); asexual reproduction is preferred for consistent, predictable yields
That's the notes covered.
Carry on to the next subtopic.