Production of Haploid Gametes and the Diploid Zygote
In sexual reproduction, each parent contributes half of their genetic material to offspring through specialised cells called gametes. This half-contribution is essential to maintain a stable chromosome number across generations.
Gamete: A haploid reproductive cell (sperm or egg) containing one copy of each chromosome (n), produced by meiosis.
Zygote: The diploid cell (2n) formed when a male and female gamete fuse during fertilisation.
The key relationship is:
- Gametes are haploid (n): Each gamete carries only one chromosome from each homologous pair.
- Zygote is diploid (2n): Fusion of two haploid gametes restores the full chromosome complement.
In humans, diploid body cells contain 46 chromosomes (23 pairs). Meiosis halves this to produce gametes with 23 chromosomes each. Fertilisation then reunites two sets of 23, restoring the diploid number of 46.
The mosquito (Aedes aegypti) has 6 chromosomes in diploid cells (2n = 6). After meiosis, each gamete contains only 3 chromosomes (n = 3). When sperm and egg fuse, the resulting zygote has 6 chromosomes again.
Why does this matter? Without meiosis halving the chromosome number, each successive generation would double the chromosome count , quickly becoming incompatible with life. Meiosis and fertilisation together keep the chromosome number constant across generations.
Conducting Genetic Crosses in Flowering Plants
Flowering plants are ideal model organisms for studying inheritance because their reproduction can be precisely controlled. By manipulating pollination, scientists can track exactly which traits come from which parent.
Genetic cross: The intentional breeding of two individuals to study how traits are inherited by their offspring.
Types of Pollination:
- Self-pollination: Pollen from the anther fertilises ovules on the same plant. Produces genetically similar offspring.
- Cross-pollination: Pollen from one plant fertilises ovules of a different plant. Promotes genetic diversity.
Steps for a Controlled Genetic Cross:
- Select parent plants with contrasting traits (e.g., tall vs. dwarf).
- Prevent self-pollination: Remove the anthers (emasculation) from the plant designated as the female parent before pollen is shed.
- Enclose the flower in a paper bag to block external pollen.
- Transfer pollen from the chosen male parent to the stigma of the emasculated flower using a fine brush.
- Allow seed development: The pollen tube grows down the style, delivering male gametes to the ovules. Fertilised ovules develop into seeds carrying the next generation's genotype.
Naming Generations:
| Generation | Name | Description |
|---|---|---|
| P | Parental | Original plants used in the cross |
| F₁ | First Filial | Offspring of P generation |
| F₂ | Second Filial | Offspring of F₁ (self- or cross-pollinated) |
Always record the direction of the cross , state which plant donated pollen (♂) and which received it (♀). This matters especially when studying sex-linked traits.