Heredity (inheritance) is the passing of characteristics from parents to offspring. Variation is the differences between individuals of the same species.
Genes, Chromosomes and DNA
- Chromosomes are thread-like structures in the nucleus that carry genetic information.
- Genes are sections of a chromosome that control particular traits (e.g. eye colour).
- DNA is the chemical that makes up genes and chromosomes.
In humans, each body cell has 46 chromosomes (23 pairs), while the sex cells (gametes) have 23 chromosomes.
Key Terms
| Term | Meaning |
|---|---|
| Genotype | The genetic make-up of an organism |
| Phenotype | The physical appearance/expression of the genes |
| Dominant gene | A gene that shows its effect even with one copy (capital letter, e.g. T) |
| Recessive gene | A gene that only shows when in a pair (small letter, e.g. t) |
| Homozygous | Two identical genes (TT or tt) |
| Heterozygous | Two different genes (Tt) |
Mendel’s Laws
Gregor Mendel studied inheritance in pea plants and established two laws:
- Law of Segregation: each parent passes only one of its two genes for a trait to its offspring.
- Law of Independent Assortment: genes for different traits are inherited independently of each other.
Types of Variation
- Continuous variation: shows a range with no distinct categories (e.g. height, weight). Caused by genes and environment.
- Discontinuous variation: falls into distinct groups (e.g. blood groups, ability to roll tongue). Controlled by genes only.
Example: A cross between a tall plant (TT) and a dwarf plant (tt) produces all tall offspring (Tt) because tall is dominant.
Summary
Genes on chromosomes (made of DNA) control inherited traits. Dominant genes mask recessive ones. Mendel’s laws describe how genes separate and sort during reproduction. Variation may be continuous or discontinuous.
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