Heredity and variation
Heredity is the passing of characters from parents to offspring through genes. Variation is the difference between individuals of the same kind.
Somatic vs germinal variation
Variations help living things adapt to change. They are the raw material for evolution.
Words you must know
| Term | Meaning | Example |
|---|---|---|
| Gene | Unit of heredity that controls a character | The gene for height in pea |
| Allele | One of the two forms of a gene | T (tall) and t (dwarf) |
| Dominant | The trait that shows in the hybrid (F₁) | Tall (T) |
| Recessive | The trait that stays hidden in the hybrid | Dwarf (t) |
| Homozygous | Both alleles the same (pure) | TT or tt |
| Heterozygous | Two different alleles (hybrid) | Tt |
| Genotype | The gene make-up | TT, Tt or tt |
| Phenotype | The visible character | Tall or dwarf |
Why Mendel chose the garden pea
- It has many clear pairs of contrasting characters, like tall and dwarf.
- Its flowers are bisexual and normally self-pollinate, so pure lines are easy to keep.
- It can be cross-pollinated by hand when needed.
- It has a short life cycle, so many generations can be studied quickly.
- It makes many seeds, giving large numbers to count.
- It is easy to grow.
Mendel's seven pairs of contrasting characters
| Character | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet (purple) | White |
| Flower position | Axial (along the stem) | Terminal (at the tip) |
| Pod shape | Inflated (swollen) | Constricted (narrowed) |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
Children of the same parents look alike but are never exactly the same, except identical twins: that is variation. A child from a tall family may stay short if poorly fed, because food also affects height, but this somatic change is not passed on.
1 marks: “The visible character of an organism is called its ______.” Phenotype.
2 marks: “Distinguish between somatic and germinal variation.” Somatic: in body cells, due to surroundings, not inherited. Germinal: in gametes, inherited.
2 marks: “Why did Mendel choose the pea plant for his experiments?” Clear contrasting characters; bisexual, self-pollinating flowers; easy to cross by hand; short life cycle; many seeds (any two).
Genotype = the Genes you carry; Phenotype = the Physical look you show.