Accumulation of variation during reproduction
When organisms reproduce, the copying of DNA is never perfect, so every new generation carries small changes. In asexual reproduction (like bacteria dividing) the differences are tiny. In sexual reproduction half the genes come from each parent and they get mixed in new combinations, so variation is much greater.
These variations pile up generation after generation. Not every variation helps, but some give a better chance of survival in a changing environment. That is why variation is the raw material for evolution.
Heredity and inherited traits
Heredity means the passing of traits (features) from parents to offspring. A trait is any feature you can see or measure: height, eye colour, blood group, earlobe shape. Children get a copy of the basic body design from their parents, plus small variations.
An inherited trait is controlled by genes and passes to the next generation. A trait you pick up in your own life (a scar, muscles from exercise) is acquired and is not passed on.
Mendel's contributions: rules for inheritance
Gregor Johann Mendel, a monk in the 1860s, studied garden peas. Peas were a smart choice: they grow fast, give many seeds, can self-pollinate or be cross-pollinated by hand, and have clear contrasting traits (tall/short, round/wrinkled seed, yellow/green seed, violet/white flower).
He called the hidden units factors; today we call them genes. Different forms of a gene are alleles (T and t). A plant with two same alleles (TT or tt) is pure-breeding / homozygous; with two different ones (Tt) it is hybrid / heterozygous.
Dominant and recessive traits
The trait that shows in the hybrid is dominant (tall, T). The trait that stays hidden is recessive (short, t). A recessive trait shows only when both copies are recessive (tt). The genetic make-up (TT, Tt, tt) is the genotype; what you see (tall or short) is the phenotype.
Monohybrid cross: one trait, 3:1
Parents (P): pure tall TT × pure short tt. Gametes: T and t. F1: all Tt, all tall. Self-pollinate F1 (Tt × Tt): gametes T, t from each side.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
F2 phenotype ratio: 3 tall : 1 short. Genotype ratio: 1 TT : 2 Tt : 1 tt. The short trait vanished in F1 but reappeared in F2, which proves the factor was never lost or blended. This is the law of dominance and the law of segregation: the two copies separate when gametes form.
Dihybrid cross: two traits, 9:3:3:1
Parents: round yellow (RRYY) × wrinkled green (rryy). F1: all RrYy, round and yellow (round and yellow are dominant). F1 × F1: each plant makes four kinds of gametes (RY, Ry, rY, ry), so the Punnett square has 4 × 4 = 16 boxes.
F2: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Two brand new mixes appear (round green and wrinkled yellow), so seed shape and seed colour are passed on independently of each other: the law of independent assortment. Check: each trait alone still gives 12:4 = 3:1.
How do traits get expressed? Genes, DNA and proteins
Inside the nucleus, DNA is packed into chromosomes. A gene is a section of DNA that holds the instructions to make one protein. Proteins do the work that builds a trait.
Example: plant height depends on a growth hormone. A gene makes an enzyme (a protein) that produces this hormone. If the gene copy works well, lots of enzyme, lots of hormone, the plant grows tall. If the copy is less efficient, less hormone, the plant stays short. So gene → protein (enzyme) → hormone → trait.
Each body cell has two copies of every chromosome, one from each parent. Gametes have only one set, so when egg and sperm fuse the pair is restored and the chromosome number stays the same generation after generation.
Board exam focus
- 1-mark: define dominant/recessive, genotype/phenotype; F2 ratios.
- 3-mark: draw a monohybrid cross with a Punnett square; show that traits are inherited independently.
- 5-mark: dihybrid cross with ratio and conclusion; how genes control traits.
Sex determination is part of this chapter too: see Sex determination in humans.
Key formulas and definitions
- Gene: a section of DNA that codes for one protein
- Allele: one form of a gene (T or t)
- Genotype: gene make-up (TT, Tt, tt); Phenotype: what you see (tall/short)
- Homozygous: TT or tt; Heterozygous (hybrid): Tt
- Monohybrid F2: phenotype 3 : 1, genotype 1 : 2 : 1
- Dihybrid F2: 9 : 3 : 3 : 1 (16 combinations)
- Number of gamete types = 2^n (n = number of hybrid gene pairs)
Worked examples
1. A pure violet-flowered pea (VV) is crossed with a pure white-flowered pea (vv). What will F1 look like?
Gametes: V from one parent, v from the other. F1 = Vv. Violet is dominant, so all F1 plants have violet flowers.
2. Two Tt pea plants are crossed and 400 seeds are grown. About how many will be short?
Punnett square: TT, Tt, Tt, tt. Short = tt = 1 out of 4. 400 × 1/4 = 100 short plants (and 300 tall).
3. A tall plant of unknown genotype is crossed with a short plant (tt). Half the children are short. What was the tall parent?
The short children are tt, so each got a t from the tall parent. The tall parent must be Tt. (If it were TT, all children would be Tt, tall.) This is called a test cross.
4. In a dihybrid cross RrYy × RrYy, 160 seeds are produced. How many are expected to be wrinkled and green?
Wrinkled green = rryy = 1 out of 16. 160 × 1/16 = 10 seeds.
5. Explain why the short trait disappears in F1 but reappears in F2.
F1 is Tt. T is dominant and hides t, so all look tall. In F2 two Tt plants each can give t, and 1 in 4 children gets t + t = tt, which is short. The t factor was hidden, never lost.
Common mistakes
- Mixing genotype and phenotype: Tt is a genotype; 'tall' is the phenotype.
- Thinking the recessive trait is weaker or rarer. Recessive only means it is hidden when a dominant copy is present.
- Writing gametes with two letters in a monohybrid cross. A gamete carries only one copy: T or t, not Tt.
- Quoting 1:2:1 as the phenotype ratio. 1:2:1 is the genotype ratio; the phenotype ratio is 3:1.