Heredity

How traits pass from parents to offspring: Mendel's experiments, dominant and recessive traits, ratios, and sex determination.

Key ideas

Each trait is controlled by a pair of genes, one from each parent.
A dominant trait shows even with one copy of its gene; a recessive trait shows only with two copies.
Mendel crossed pure tall and dwarf pea plants: all of the first generation were tall, and the second generation was 3 tall : 1 dwarf.
With two traits, the second generation shows a 9 : 3 : 3 : 1 ratio, so the traits are inherited independently.
Genes are on chromosomes; each gamete gets one gene of each pair.
Humans: females are XX and males XY. The father’s sperm (X or Y) decides the sex of the child.

Remember

Monohybrid cross	Tt × Tt → TT : Tt : tt = 1 : 2 : 1; tall : dwarf = 3 : 1
Dihybrid ratio	9 : 3 : 3 : 1
Sex determination	XX × XY → half XX (girl), half XY (boy)

Watch out

Saying the mother decides the sex of the child.
Mixing up the 3 : 1 (what they look like) and 1 : 2 : 1 (gene pairs) ratios.
Thinking a recessive trait disappears for ever (it can reappear in later generations).

Worked example

Cross iiGg with iiGg (I = inflated, i = constricted; G = green, g = yellow). List the kinds of offspring (phenotypes) and give their ratio. Show the working for each trait separately.

Working: pod shape: all constricted; pod colour: 3 green : 1 yellow; multiply

Answer: constricted green pods : constricted yellow pods = 3 : 1