Heredity (A): genes, alleles and Mendel’s first law
Answers are for parents and teachers.
1
Write the genotypes of a homozygous recessive and a heterozygous plant for seed shape (R round, r wrinkled).
rr (wrinkled) and Rr (round).
2
Cross Yy × yy. Give the genotypes and phenotypes of the offspring.
1 Yy (yellow) : 1 yy (green).
3
A friend says “brown eyes are dominant, so most people have brown eyes because of one gene”. What is wrong?
Dominant does not mean most common, and human eye colour depends on several genes, so one-gene models are only simple.
4
In an F₂ of 120 pea plants with a 3 : 1 ratio, how many are expected to be dominant and how many recessive?
120 ÷ 4 = 30 recessive; 120 − 30 = 90 dominant.
5
Draw a Punnett square for Tt × Tt and fill it in.
P: ♀ Tt × ♂ Tt
| ♀ \ ♂ | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotype: 1 TT : 2 Tt : 1 tt
Phenotype: 3 : 1
Phenotype: 3 : 1
TT, Tt, Tt, tt: 3 tall : 1 short.