Mendel's hybridisation experiments on garden pea showed that traits are controlled by paired factors (genes) that segregate into gametes and, for different traits, assort independently.
How it's asked: F2 phenotypic and genotypic ratios, test-cross ratios, fraction-of-progeny probability questions from a dihybrid cross, and which of the seven pea traits is dominant.
Why this is true
Each diploid plant carries two copies (alleles) of a gene, one inherited from each parent. During gamete formation the two alleles separate so that each gamete carries only one, which is why a heterozygote Tt makes T and t gametes in equal numbers. Random fusion of these gametes gives 1 TT : 2 Tt : 1 tt, and because the dominant allele masks the recessive one in Tt, the phenotypes appear as 3 tall : 1 dwarf. When two genes lie on different chromosomes, the way one pair of alleles separates does not influence the other, so the 3:1 ratios of each trait multiply to give 9:3:3:1. A test cross with the recessive parent reveals the hidden genotype, because the recessive parent contributes only recessive alleles and lets every allele of the unknown plant show in the progeny.
Key points
Working a dihybrid cross
- 1ParentsRRYY (round yellow) × rryy (wrinkled green)
Mendel's seven pea traits
| Character | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated (full) | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
Common traps
- Pod colour green is dominant but seed colour green is recessive; students swap them.
- Of the 9 round yellow F2 plants only 1 (RRYY) breeds true; true-breeding round yellow is 1/16 of the F2, not 9/16.
Worked example
A tall pea plant of unknown genotype is crossed with a dwarf plant. Of 120 progeny, 62 are tall and 58 are dwarf.
High-yield
Monohybrid F2 3:1 (1:2:1), dihybrid F2 9:3:3:1, test crosses 1:1 and 1:1:1:1.
Quick check
Q1.In a dihybrid cross RrYy × RrYy, what fraction of the progeny will be wrinkled and yellow?
Q2.Which cross is a test cross?