Why Two Tall Pea Plants Can Produce a Short One

Children often share features with their parents, yet sometimes a trait skips a generation entirely. Careful crosses between pea plants revealed the simple rules behind this pattern of inheritance.

This lesson covers Mendel's principles, monohybrid and dihybrid crosses with Punnett squares, and the key terms of genetics.

Mendel's principles are the law of dominance, the law of segregation and the law of independent assortment, and the pea experiments succeeded because of a well-chosen plant, clearly contrasting traits, true-breeding lines, large numbers and mathematical analysis.

Law of dominance:

- Characters are controlled by discrete units called factors, which occur in pairs
- In a pair of dissimilar factors, one — the dominant — is expressed, while the other — the recessive — stays hidden

Law of segregation:

- The two factors of a pair separate during gamete formation, so each gamete receives only one
- The factors never blend, so a recessive trait can reappear in the next generation

Law of independent assortment. When two pairs of traits are combined, the factors of one pair segregate independently of those of the other pair.

Why the experiments worked:

- Garden pea — a short life cycle, many seeds, and flowers that self-pollinate but can be crossed by hand
- Seven pairs of clearly contrasting traits, such as tall versus dwarf plants and round versus wrinkled seeds
- True-breeding lines, confirmed by repeated self-pollination
- One or two traits at a time, with large samples counted and analysed mathematically

An everyday example. Plant breeders at Indian agricultural universities still begin new crosses with true-breeding parent lines and large numbers of plants — the same design principles.

The substance. Segregation always holds, but independent assortment does not — genes lying close together on one chromosome tend to be inherited together.

A Punnett square lists the gametes of one parent along the top and those of the other down the side, and filling the grid gives every offspring genotype; a monohybrid cross gives a 3:1 phenotypic ratio in the F2, a dihybrid cross gives 9:3:3:1, and a test cross is a cross with the recessive parent.

Monohybrid cross — tall (TT) × dwarf (tt):

- F1: all Tt, all tall
- Selfing the F1, $Tt\times Tt$, each parent gives gametes T and t
- F2 genotypes: 1 TT : 2 Tt : 1 tt
- F2 phenotypes: 3 tall : 1 dwarf

Dihybrid cross — round yellow (RRYY) × wrinkled green (rryy):

- F1: all RrYy, round and yellow
- Each F1 plant makes four kinds of gamete in equal numbers: RY, Ry, rY and ry
- A 4 × 4 Punnett square gives 16 combinations
- F2 phenotypes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Back cross versus test cross:

- Back cross — the F1 crossed with either parent
- Test cross — a plant with the dominant phenotype crossed with the homozygous recessive parent to reveal its genotype
- All offspring dominant means the plant was homozygous (TT); 1 dominant : 1 recessive means it was heterozygous (Tt)
- A dihybrid test cross, RrYy × rryy, gives 1 : 1 : 1 : 1

An everyday example. A seed farm checking whether a tall pea plant is pure-breeding crosses it with a dwarf plant — even a few dwarf offspring show the tall plant was Tt.

The substance. Every test cross is a back cross, but not every back cross is a test cross — only a cross with the recessive parent reveals a hidden genotype.

Alleles are alternative forms of a gene; a dominant allele shows its effect even in a single copy and a recessive allele only in two copies; the genotype is the combination of alleles and the phenotype is the observable trait; and an organism is homozygous with two identical alleles and heterozygous with two different ones.

Key terms:

- Gene — a unit of inheritance that controls a character
- Alleles — alternative forms of a gene, such as T for tall and t for dwarf
- Dominant allele — expressed in both TT and Tt, and written with a capital letter
- Recessive allele — expressed only in tt, and written with a small letter
- Genotype — the genetic makeup, such as TT, Tt or tt
- Phenotype — the observable character, such as tall or dwarf
- Homozygous — identical alleles, such as TT or tt; true-breeding
- Heterozygous — different alleles, such as Tt; a hybrid

Worked example. In $Tt\times Tt$, three genotypes appear — TT, Tt and tt — but only two phenotypes, tall and dwarf, because TT and Tt look the same.

An everyday example. Two healthy parents who both carry the sickle-cell allele can have a child with sickle-cell anaemia, because each passes on a hidden recessive allele.

The substance. Dominant does not mean common or better — it only means the allele is expressed in a heterozygote, and some harmful conditions are caused by dominant alleles.