Why Crossing Red and White Flowers Can Give Pink

Children inherit features from both parents, yet brothers and sisters are rarely identical. The basic rules can be seen by crossing garden peas and counting their offspring — and the same rules predict inheritance in plants, animals and people.

This part covers Mendel's monohybrid crosses, the laws of dominance and segregation, the test cross, and deviations from simple dominance.

**Mendel chose Pisum sativum because it has clear contrasting traits, true-breeding lines, flowers that can be self- or cross-pollinated under control, a short life cycle and many seeds; each monohybrid cross followed one trait over several generations, and large counts were analysed statistically.

The seven traits: stem height (tall or dwarf), flower colour (violet or white), flower position (axial or terminal), pod shape (inflated or constricted), pod colour (green or yellow), seed shape (round or wrinkled) and seed colour (yellow or green).

How the crosses worked:

- Two true-breeding parents differing in one trait form the parental generation
- All F${}_1$** plants resemble one parent
- Self-pollinated F${}_1$ plants give an **F${}_2$** in which both parental traits reappear
- Counting large F${}_2$ samples reveals a steady 3 : 1 ratio

Worked example. An F${}_2$ of $1064$ plants in a $3:1$ ratio should contain

$$\frac{3}{4}\times 1064=798\text{ tall},\frac{1}{4}\times 1064=266\text{ dwarf}$$

An everyday example. Farmers who save seed from a true-breeding rice variety get the same traits season after season — the same idea as a pure line.

The substance. Large samples were essential — with only a few plants, chance variation would hide the 3 : 1 pattern.

**The Law of Dominance says that when two different alleles are present, the dominant one is expressed and the recessive one stays hidden; the Law of Segregation says the two alleles separate during gamete formation so each gamete carries only one; a Punnett square combining the gametes shows a 1 : 2 : 1 genotypic and 3 : 1 phenotypic ratio in the F${}_2$.

Key terms:

- Gene — the unit of inheritance; alleles — its alternative forms, such as T and t
- Homozygous — two identical alleles (TT or tt); heterozygous — two different alleles (Tt)
- Genotype — the allele combination; phenotype — the observable trait

Law of Dominance:

- Characters are controlled by discrete units, called factors, that occur in pairs
- In a dissimilar pair, one factor dominates the other

Law of Segregation:

- Alleles do not blend; they separate when gametes form
- A homozygous parent makes one kind of gamete; a heterozygous parent makes two kinds in equal numbers

Punnett square for Tt × Tt:

- Gametes from each parent: T and t
- Genotypic ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic ratio: 3 tall : 1 dwarf

An everyday example. A tall pea plant grown from a mixed packet of seed may carry a hidden dwarf allele that shows up only among its offspring.

The substance. Dominance describes expression, not frequency** — a dominant allele is not necessarily the more common one in a population.

A test cross crosses an organism showing the dominant phenotype with a homozygous recessive one; if all the offspring show the dominant trait, the organism is homozygous dominant, but if about half show the recessive trait, it is heterozygous.

The two outcomes:

- TT × tt — all offspring are Tt, and all are tall
- Tt × tt — offspring are Tt or tt, in the ratio 1 tall : 1 dwarf

Worked example. A violet-flowered pea plant crossed with a white-flowered plant gives $48$ violet and $52$ white offspring. That is close to $1:1$, so the violet parent was heterozygous, Ww. Had it been WW, all $100$ offspring would have been violet.

An everyday example. Cattle breeders check whether a black bull carries the hidden recessive red coat allele by mating it with red cows and looking at the calves — the same logic as a test cross.

The substance. Only a recessive parent works as the tester — crossing with a dominant plant would mask the very alleles you are trying to detect.

**In incomplete dominance the heterozygote shows an intermediate phenotype, as in pink snapdragons; in co-dominance both alleles are fully expressed together, as in blood group AB; and multiple allelism means a gene has more than two alleles in a population, as with the three alleles I${}^A$, I${}^B$ and i of the ABO blood groups.

Incomplete dominance** — in the dog flower, Antirrhinum, or four o'clock plant, Mirabilis:

- Red (RR) × white (rr) gives all pink (Rr) plants in F${}_1$
- F${}_2$ shows 1 red : 2 pink : 1 white, so phenotypic and genotypic ratios are the same

Co-dominance. The F${}_1$ resembles both parents rather than an intermediate. I${}^A$ and I${}^B$ are co-dominant, so I${}^A$I${}^B$ people carry both A and B sugar polymers on their red blood cells — blood group AB.

Multiple alleles — the ABO gene I:

- Three alleles, **I${}^A$, I${}^B$ and i**, though each person carries only two
- I${}^A$ and I${}^B$ are both dominant over i
- I${}^A$I${}^A$ or I${}^A$i gives A; I${}^B$I${}^B$ or I${}^B$i gives B; I${}^A$I${}^B$ gives AB; ii gives O
- Six genotypes produce four blood groups

An everyday example. Blood donation camps record every donor's blood group, because receiving the wrong group can cause a dangerous reaction.

The substance. Incomplete dominance is not blending — the red and white alleles reappear unchanged in the F${}_2$.