Factors and symbols
To explain what he had counted, Mendel proposed something he could not see. He called it a factor, and the whole notation used from here on rests on it.
Mendel's proposal
Every characteristic is controlled by two factors, which occur as a pair in the plant.
- One factor of the pair comes from the mother, through the ovum.
- The other comes from the father, through the pollen.
Writing them down
Each factor gets a letter. For stem height, a capital T stands for the dominant factor for tallness and a small t for the recessive factor for dwarfness. Since every plant carries two, three combinations are possible.
| Pair of factors | The plant looks | Because |
|---|---|---|
| TT | Tall | Both factors are for tallness |
| Tt | Tall | T is dominant, so it shows and t is hidden |
| tt | Dwarf | Neither factor is dominant, so the recessive form shows |
The conventions
- The same letter is used for both forms of one character, so a pair always belongs together.
- The capital letter is the dominant factor; the small letter is the recessive one.
- The letter chosen is usually the first letter of the dominant form — T for tall, not d for dwarf.
Tt and tt look completely different on paper and identical to count
A Tt plant is tall and a TT plant is tall, and standing in the field you cannot tell them apart. That is the difficulty Mendel had to get round, and it is why the F2 ratio is three to one rather than something simpler.
Two factors, one from each parent
Every characteristic is controlled by a pair of factors, one inherited from each parent. Write them with the same letter — capital for dominant, small for recessive.