The cross diagram
A cross is worked out on paper in a fixed order. Follow the same four steps every time and you will not lose track of which letter came from where.
The four steps
- Write the two parents, each with both of its factors.
- Below each parent, write the gametes it can make. A gamete carries one factor of the pair, never both.
- Combine each gamete from one parent with each gamete from the other.
- Read off the offspring and group them.
Worked: pure tall crossed with pure dwarf
Parents
tall × dwarf TT × tt
Gametes
T, T and t, t
↓
Offspring (F1)
Tt — every plant tall
Figure to be added
Why the gametes carry only one factor
This is meiosis, which you met in the chapter on cell division. Gametes are formed by meiosis, so each gets one chromosome of every homologous pair and therefore one factor of every pair. Fertilisation puts two gametes together, and the pair is restored in the offspring.
A gamete never carries a pair
Writing Tt as a gamete is the commonest mistake on this page. A Tt plant makes gametes carrying T and gametes carrying t — separate gametes, half of each — never a single gamete holding both.
Parents in pairs, gametes singly
Each parent carries two factors and each gamete carries one. Fertilisation brings two gametes together, so the offspring has two again.