Principles of Inheritance
Test cross
A tall pea plant looks the same whether it is TT or Tt. A test cross tells you which one it is.
When a plant shows the dominant trait, you cannot tell by looking if it has one dominant copy or two. So you cross it with a pure recessive plant (tt) and let the offspring show the answer.
If all the offspring are tall, the parent was TT (pure). If about half are tall and half are short, the parent was Tt (a carrier).
The short offspring are the clue. They can appear only if the tall parent was carrying a hidden recessive copy.
Where students slip up
A test cross does not always give a 1 : 1 ratio. It gives 1 : 1 only when the unknown parent is Tt. If it is TT, every offspring is tall. Read the ratio you actually get. That is the answer, not a fixed rule.
Where this connects
A test cross is the Law of Segregation turned into a tool. The same move, cross with a known recessive and read the offspring, is how breeders still unmask hidden carriers in crops and cattle.
The split that makes it work →Quick check
A tall plant is crossed with a short (tt) plant. Half the offspring are short. The tall parent was:
- ATT
- BTt
- Ctt
- DCannot tell