Principles of Inheritance
Law of Dominance
When two different versions of a gene are present, usually only one shows. That one is dominant. The hidden one is recessive.
Cross a pure tall pea with a pure short pea, and all the plants in the next generation are tall. The ‘short’ instruction did not disappear. It is just hidden. One allele shows in the plant (dominant). The other stays hidden while a dominant copy is present (recessive).
A recessive trait shows only when both copies are recessive.
Where students slip up
Dominant does not mean stronger, more common, or better. It only means ‘the one that shows when both are present’. Many dominant traits are rare, and many recessive traits are common. Dominance is about what you see, not about strength.
Hold onto this
Dominant is the one you see. Recessive hides until it is alone.
Why it helps in the exam: this is why the F₁ of a monohybrid cross all look the same, and why a recessive trait can skip a generation and come back later.
Where this connects
This is why a trait can vanish for generations and then reappear. The recessive copy was never gone, only outvoted. That gap between what is carried and what is seen is the whole idea behind genotype and phenotype.
Genotype vs phenotype →Quick check
A pure tall (TT) pea is crossed with a pure short (tt) pea. The F₁ plants are:
- AAll short
- BAll tall
- CHalf tall, half short
- DAll medium height