Principles of Inheritance
Law of Segregation
You have two copies of every gene, one from each parent. When you make egg or sperm cells, the two copies separate, and each cell gets only one.
A gene comes in two copies. When the body makes gametes (egg or sperm cells), the two copies split apart. So each gamete gets just one copy, chosen at random. Later, fertilisation joins two gametes and makes a new pair. This clean split is the whole law.
Where students slip up
Students often think the two copies mix together, or go into the same gamete. They do not. They separate fully, one copy per gamete. This is why a tall pea plant that carries a hidden ‘short’ copy can still have short offspring. The hidden copy was always there, and it goes into a gamete alone.
Hold onto this
Two copies go in. One copy comes out. Every gamete, always.
Why it helps in the exam: the 3 : 1 ratio in the F₂ generation comes straight from this. Once you understand the split, you can work the ratio out instead of memorising it.
Where this connects
This clean split is really a rule of chance. Each gamete is a coin toss, and the 3 : 1 ratios you count later are just those tosses added up over many offspring.
See it become the 3 : 1 ratio →Quick check
A pea plant with genotype Tt makes gametes. What fraction carry the T allele?
- AAll of them
- BHalf
- COne quarter
- DNone