The Simple Story
Imagine every living thing has instruction cards for each trait — like height. You have TWO cards for height (one from Mum, one from Dad). When it's time to make a baby cell (gamete), the two cards separate: each gamete gets only ONE card. When sperm meets egg, the baby gets one card from Mum and one from Dad — back to two again.
That's Mendel's Law of Segregation: cards (alleles) travel in pairs in body cells, but separate into singles for gametes.
What About Two Traits at Once?
Now imagine you have two different types of cards: HEIGHT cards and COLOUR cards. When you make gametes, the HEIGHT cards and COLOUR cards don't care about each other — they separate randomly and independently. A gamete might get your Tall HEIGHT card with your Red COLOUR card, or your Tall HEIGHT card with your White COLOUR card — completely random.
That's the Law of Independent Assortment: different types of cards (genes on different chromosomes) sort into gametes independently of each other.
Why Does 3:1 Happen?
You're Tt (one Tall card + one Dwarf card). Your partner is also Tt. You make T and t gametes equally. Your partner makes T and t gametes equally. Random mixing:
- T+T = TT (Tall) — 1 chance
- T+t = Tt (Tall) — 2 chances (T from you + t from partner, OR t from you + T from partner)
- t+t = tt (Dwarf) — 1 chance
3 Tall : 1 Dwarf. The maths is just probability — like flipping two coins.
Where the Simple Story Breaks Down
- Sometimes cards on the SAME chromosome are stapled together (linked genes) — they can't separate easily → violates Independent Assortment
- Sometimes neither card is fully "louder" than the other → blended result (incomplete dominance) → violates simple Dominance
- One card can produce multiple different effects (pleiotropy) — one instruction card, many results
The Key Insight
Mendel never knew about chromosomes, DNA, or genes. He just counted peas and trusted the numbers. The laws he discovered from counting peas turned out to describe the behaviour of chromosomes during meiosis — discovered 40 years later. That's the power of careful observation + statistics.