Mendel's Laws of Inheritance Explained Simply
Gregor Mendel spent years crossing pea plants in a monastery garden in Brno and discovered something nobody at the time thought possible: inheritance follows fixed numerical ratios. His three laws still explain today why a child can have blue eyes although both parents have brown ones.
Gene — the stretch of DNA that determines a trait.
Allele — one version of a gene, for example "red flower" or "white flower".
Dominant — prevails even when present only once. Written with a capital letter.
Recessive — only shows when present twice. Written in lower case.
Homozygous — both alleles the same, so AA or aa.
Heterozygous — both alleles different, so Aa.
Genotype — the combination of alleles inside the cell.
Phenotype — what you can see from the outside.
P, F1, F2 — parental generation, first and second filial generation.
The first law: uniformity
Cross two homozygous parents that differ in one trait and all the offspring of the F1 generation look the same. Mendel crossed red-flowering peas (AA) with white-flowering ones (aa). Every offspring received one A and one a, so it was Aa — and flowered red, because red is dominant. The white had not disappeared, it was merely masked.
The second law: segregation
Cross those F1 plants with one another and the hidden trait reappears, in a fixed ratio. Aa × Aa produces four equally likely combinations: AA, Aa, aA and aa. Three of them flower red, one white — the famous 3 : 1 ratio in the phenotype. In the genotype it is 1 : 2 : 1.
| A | a | |
|---|---|---|
| A | AA red | Aa red |
| a | aA red | aa white |
This grid is called a Punnett square. The possible germ cells of the parents go along the top and down the side, and the combinations fill the boxes. It is the most useful tool in the whole topic because it makes arithmetic slips almost impossible.
The third law: independent assortment
Look at two traits at once — say flower colour and seed shape — and they are inherited independently of one another. A cross AaBb × AaBb gives the phenotype ratio 9 : 3 : 3 : 1. Nine plants show both dominant traits, three each show one of the two, and a single one shows both recessive traits.
16 equally likely boxes in the Punnett square
9 × A_B_ | 3 × A_bb | 3 × aaB_ | 1 × aabb
= 9 : 3 : 3 : 1
Important: the third law only holds if the two genes sit on different chromosomes. If they lie close together on the same chromosome they are usually inherited together. Mendel did not yet know about this gene linkage — he was simply lucky with his seven pea traits.
💬 Mia asks Grandpa Theo
If three out of four flower red — does that mean four plants are guaranteed to include one white?
No, and that is the most important point. 3 : 1 is a probability, not a guarantee. With four plants you might get four red ones by chance. Only with hundreds does the result approach the ratio. That is why Mendel counted tens of thousands of plants.
And why do both my parents have brown eyes while my sister has blue ones?
Because both of them are heterozygous. Each carries a hidden allele for blue. When the two hidden ones meet, it becomes visible. In humans, though, several genes are involved, so it is more complicated than in peas.
Mixing up genotype and phenotype. AA and Aa look the same but are different.
Reading the ratio as a guarantee. 3 : 1 holds statistically, not for every group of four.
"Dominant means common." Dominance says nothing about how widespread an allele is. Six fingers is dominant and still rare.
Overlooking gene linkage in the third law. On the same chromosome, 9 : 3 : 3 : 1 does not apply.
Overlooking incomplete dominance. In some species red × white gives pink — then the ratio is 1 : 2 : 1 in the phenotype as well.
Brown eyes (B) are dominant over blue (b). One parent has blue eyes, the other has brown eyes and is heterozygous. Which eye colours are possible in the children, and with what probability?
Show solution
Parental genotypes: bb (blue) and Bb (brown, heterozygous).
Germ cells: bb supplies only b. Bb supplies B half the time and b half the time.
Punnett square:
b × B = Bb → brown
b × b = bb → blue
Result: 50 % brown (Bb), 50 % blue (bb).
A cross like this is called a test cross: you cross with the homozygous
recessive partner to find out whether the dominant one is AA or Aa. With AA all the
children would be brown-eyed; with Aa about half would be blue-eyed. This was Mendel's
real stroke of method, and animal breeders still use it in exactly the same way.
Mendel published his results in 1866 — and was overlooked for almost 35 years. Only in 1900 did three researchers independently rediscover the same laws and in doing so find his work again.
Mitosis and meiosis · Chromosomes · DNA · Theory of evolution