Inheritance and DNA · Process
Passing it on: heredity
A characteristic can vanish for a whole generation and come back unchanged. Whatever carries it cannot be a fluid that mixes — and working out what it is instead is the beginning of genetics.
Start here
Cross a tall pea plant with a short one and you get tall plants.
Not medium plants. Tall ones, every time. Then breed those tall plants together and the short ones come back, in about a quarter of the offspring — from parents that were all tall, carrying something that had not shown itself for a generation.
What does the reappearance tell you?
The instruction for short was still there the whole time — carried, complete and unaltered, by plants that were tall. Inherited information comes in separate units that keep their identity from one generation to the next. Nothing blended, and nothing was diluted. That single observation is the foundation of genetics.
At the bench · two parents, one gene
Grow seeds and count them
no seeds grown
Flower colour in pea plants. Each parent carries two copies of the gene and passes one of them, chosen at random, into each seed. P gives purple and beats p; a plant is white only if it has p from both parents.
Parent plant 1
Parent plant 2
Pp crossed with Pp
Most recent seed
Purple flowers
0
White flowers
0
Both parents carry two P versions, so there is no p anywhere in this cross to pass on. Every seed is purple, and none of them is carrying anything hidden — grow a thousand and the generation after will be purple too.
One parent has only P and the other only p, so every seed receives one of each and every seed is Pp. All purple — and every single one of them is carrying p. Cross two of these and the white flowers come back.
One parent carries two P versions, so every seed receives at least one P. Every offspring is purple however many you grow — and some of them are quietly carrying p, which will show up in the generation after.
Both parents carry only p, so every seed gets p from both sides. All white, every time. A hidden version stops being hidden when there is nothing left to override it.
Both parents are purple and both carry p. Each seed has a one-in-four chance of receiving p from both — so about a quarter come out white, from two purple parents. This is Mendel’s 3:1, and it is the result that made him think in particles.
One parent carries two of the same version and the other carries one of each. Watch the proportion rather than any individual seed — chance decides each one, and only the totals show the pattern.
How the information travels
Halved, combined, then copied into everything.
Every body cell has two of each
Chromosomes come in pairs — 23 pairs in a human — so every gene is present twice, once on each chromosome of the pair. The two copies may be the same version or different ones.
Gametes get one of each
When sex cells are made, each gamete receives one chromosome from every pair, so it carries a single version of each gene. Which one it gets is decided by chance, which is why siblings differ.
Fertilisation restores the pair
One gamete from each parent fuses and the full number is back — 46 in humans, half from each parent. This is the moment the new combination is fixed.
Every cell is a copy
The fertilised egg divides again and again, copying its DNA exactly each time, so every cell of the new organism carries the same instructions it started with.
Key fact
Heredity is the transfer of genetic information from parents to offspring. Each parent passes half their chromosomes in a gamete, so offspring carry two versions of every gene, one from each parent. Versions are not blended: one may be hidden for generations and still be passed on unchanged.
Think again
“Characteristics blend — a tall parent and a short parent give a medium child.”
This was the accepted view for most of the nineteenth century, and it has a fatal problem that Darwin himself worried about: if characteristics blended, then variation would halve with every generation and after a few dozen generations every individual in a species would be identical. Look around a classroom; that has not happened. Mendel's peas showed why. Cross a tall variety with a short one and the offspring are all tall, not medium — and the shortness reappears, undiluted, in the generation after. Information is carried in discrete units that keep their identity, and what you see is the result of which versions a plant happens to be carrying. Height in people looks like blending because hundreds of genes are involved, which is the smooth curve in Variation: continuous and discontinuous; underneath, each of those hundreds of genes is being passed on whole.
“It skipped a generation, so the gene must have disappeared and come back.”
Nothing disappeared. A characteristic that is hidden in one generation was being carried the whole time by parents who did not show it, because they also carried a version that overrides it. Set both parents on the bench to Pp — both purple — and roughly a quarter of the seeds come out white. The white version was in both parents, in every one of their cells, doing nothing visible. This matters far beyond peas: it is why two people with no family history of a condition can have a child who inherits it, and why breeders and doctors distinguish between what an organism shows and what it carries. The two questions what does it look like and what is it carrying have different answers, and only the second one predicts the next generation.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · The numbers
A human body cell has 46 chromosomes. How many are in a sperm cell, and why?
Rung 2 · The one that catches people
Two purple-flowered pea plants are crossed and about a quarter of the offspring are white. What does that show?
Rung 3 · Explain the skipped generation
A characteristic appears in a grandparent, is absent in the parents, and appears again in a grandchild. Explain how, using what you know about versions of genes and how gametes are made.
Rung 4 · Take it somewhere new
A dog breeder wants white-coated puppies, and white is the hidden version of the coat-colour gene. She has two coloured-coated dogs that have each produced a white puppy in an earlier litter. Explain what she can predict, what she cannot, and why breeding for a hidden characteristic is easier than breeding one out.
Key note
Heredity is the passing of genetic information from one generation to the next. Gametes carry half the chromosomes — 23 in humans — so a fertilised egg has a full set of 46, half from each parent, and every cell of the new organism is a copy of that. Genes come in versions; an organism carries two of each and shows one, so a version can be carried without being shown and can reappear generations later.
Going further
Gregor Mendel was a monk running a garden in what is now the Czech Republic, and between 1856 and 1863 he grew something like 28,000 pea plants, counting the offspring of every cross. Peas were an inspired choice: they self-pollinate, so he could establish pure-breeding lines; they have several characteristics that come in two clean forms; and they grow fast. Counting was the real innovation. Other people had crossed plants before and described the results; Mendel counted them, found ratios close to 3:1, and realised the ratio implied particles rather than fluids. He published in 1866 and was almost entirely ignored — his paper was cited a handful of times in thirty-four years — and he died in 1884 without knowing. Three separate botanists rediscovered the same rules in 1900 and found his paper already there. It is the clearest case in science of a correct answer arriving before anyone had a use for it.
Before this lesson
Connects to
At GCSE this becomes
- Alleles, dominant and recessive, homozygous and heterozygous, Punnett squares and probability, and inherited disorders.
Where to next
Ask Mr Badmus AI
Want to work out what a cross would produce before you run it?
The bench uses one gene with two versions and a clean dominance rule, which is the simplest case and the one Mendel chose deliberately. Most characteristics involve several genes, several versions and no clean dominance, and a real 3:1 ratio only emerges over large numbers — which is why the hundred-seed button exists.
Lesson content © MrBadmusAI.