Not just genes. A living cell.

What did your ancestors pass on?

Your ancestors did not pass on only their genes. Each of them gave life to the next generation by giving a living cell of themselves. For roughly four billion years, perhaps a little less, life has travelled this way, from one living cell to the next, until it reached you.

Four frames

Every link is a cell

Each of your ancestors gave life to the next generation by giving a living cell of themselves.

Illustration generated with AI. Schematic, not a scientific record.

The whole idea in five sentences

Your ancestors did not pass on only genes. Each gave life by giving a living cell of themselves.
A child starts as an egg that is already alive, not as a list of genes.
DNA is not a drawing of the body. The cell uses it like a box of parts, and the body grows step by step.
Natural selection never saw a gene. It saw whole living things that worked.
The chain never had to be perfect. A body can often be built in more than one way, so most single mutations do not end a line.

Genes matter a great deal. This page follows one side of a scientific debate; “a box of parts” is only a comparison. The details and the limits are in the full version.

What is really passed on

A family tree usually follows genes: half from your mother, half from your father. Switch the view to see what else came with the egg.

Illustration generated with AI. Schematic, not a scientific record.

From the mother

Half of the DNA in the nucleus
Mitochondria, with their own small piece of DNA (the cell’s power plants)
Everything else in the egg: its fluid, its outer skin and its tiny machines

From the father

Half of the DNA in the nucleus

A child does not start as a bare list of genes. It starts as an egg cell that is already alive, with all its own parts, including its mitochondria. The sperm adds little besides its DNA (and a small structure that helps the first cell divisions).

Two cells join to make one. That one cell divides again and again until it becomes a body.Figure 11.9. OpenStax, Biology 2e · CC BY 4.0 · Used unchanged.

DNA is not a drawing of the body

People often say DNA is like a blueprint. It is closer to a box of parts. The cell uses the parts to build itself, and the result depends on what happens along the way. Three examples.

Splicing

One gene, several proteins

A cell can cut and join the same gene in different ways, so one gene can make several different proteins. The cell decides which one, not only the DNA.

Pattern

Five fingers

There is no gene that says “five”. While the hand grows, chemical signals, made by many genes working together, spread through it and help set where each finger forms. Change a signal, and the number of fingers can change.

Chance

The colours of a cat

A tortoiseshell cat has orange and black patches. Early in the embryo, each cell picks one of its two X chromosomes at random and switches it off, and all its descendants keep that choice. The patches are a map of that chance.

One gene, cut two ways. The cell chooses which pieces to keep.Figure 16.11. OpenStax, Biology 2e · CC BY 4.0 · Used unchanged.

Each patch grew from cells that switched off the same X. Where the patches fall comes from chance, not from a plan.Figure 13.9. Photo: Michael Bodega. OpenStax, Biology 2e · CC BY 4.0 · Used unchanged.

One gene, one trait? Rarely

At school a gene often comes with a trait: a gene for a blood group, a gene for a disease. A few traits do work almost like that, for example Huntington’s disease and cystic fibrosis. Most common traits, such as height, depend on thousands of small gene differences and on the environment. Five examples show how much lies between a gene and a trait.

A network

Clotting is a team of proteins

Blood clots when more than a dozen proteins act one after another, and each is made under the control of at least one gene. Losing one of them can matter a lot, a little, or not at all: without factor VIII the result is hemophilia A, while without factor XII there is usually no bleeding problem.

Modifiers

One change, many outcomes

Sickle cell disease comes from one change in the gene for beta-globin, a part of hemoglobin. People who inherit it from both parents can still be very differently ill, because other genes, for example those that keep fetal hemoglobin switched on, soften it. Infections and the care a person receives matter too.

Dose

Three copies, no new gene

In Down syndrome there is a third copy of chromosome 21. No new gene appears. Genes that normally come in two copies come in three, and that is enough to change how a body develops. How much of a gene is made matters, not only whether it is there. Which of the roughly 200 genes on chromosome 21 cause which features is still being worked out.

Beyond proteins

A gene that makes no protein

The gene XIST sits on the X chromosome. It makes an RNA, not a protein, that coats one X chromosome and largely switches it off. That switch, together with two different colour variants on the cat’s two X chromosomes, makes the patches of a tortoiseshell cat. Only about 1 to 2 percent of the human genome is the part that codes for proteins. Biologists still argue about how much of the rest does a job, but some of it clearly does.

A genome is not a cell

Dolly needed an egg

Dolly the sheep was born in 1996. She was cloned by fusing an adult cell with an egg cell whose own nucleus had been removed. It worked in only a very small fraction of attempts. The DNA alone was not enough: the egg brought the living machinery that started development.

Working clotting factor VIII in the blood

Grades used for hemophilia A. About 50 to 150 percent is typical. The same missing protein gives a spectrum, not an on-off switch.

Ask “which genes, how much of each, and in which cell?” rather than “which gene?” A “gene for X” is shorthand for “a difference at this spot changes X under usual conditions”, and gene-centred biologists, Dawkins included, mean it that way too.

If a condition runs in your family, a genetic counsellor can tell you what applies to you.

Natural selection never saw a gene

Now back to the chain.

Natural selection never saw a gene. It saw your ancestors, and each of them was a whole living thing that worked.

What survives, or does not, is a whole living thing. A gene matters only through the body it helps to build.

Nobody is picking winners in advance. Lines whose bodies worked well enough carry on, and others stop, partly by chance. You belong to a line that never stopped.

Why the chain does not need to be perfect

Mutations, small changes in DNA, happen all the time. Many do no harm, and a body can often be made in more than one way. Some mutations do end a line, and selection removes them. Yours is a line that kept going.

Switch genes off. Route 1 needs gene A and gene B together. Route 2 needs only gene C. The link holds as long as one route is still open. This is a toy model, not a real pathway.

Route 1Needs A and B · open

Route 2Needs only C · open

Routes still open · 2

The link holds.

One broken gene is nothing unusual: every person carries around a hundred broken genes. A spare route or a second working copy means many single failures do not end a lineage.

The limits of this chapter

Genes matter a great deal. This chapter only says that genes are not a full plan for the body. Here is what we are sure of, and six things we are not saying.

What we are sure of

The cell is the basic living unit, not the gene.
DNA does not describe a body the way a plan describes a house.
Genes matter a great deal. A change in a single gene can cause a disease or a new trait.
Natural selection mostly works on whole living things: it acts on the traits of bodies, not on genes directly.

What we do not claim

That genes do not matter.

They do. The point is only that DNA is not a complete drawing of the body. The cell, and what happens while it grows, count too.

That mothers matter more than fathers.

Each parent gives half of the DNA in the nucleus. The egg also brings the cell’s own machinery, but that is a fact about cells, not about who matters more.

That “a box of parts” is the whole truth.

It is only a comparison. DNA also holds the instructions for making proteins and for switching genes on and off. It is more than a passive box.

That hardship is passed down as a memory.

People who were in the womb during the Dutch famine of 1944–45 carry small chemical marks on their DNA, mostly if the famine hit around the time of conception. But most such marks are erased when egg and sperm are made. Studies of the grandchildren show small, inconsistent effects, and nobody has shown that a mark on the DNA is what carries them.

That the gene-centred view is useless.

For some questions about evolution it is still the right tool. It just does not show how a body is made.

That the debate is over.

Biologists still disagree about how much each part matters. This page follows one side, the one in Philip Ball’s book. The best-known case for the gene-centred side is in the book by Richard Dawkins, listed below. He too says DNA is a recipe, not a blueprint; the argument is about which level explains most.

What did your ancestors pass on?

The whole idea in five sentences