Reading your own genome

Genetic testing, ancestry inference, hereditary risk, and counselling: Module 6.

DNA testing has moved from research labs into everyday life, from ancestry kits to medical screening. But reading a genome is not like reading a fortune: most results are about probabilities, not certainties. This section covers the main kinds of genetic tests, how ancestry estimates work, what risk numbers mean, and why genetic counselling matters.

What a genome can tell you

Your genome is your complete set of DNA. Tests read some or all of it and compare it to what scientists have learned from many other people.

For a few conditions caused by a single gene, a test can give a fairly clear answer. For most common traits and diseases, which are polygenic (many genes) and shaped by environment, a test can only estimate a tendency: not a fixed outcome.

Probability, not prophecy

Most genetic results are probabilities, like a weather forecast. A test might say your risk of a condition is higher or lower than average, but that is a chance, not a verdict.

Lifestyle, environment, and luck all still play a role. This is why a result should rarely be read alone, without context or expert help.

Variants, SNPs, and what results mean

The most common type of variant a test finds is a SNP (single nucleotide polymorphism), a single-letter swap at one position in the genome. Most SNPs are completely harmless and simply reflect normal human diversity. A small number are linked to disease risk; an even smaller number directly cause disease.

Tests cross-reference your variants against curated databases. Each variant gets a classification: pathogenic (known to cause disease), benign (known to be harmless), or variant of uncertain significance (VUS), meaning there is not yet enough evidence to say either way. The VUS category is large, and classifications can change as more data is gathered, so an "uncertain" result today may be resolved in future.

Sibling 50%, chimpanzee 98%, why both are true

The two numbers measure completely different things, which is why they do not contradict each other.

Any two unrelated people share about 99.9% of their DNA sequence; almost everything is identical. The small fraction that varies, roughly 3–4 million positions called SNPs, is what genetic tests actually read. A sibling inherits each of those variable positions from the same two parents, so by chance they end up with the same version at about 50% of those spots. That 50% refers only to the variable positions, not the whole genome.

The 98% figure for chimpanzees compares total sequence identity, including all the conserved stretches that have not changed since humans and chimps shared a common ancestor about 6 million years ago. Because most of the genome is conserved in both species, the total similarity is high. A 2% difference still represents tens of millions of letter changes, enough to account for all the biological differences between species.

But how do we know?

The Human Genome Project and subsequent comparative genomics studies have measured substitution rates directly between human and chimpanzee reference genomes. The ~1.2–1.5% single-base substitution rate (plus ~3% from indels) represents the accumulated mutations since divergence ~6 million years ago. Within the human population, nucleotide diversity (π) is approximately 0.1%, about 10× lower. This contrast shows that the chimp comparison counts a much larger proportion of shared conserved sequence.