Genetics glossary

A place where your DNA differs from the standard human reference, such as one changed letter or a missing piece. Most variants are harmless.

One version of a gene or of a single position in your DNA. You carry two alleles at each position, one from each parent.

Heterozygous means you carry a variant on one of your two copies of a gene. Homozygous means you carry it on both copies.

How often people who carry a variant go on to develop the condition linked to it. It can be close to 100% for some variants and 30% or less for others.

The five grades clinical laboratories give a variant, under rules from the American College of Medical Genetics and Genomics: pathogenic, likely pathogenic, uncertain significance, likely benign and benign. Your report uses only the grades listed in ClinVar.

A free public database, run by the US National Institutes of Health, of what each variant is known to mean and how strong the evidence is.

One number that adds up the small effects of many variants on a single trait, such as LDL cholesterol or type 2 diabetes. It compares your total with a reference group. It is most accurate for people whose ancestry matches the studies behind it. Your report shows a score only if it held up when checked against people of your ancestry.

The two letters you carry at one position in your DNA, one from each parent, written like C/T. Consumer DNA files, such as those from 23andMe or AncestryDNA, record your genotype at several hundred thousand positions.

A test that sequences nearly all of your DNA, about 3 billion letters. Each position is sequenced about 30 times on average. A few hard regions can stay unclear.

Whole genome vs a DNA array

How your genes change the way your body handles medicines. That is why a standard dose can be too strong or too weak for some people. Show these results to your doctor or pharmacist before you start a new medicine.

A person with one changed copy of a gene for a recessive condition, who usually has no symptoms. If both parents carry a change in the same gene, each child has a one-in-four chance of having the condition.

Every gene in our carrier screen

A branch of the human family tree, traced along one line of descent. Your maternal haplogroup follows your mother's line back through the generations. If you have a Y chromosome, your Y haplogroup follows the father-to-father line.

Estimating DNA positions a test did not measure, using reference data from people whose whole genome was sequenced. DNA arrays rely on it. Whole-genome sequencing measures nearly every position directly.

The letter the standard human reference genome (GRCh38) has at a given position. A risk allele is the version linked to a trait or condition. Carrying it does not mean you have that trait or condition.