Genetics · BEGINNER LESSON
Rare variant
A rare variant is a genetic DNA change that appears at low frequency in a population, often defined here as minor allele frequency below 1%. Such variants can have little effect, or they can contribute strongly to disease risk, including inherited cancer predisposition and childhood acute lymphoblastic leukemia (ALL).
What it is and what is measured
A rare variant is a DNA sequence change whose frequency in a population is low. The sources use minor allele frequency, or MAF, meaning the proportion of sampled DNA copies carrying the variant. Here, rare variants are defined as MAF below 1%, while common variants are above that threshold in one source and above 5% in another study design. Scientists measure how often the variant appears and whether people carrying it have a different chance of developing a trait or disease, often reported with an odds ratio (OR), which compares odds between groups.
[1] Role of common host genome variants in Childhood Acute Lymphoblastic Leukemia [2] Low frequency and rare coding variants affect susceptibility ... [3] MIRAGE: A Bayesian statistical method for gene-level rare ...
How rare variants are studied
Rare variants are often studied with DNA sequencing, such as whole-exome sequencing (WES), which reads protein-coding regions. Because one rare change can be hard to test alone, researchers sometimes combine variants across a gene. Gene-based methods try to increase statistical power, which is the ability to detect a real association. Some methods also use functional annotations, meaning outside information that predicts whether a variant may affect a protein or gene.
[2] Low frequency and rare coding variants affect susceptibility ... [3] MIRAGE: A Bayesian statistical method for gene-level rare ...
Mechanisms and biological meaning
Rare variants can matter because harmful alleles are often kept uncommon by purifying selection, the removal of deleterious changes over time. The sources note that rare variants may have larger effects than common ones, and some rare germline variants underlie a fraction of childhood ALL cases. However, not every rare variant is harmful: some are silent, some change protein sequence, and some have uncertain impact. Incomplete penetrance means a person can carry a risk variant without developing the disease, which helps explain why risk variants can also be seen in controls.
[1] Role of common host genome variants in Childhood Acute Lymphoblastic Leukemia [2] Low frequency and rare coding variants affect susceptibility ... [3] MIRAGE: A Bayesian statistical method for gene-level rare ...
Limitations of detection and interpretation
Rare-variant studies face small numbers of carriers, so estimates can be unstable and effect sizes can look exaggerated in small samples. Multiple testing, the many statistical comparisons made at once, raises false-discovery risk. Genome-wide association studies (GWAS), which scan many single-nucleotide polymorphisms (SNPs, single-base DNA changes), are good for common variants but are less suited to rare ones. Grouping variants can improve power, but collapsing different variants together may hide important differences because not all variants in a gene act the same way.
[1] Role of common host genome variants in Childhood Acute Lymphoblastic Leukemia [2] Low frequency and rare coding variants affect susceptibility ... [3] MIRAGE: A Bayesian statistical method for gene-level rare ...
SEE THE IDEA
How rare-variant analysis works
A variant found in only a small fraction of the population.
Scientists estimate minor allele frequency and test disease association.
Single-variant tests or gene-based collapsing tests are used.
Results are checked against biology, annotations, and replication.
A rare variant is first counted, then tested for association, then interpreted with methods that account for low frequency and mixed effects.
Rare variants are difficult to detect one by one, so analysis often moves from frequency measurement to association testing to biological interpretation.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.