Roughly two in three people carry at least one recessive genetic variant that would cause serious disease in a child — if their partner happened to carry the same one. Individually, carriers are healthy and unaware. Together, they have a 25% chance of passing on the condition with each pregnancy. Carrier screening surfaces these variants before conception, so you know what you're planning around.
Traditional obstetric carrier screening panels test 3–175 genes depending on how much you pay and where you live. Whole genome sequencing tests everything — approximately 20,000 genes, including every clinically relevant recessive variant that has ever been catalogued. Here's what that changes for expecting or planning parents.
How recessive inheritance actually works
Everyone inherits two copies of most genes — one from each parent. Recessive conditions require both copies to be affected. If only one copy has a pathogenic variant, you're a carrier: healthy, and unaware unless you're tested.
When two carriers of the same recessive condition conceive together:
- 25% chance the child inherits two normal copies (unaffected, not a carrier)
- 50% chance the child inherits one normal, one variant copy (carrier, unaffected)
- 25% chance the child inherits two variant copies (affected by the condition)
The 25% risk per pregnancy sounds low until you multiply by the number of children a couple might have. It's also independent per pregnancy — the odds do not "even out" across multiple children.
Conditions worth screening for
The most commonly-screened recessive conditions in Western populations, with typical carrier frequencies:
| Condition | Gene | Carrier frequency |
|---|---|---|
| Cystic fibrosis | CFTR | 1 in 25 (European) |
| Spinal muscular atrophy | SMN1 | 1 in 50 (all populations) |
| Sickle cell disease | HBB | 1 in 12 (Sub-Saharan African) |
| Tay-Sachs disease | HEXA | 1 in 27 (Ashkenazi Jewish) |
| Fragile X syndrome | FMR1 | 1 in 150 (female) |
| Thalassemias | HBA1/2, HBB | 1 in 20 (Mediterranean) |
| Alpha-1 antitrypsin | SERPINA1 | 1 in 25 (European) |
| Wilson's disease | ATP7B | 1 in 90 |
Depending on ancestry, expanded carrier screening panels catch 90%+ of the pathogenic variants that traditional panels miss. WGS catches essentially everything catalogued, plus novel variants that would not appear on a fixed panel.
Why WGS over a targeted panel
Traditional carrier screening panels have three limitations WGS solves:
1. Panel selection bias
Standard obstetric panels are built around common variants in majority populations. If your ancestry is mixed or non-European, standard panels miss disproportionately more of your relevant variants. WGS treats every position equally.
2. Novel variants
Carrier screening panels catch only variants that were on the panel when it was designed. A novel variant that arose in your family isn't on any panel. WGS reads every letter and reports anything that differs from the reference, including private family variants.
3. Structural variants and CNVs
Some carrier conditions (SMA, alpha-thalassemia) are caused by large deletions or copy number changes, not point mutations. Short-read WGS is imperfect at detecting these but still substantially better than SNP-based carrier panels.
What to do when you get the results
The best-case result is what most carriers get: you carry one or two recessive variants, your partner does not carry the same ones, and your reproductive risk is essentially the same as the background population.
If you and your partner both carry the same recessive condition — the 1–3% case — you have several options depending on where you are in the planning process:
- Genetic counselling. Non-negotiable. A board-certified genetic counsellor will review the specific variants, their penetrance, and your options in your jurisdiction.
- Preimplantation genetic testing (PGT-M). IVF with embryo screening to select embryos that did not inherit both variants. Effective, expensive, invasive.
- Prenatal diagnostic testing. Chorionic villus sampling or amniocentesis during pregnancy to determine whether the fetus is affected.
- Donor gametes. Using donor sperm or egg from a screened donor who does not carry the same variant.
- Adoption. A valid choice, made with full information.
- Proceed with informed acceptance. Many couples proceed with knowledge of the risk and appropriate prenatal monitoring.
When to test
Ideal window is preconception — before pregnancy begins — because it maximises the options above. Second-best is early in the first trimester, which still allows for prenatal diagnostic testing before decisions become more constrained.
WGS takes 3 months from order to results. Plan accordingly: order at least 4–5 months before you want to be actively trying, so you have results in hand and time for genetic counselling if needed.
Both partners, or one?
For most recessive conditions, both partners need to be tested to know your reproductive risk. Testing only one partner tells you nothing about the joint risk. If budget forces prioritisation, test the partner with the ancestry that carries the higher-frequency conditions in your context first, then decide whether to test the second partner based on the first partner's results.
For X-linked conditions like fragile X, the female partner's status is what matters for risk to sons. For autosomal dominant conditions (BRCA1/2, Huntington's), either partner testing tells the story.
Bottom line
Whole genome sequencing is one of the most cost-effective preconception interventions available. Two kits — one per partner — run €300–800 depending on sale timing, and give you carrier screening for essentially every catalogued recessive condition plus all the other reports WGS provides. Order early, budget for genetic counselling if a joint carrier finding surfaces, and use the results to inform planning — not to trigger anxiety about the vast majority of low-risk results.
Ready to sequence your genome?
Save 10% at Dante Labs with the code below — stacks on any current sale price.