Strong Genetic

Cystic Fibrosis

GeneCFTRrsIDrs113993960, rs75527207, rs78655421, rs74503330, rs80034486InheritanceAutosomal recessiveCarrier frequency~1 in 25 Northern European, ~1 in 29 Ashkenazi Jewish, ~1 in 46 Hispanic/Latino, ~1 in 65 African AmericanSystemCarrier Screening

Summary

Cystic fibrosis is a progressive condition that affects the lungs and digestive system. A defective CFTR protein causes mucus throughout the body to become thick and sticky, leading to chronic lung infections, pancreatic insufficiency, and impaired nutrient absorption. With modern treatments including CFTR modulator therapies like trikafta, median survival has improved to around 50 years and continues to rise — a dramatic shift from the outlook even a generation ago.

Practical takeaway

This becomes relevant when you're planning to have children. The single most useful step is partner carrier testing — a straightforward blood or saliva test ordered through a GP or genetic counsellor.

If your partner tests negative on a standard panel, residual risk drops significantly but doesn't reach zero (because panels don't cover every rare variant). A genetic counsellor can quantify the remaining risk based on your partner's ancestry and the specific panel used. If your partner also tests positive as a carrier, a genetic counsellor becomes essential — they'll walk through options including prenatal testing, preimplantation genetic diagnosis (PGD), and what CF care looks like today.

ACMG recommends universal carrier screening for CF regardless of ancestry. This isn't about alarm — it's about information.

Evidence detail

What Carrier Status Means For You

You are not affected by cystic fibrosis. You carry one working copy of the CFTR gene and one copy with a pathogenic variant — that single working copy is enough for normal CFTR protein function. You will not develop CF symptoms.

Where this matters is family planning. If your partner also carries a CFTR variant, each pregnancy carries a 25% chance of the child having cystic fibrosis, a 50% chance of the child being a carrier like you, and a 25% chance of the child inheriting no variants. These are per-pregnancy odds — they don't change based on previous children.

Population Context

Cystic fibrosis carrier frequency varies substantially by ancestry because the most common variant (F508del, rs113993960) originated in European populations and was likely maintained at high frequency through heterozygote advantage — carriers may have had increased resistance to certain infectious diseases.

Northern European populations have the highest carrier rates (~1 in 25). Ashkenazi Jewish populations are similar (~1 in 29). Hispanic/Latino (~1 in 46) and African American (~1 in 65) populations have lower but still clinically relevant carrier frequencies, often with different variant profiles that standard panels may cover less completely.

These frequency differences mean that "not detected" results carry different residual risk depending on your ancestry — another reason genetic counselling adds value beyond the raw test result.

Limitations

23andMe tests 29 of the more than 2,000 known pathogenic CFTR variants. These 29 were chosen because they account for approximately 90% of carriers in European-descent populations, but coverage drops for other ancestries — particularly African and Asian populations where rarer variants are more prevalent.

A "not detected" result does NOT mean you are not a carrier. It means none of the 29 tested variants were found. Residual carrier risk after a negative 29-variant panel ranges from roughly 1 in 240 for Northern Europeans to 1 in 170 for African Americans (because the panel misses a larger fraction of variants in that population).

Clinical-grade expanded carrier screening panels test hundreds of CFTR variants and provide substantially lower residual risk. If family planning decisions depend on this result, clinical confirmation through an expanded panel is recommended.

Sources (5)

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