Population Study Challenges Assumptions About Neutrality of Recessive Gene Carriers

Carriers of recessive genetic disorders, as a group, appear to be slightly less healthy and less likely to have children. The disadvantage is most pronounced among carriers of recessive variants linked to intellectual disability, who also tend to spend fewer years in education and are more often childless, according to research from Radboud University Medical Center (Radboudumc). The findings suggest that classic assumptions about recessive genes may need to be reconsidered.

Spontaneous Mutations and Intellectual Disability

In a 2014 study, Radboudumc researchers reported that inherited gene variants were rarely the direct cause of intellectual disability. In most cases, the condition was linked to spontaneous mutations — genetic changes that arise in the child and are not present in either parent.

Each child is estimated to acquire around 100 spontaneous mutations across their DNA. On average, only about one of these affects a gene, and only a small fraction occur in genes associated with intellectual disability. Even so, these rare events happen frequently enough to account for most cases.

Why So Few Recessive “Double Hits”?

“We felt there was something else going on,” said Christian Gilissen, Professor of Genome Bioinformatics and first author of the 2014 study.

People inherit two copies of each gene — one from each parent. If a mutation in just one copy can cause a condition, it is typically described as dominant. Other conditions involve recessive genes, where a person usually needs harmful mutations in both copies for the disorder to develop.

In clinical practice, the team observed very few cases in which children with intellectual disability carried two damaging variants in the same recessive gene. That was unexpected because more recessive genes linked to intellectual disability are known than dominant ones. Researchers began investigating why these recessive variants appeared to be relatively rare.

Large-Scale Evidence From the UK Biobank

The question led to a broader investigation, now published in Nature Human Behaviour. Researchers analyzed genetic and health data from more than 300,000 participants in the UK Biobank to estimate how frequently recessive disease variants occur in the general population.

They found that each person carries, on average, two harmful variants across approximately 1,900 recessive disease genes.

Traditional genetic teaching suggests that carriers — people with only one altered copy of a gene — should not experience negative effects. However, the data suggested otherwise. As a group, carriers had more medical diagnoses and slightly fewer children, reducing the likelihood of passing these variants on to future generations.

Education as an Unexpected Clue

When researchers examined the data more closely, recessive variants associated with intellectual disability appeared especially underrepresented compared with other recessive genes.

The UK Biobank analysis offered a possible explanation. Carriers of recessive variants linked to intellectual disability tended to spend fewer years in education, suggesting slightly lower average educational attainment — despite the long-standing assumption that carriers are unaffected.

Fewer Children and the Role of Selection

Overall, carriers of recessive variants linked to intellectual disability showed reduced reproductive success along with shorter educational trajectories.

Han Brunner, Professor of Clinical Genetics, noted that Darwin’s ideas about selection extend beyond survival alone. In addition to natural selection, Darwin also described sexual selection, where reproductive outcomes are influenced by mating patterns and partner choice.

Researchers emphasize that the study cannot prove a single mechanism behind the findings. However, they suggest that sexual selection may help explain why certain recessive variants — particularly those associated with intellectual disability — become less common in the population over time.

Implications for Genetics and Evolution

The study suggests that carriers of recessive disease variants may experience measurable disadvantages at the population level.

Although similar effects have previously been discussed for specific disorders, researchers argue that this is among the first demonstrations of a broad selection signal across recessive disorders in general.

According to the team, the findings also challenge the idea that modern medicine and living conditions have effectively stopped human evolution. Instead, the genetic landscape continues to change over time.

As Gilissen stated: “Humans will never be perfectly adapted to whatever challenges come next.”