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Sperm quality in men as young as 30 deteriorating, raising risk of harmful genetic mutations in children
Thursday, October 23, 2025
New research reveals a surge in harmful genetic mutations—linked to conditions like cancer and autism—in sperm, urging aspiring fathers to reconsider delaying fatherhood.
Photo credit: Shutterstock
By Mary Wangari
Correspondent
What you need to know:
- Causes linked to declining male fertility globally and in Kenya include sexually transmitted infections, hormonal abnormalities, lifestyle factors such as obesity, stress, alcohol and smoking, as well as some pesticide and chemical exposure.
New scientific evidence suggests that men who delay fatherhood until their early thirties are more likely to transmit several harmful mutations to their offspring.
The research, published this month in Nature journal, found that the risk of transmitting multiple harmful mutations surges with the father's age. UK-based researchers used advanced DNA sequencing to analyse over 1,000 sperm samples from 81 men aged 24 to 75.
Also read: How alcohol robs men of sex, fatherhood, and family
The study dubbed ‘Sperm sequencing reveals extensive positive selection in the male germline’ explains that mutations occurring in cell lineages of sperm or eggs can be passed to offspring. "In humans, positive selection of driver mutations during spermatogenesis can increase the birth prevalence of certain developmental disorders," the study states, adding: “Until recently, characterising the extent of this selection in sperm has been limited by the error rates of sequencing technologies.”
The results were striking. The DNA analysis indicated that one in 50 sperm samples (two per cent) from healthy men in their early 30s already harboured harmful mutations known to cause health disorders in children. This rate increased significantly with age, rising to between three and five per cent in men aged 43 to 74.
Subjects aged 70 years recording a 4.5 per cent of deadly mutations in their sperm.
The researchers emphasised a clear link between paternal age and genetic risk for offspring, urging aspiring fathers to consider these findings.
"We show that positive selection during spermatogenesis drives a two to three-fold increased risk of known disease-causing mutations, which results in three to five percent of sperm from middle-aged to older individuals with a pathogenic mutation across the exome.”
Experts warn these findings “shed light on germline selection dynamics and highlight a broader increased disease risk for children born to fathers of advanced age than previously appreciated.”
The surge in harmful mutations did not solely originate from the accumulation of random DNA errors over time, the scientists explained.
They pointed to a subtle, natural selection process within the testes that seemingly gives reproductive advantage to certain mutations, which enables them to be more common during the formation of sperm.
“We detected 40 genes (31 newly identified) under significant positive selection in the male germline that have activating or loss-of-function mechanisms and are involved in diverse cellular pathways.”
Harmful gene mutations are linked to various health complications, including a higher risk of cancer and autism in children. While the rate of harmful mutations in sperm increased with age, scientists clarify that not all toxic mutations lead to a successful pregnancy, as some cause miscarriages, hinder fertilisation, or impede normal development of the embryo.
The experts called for extensive research to investigate how children's health outcomes are affected by rising sperm mutations.
Also read: Why women’s eggs cost more than sperm at Kenya fertility clinics
The findings come at a time when delaying fatherhood and motherhood is an increasing trend. The study provides significant insights into ageing-related reproductive health risks in men, which have historically been inadequately explored compared to women, where older maternal age is linked to a higher likelihood of birth defects due to chromosomal abnormalities in eggs. The findings reiterate the critical need to "recognise that the age of both parents contributes to risk of disorders in offspring."
“Future research will refine our understanding of selective pressures and disease risk associated with germline mutation, clarifying their implications for human reproduction and health.”
Decreasing male fertility (sperm count)
This genetic deterioration occurs amid a global trend of declining male fertility. While women are born with all their lifetime eggs, sperm production in men begins around age 10−12 and continues throughout life, with an average man producing millions of sperm daily, which mature fully within three months.Sperms are quite fragile despite their ability to survive outside the body, with just minimal body chemistry changes triggering a significant impact on the ability to fertilise an egg, move and grow.
Increasing scientific evidence indicates that the global average sperm count has fallen by over 60 per cent since the 1970s, with an accelerated decline of about one per cent per year since 2000.
This significant drop is also reported among African men, including Kenyans, with a 2022 review noting deteriorating sperm volume and quality since 1965. Causes linked to declining male fertility globally and in Kenya include sexually transmitted infections, hormonal abnormalities, lifestyle factors such as obesity, stress, alcohol and smoking, as well as some pesticide and chemical exposure.
mwnyambura@ke.nationmedia.com