From sperm quality and testosterone to testicular size, a study of Finnish and Danish father-son pairs investigates how closely men's reproductive characteristics resemble those of their fathers.
Study: Father–Son Comparison of Reproductive Traits. Image Credit: Pixel-Shot / Shutterstock
In a recent study published in the journal Andrology, researchers compared reproductive traits in father-son pairs from Denmark and Finland.
Total fertility rates have substantially declined in several countries, while previous research has documented deteriorating semen quality in some populations. Longitudinal analyses indicate a decline in semen quality in Finland, with no persistent trends in Denmark in recent decades. Lifestyle, genetic, and environmental factors are among the possible contributors to changes in semen quality. The genetic contribution to reproductive health remains poorly understood, with some evidence suggesting that reproductive hormone levels in males are heritable.
About the study
The researchers conducted a cross-sectional comparison of father-son pairs. Young males from the Finnish-Danish Boy birth cohort, born in Finland or Denmark during 1997–2002, attended a follow-up visit with their fathers at age 18–23 years (i.e., during 2017–21). At the visit, the researchers evaluated subjects' height, weight, spermatic cord palpation, testicular size and position, penile width and length, anopenile distance (APD), and anoscrotal distance (ASD).
Testicular size was measured using ultrasonography (USG) and an orchidometer. APD, ASD, and penile width were measured using a caliper, and penile length was measured using a ruler. Questionnaires were administered before the study visit to collect information on general health, three-month smoking habits, alcohol intake, fever, history of gonorrhea, chlamydia, or cryptorchidism, and maternal smoking during pregnancy.
Reproductive hormones were measured in fasting serum samples. Assessed hormones were testosterone, inhibin B, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and sex hormone-binding globulin (SHBG). Hormone concentrations were determined using immunoassays, while free testosterone was calculated using the Vermeulen equation.
Inhibin B/FSH, testosterone/LH, and free testosterone/LH ratios were calculated. Participants were instructed to abstain from ejaculation for two to seven days before providing semen samples, but some reported shorter intervals. The researchers assessed semen volume, sperm motility, sperm count, and morphology. The Wilcoxon signed-rank test was used to examine paired differences between sons and fathers. Spearman's correlation was performed to examine relationships among reproductive traits between sons and fathers.
Linear mixed models were used to assess familial resemblance. Models were adjusted for various explanatory factors, including height, weight, body mass index (BMI), nationality, history of cryptorchidism, age, varicocele, smoking, abstinence time, and examiner, depending on the outcome. Time from ejaculation to semen analysis was considered for sperm motility. In sensitivity analyses, five pairs with a history of surgically corrected cryptorchidism were excluded.
Findings
The study included 117 father-son pairs: 79 from Denmark and 38 from Finland. The median age of fathers and sons was 52.2 and 19.6 years, respectively. Fathers had a higher BMI (26.1 kg/m2 vs 21.5 kg/m2) and reported a median weekly alcohol consumption of six units, compared with one among sons. In addition, fathers more often rated their health as excellent or good than sons.
Daily smoking in the past three months was more prevalent among sons. Sperm concentration was below 1 million/mL for five participants and below 15 million/mL for 23 additional subjects. Fathers had higher FSH and SHBG levels, lower free and total testosterone levels, larger testes, and lower inhibin B/FSH, testosterone/LH, and free testosterone/LH ratios than sons.
There were no significant generational differences in inhibin B or LH levels. Fathers had higher total sperm counts and sperm concentrations, whereas sons had a greater proportion of progressively motile sperm. The proportion of sperm with normal morphology and semen volume did not differ between sons and fathers. Fathers had larger APD, while sons had larger ASD. Sensitivity analyses yielded similar results.
Familial resemblance analysis revealed significant correlations between sons and fathers for ultrasound-measured testicular size, LH levels, SHBG levels, inhibin B levels, FSH levels, sperm concentration, penile width, inhibin B/FSH ratio, free testosterone/LH ratio, testosterone/LH ratio, and APD. These associations were weak, with adjusted intraclass correlation coefficients ranging from 0.17 to 0.30. In adjusted analyses, the researchers observed no significant familial resemblance for total or free testosterone, total sperm count, sperm motility, semen volume, normal sperm morphology, penile length, or ASD. In sensitivity analyses, the correlations for penile width and free testosterone/LH ratio became borderline significant, while orchidometer-measured testicular size became statistically significant.
Conclusions
The study identified limited familial resemblance in reproductive traits, such as testicular size, sperm concentration, penile width, select hormones, and APD, in father-son pairs. Researchers could not distinguish genetic influences from shared environmental or other familial factors. Limitations include the small sample size, particularly from Finland, low participation rates, substantial age differences between fathers and sons, reliance on a single semen sample, and self-reported information on cryptorchidism among fathers. The sample was too small to establish generational trends in semen quality, and fathers had demonstrated fertility, potentially affecting comparisons with their sons.
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