From sperm quality and testosterone to testicular size, a study of Finnish and Danish father-son pairs recently investigated how closely men's reproductive characteristics resembled those of their fathers, with the findings published in the journal Andrology.
News-Medical.net reports that for their cross-sectional comparison, the researchers looked at young males from the Finnish-Danish Boy birth cohort born in Finland or Denmark during 1997-2002, who attended a follow-up visit with their fathers at ages 18–23 (i.e, during 2017-21).
At the visit, the study teams, led by the University of Turku, in Finland, 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 gonorrhoea, 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.
What they found …
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, 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 co-efficients 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.
Resemblances
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.
Study details
Father–Son Comparison of Reproductive Traits
Elamo H, Rodprasert W, Henriksen LS, et al.
Published in Andrology on 18 August 2026
Abstract
Background
Little is known about the relative contribution of genetic and environmental factors on male reproductive function.
Objectives
We studied in a cross-sectional setting, whether testicular function and genital size correlate in father–son pairs. The study included young men from a prospective Finnish–Danish Boy cohort study and their fathers.
Materials and Methods
We collected data on young men born between 1997 and 2002 at Turku University Hospital, Finland, and at the University Hospital of Copenhagen, Denmark. These young men were invited for an adult follow-up visit at 18–23 years together with their fathers. Altogether 38 Finnish and 79 Danish father–son pairs participated in the study. The study visits included analysis of a semen sample, measurement of serum reproductive hormones and genital size (testes, penis, anogenital distance [AGD]). Intraclass correlation coefficients (ICCs) were used to evaluate familial resemblance between fathers and sons.
Results
In the adjusted analyses, weak significant positive correlations between fathers and sons were observed in testicular volume measured by ultrasonography (ICC 0.30), concentrations of inhibin B (ICC 0.25), FSH (ICC 0.24), LH (ICC 0.22), and SHBG (ICC 0.26), sperm concentration (ICC 0.21), and anopenile distance (ICC 0.19). Penile width (ICC 0.23) showed significant father–son correlation, but it was of borderline statistical significance when pairs with cryptorchidism that had required surgical correction (n = 5) were excluded. Father–son correlation for testicular size based on orchidometer (ICC 0.16) was statistically significant only when excluding pairs with a history of cryptorchidism (ICC 0.18).
Conclusion
In conclusion, while genetics contributes to reproductive health, the studied reproductive variables showed rather weak correlations between fathers and sons.
Andrology article – Father–Son Comparison of Reproductive Traits (Open access)
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