A Two-reference Point Zone Genetic Model for the Distribution of Developmental Robustness in Human Offspring

Abstract

This study proposes a two-reference-point zone genetic model to explain variation in the offspring Composite Developmental Robustness Index (CDRI) as a function of parental age, addressing the limitations of conventional single-mean fitness models. Population-level phenotypic data from parents aged 20-35 years and ≥35 years were analyzed using Gaussian Mixture Modeling combined with bootstrap resampling, with statistical validation performed through Shapiro-Wilk tests, bimodality indices, and ANOVA/Kruskal-Wallis analyses. The results reveal a unimodal distribution centered near the population mean (~50 CDRI) among younger parents, whereas offspring of parents aged 35 years and older display a bimodal distribution comprising a lower peak (~35 CDRI), associated with chromosomal instability and deleterious genetic variation, and a higher peak (~70 CDRI), linked to rare advantageous phenotypic combinations. This bimodality is consistent with mechanisms such as age-related recombination variability, heterozygote advantage, and disruptive selection, indicating that phenotypic variance increases with advancing parental age. The proposed framework highlights the importance of distributional structure over mean-based interpretation in reproductive genetics and provides a conceptual basis for future applications in evolutionary biology, genetic counseling, and integrative studies incorporating epigenetic and environmental factors.