Same-Sex Twin Pair Phenotypic Correlations are Consistent with Human Y Chromosome Promoting Phenotypic Heterogeneity

Junk DNA has been long appreciated as an evolutionary facilitator because it can participate in the causation of genetic variation such as chromosome rearrangements and can be exapted into coding or regulatory elements. Recently, it has been proposed that junk DNA variation within natural populations indirectly causes a phenotypic heterogeneity that subsequently promotes genetic capacitance, i.e., the random fluctuation of genetic variation. Junk DNA role as capacitor might drive population traits such as sexual dimorphism, spatiotemporal dynamics, or genetic diversification leading into speciation. Whether the human species also showed junk DNA-based capacitance manifested as a junk DNA-dependent phenotypic heterogeneity that contributed to the etiology and expression of diseases or the evolutionary history of human populations is intriguing. Because the human Y chromosome is highly enriched in junk DNA, humans are sexually dimorphic for the genomic content in junk DNA. Thus, it would be expected that junk DNA-based capacitance in humans were manifested as a sexual dimorphism for phenotypic heterogeneity. Here, I gather supporting evidence for the existence of a sexual dimorphism for putative junk DNA-based phenotypic heterogeneity by analyzing same-sex twin pairs phenotypic concordance.