Beyond quantitative and qualitative traits: three telling cases in the life sciences

Abstract

This paper challenges the common assumption that some phenotypic traits are quantitative while others are qualitative. The distinction between these two kinds of traits is widely influential in biological and biomedical research as well as in scientific education and communication. This is probably due to both historical and epistemological reasons.However, the quantitative/qualitative distinction involves a variety of simplifications on the genetic causes of phenotypic variability and on the development of complex traits. Here, I examine three cases from the life sciences that show inconsistencies in the distinction: Mendelian traits (dwarfism and pigmentation in plant and animal models), Mendelian diseases (phenylketonuria), and polygenic mental disorders (schizophrenia). I show that these traits can be framed both quantitatively and qualitatively depending, for instance, on the methods through which they are investigated and on specific epistemic purposes (e.g., clinical diagnosis versus causal explanation). This suggests that the received view of quantitative and qualitative traits has a limited heuristic power-limited to some local contexts or to the specific methodologies adopted. Throughout the paper, I provide directions for framing phenotypes beyond the quantitative/qualitative distinction. I conclude by pointing at the necessity of developing a principled characterisation of what phenotypic traits, in general, are.

The DSM-5 introduction of the Social (Pragmatic) Communication Disorder as a new mental disorder: a philosophical review

“…Finally, it should be noted that the concept of threshold itself is barely defined even in more basic science like genetics and developmental biology (Serpico, 2020). 22 This brings about some arbitrariness into diagnosis as to what 'sub-clinical' means, decreasing the DSM-5's reliability.…”

“…The framing of autism-related disorders as a spectrum was also meant to accommodate the results of studies of genetic and environmental risk, which pointed at a dimensional view of psychopathology (American Psychiatric Association, 2013, p. 12; see alsoJang, 2005;Knopik et al, 2017;Plomin, Haworth, & Davis, 2009; for a critical analysis, seeSerpico, 2020).…”

“…Notably, the question of what can constitute a threshold between normal and psychopathological states is of utmost importance to achieve a clear defintion of dimensionality. Indeed, depending on whether thresholds are understood as conventional (involving, e.g., clinical or pragmatical factors) or as 'natural discontinuites,' the very concept of dimension can change (for a discussion, seeSerpico, 2020).…”

From Obesity to Energy Metabolism: Ontological Perspectives on the Metrics of Human Bodies

“…It is likely that the problem is more significant for classical research programs, such as those involved in the study of so-called qualitative traits like monogenic obesity. For a discussion on the theoretical and historical connection between the study of qualitative/monogenic traits and Mendelian methods, on the one hand, and of quantitative/polygenic traits and biometrical methods, on the other, see Serpico (2020). 11 Note that the two terms have a variety of meanings in biology.…”

The Cyclical Return of the IQ Controversy: Revisiting the Lessons of the Resolution on Genetics, Race and Intelligence

“…However, work on genetic interactions dates back to the early twentieth century; for example, see Bateson ( 1909 ); Carlborg and Haley ( 2004 ); Cordell ( 2002 ). Genotype-environment interactions were well established and discussed with respect to the study of norms of reaction and within discussions on the limitations of quantitative genetics; for example, Dobzhansky ( 1955 ); Lewontin ( 1974 ); see also Fuller et al ( 2005 ); Nelson ( 2013 ); Paul ( 1998 ); Serpico ( 2020 ).…”