Volume 4: Modelling of Human Growth. Dynamic Phenotype

Human growth modelling has particular relevance in anthropological research while successful growth models can contribute in various areas of daily practical life.

In order for a model to be successful, it should be based on an individual modelling and provide biologically understandable parameters.

This monograph brings an overview of currently available models used for human growth and introduces a new model which is unlike other models based on physiological principles of growth and provides biological output. The presented model is called Dynamic Phenotype (since it models human growth as a phenotypic representation of the genotypic growth traits – a growth program acquired from the parents and modulated by the environmental factors). Dynamic Phenotype is based on the physiological principles of growth as the balance between anabolic and catabolic processes. Another important innovation with regard to the existing growth models is the fact that it is focused on the biological individual. Along with Waddington (1957), Ludvík Novák (author of the Dynamic Phenotype model) states that genetic program for growth until maturity (along with the overall genetic makeup) is created in the moment of origin of the biological individual – the creation of the zygote from the egg and the sperm. From this moment each individual has his/her own growth program set (which is modulated by nutrition and other environmental factors) and follows his/her own growth canal. The position in the canal is ensured by the action of physiological mechanisms called homeorhesis. Other model approaches do not regard the individual part of growth modeling. We feel that individual approach, especially to longitudinal data, is crucial. Each individual and its growth are different and in summarizing approaches the variability and its causes are inevitably lost.

Another important advantage of Dynamic Phenotype is its relative ease of use and interpretation for non-mathematicians – for biologists and physicians. The model is parametric and therefore the growth curve (the actual growth process and its underlying mechanisms) is modeled from the available empirical data with use of mathematical functions; these functions are characterized by parameters. Unlike most other parametric growth models, Dynamic Phenotype allows full biological

interpretation of its parameters. According to Winsor (1932), each growth curve is characterized by three parameters; the same is true for Dynamic Phenotype. The biological parameters are: the point of origin of the growth curve, the asymptote – or the limiting value of the curve and the value of the maximum gain in the inflexion point. In agreement with most model approaches, Dynamic Phenotype divides the human growth curve into three components. These components are identical

to Karlberg’s I, C and P component. Ludvík Novák, however, arrived to the division into three components based on different physiological mechanisms which control growth in each component independently of Karlberg based on his observations and profound knowledge of human physiology. In the system of Dynamic Phenotype, the components are each controlled by a separate mathematical growth function, which are all, unlike in Karlberg’s model, completely adequate for individual modeling and can be used for any growth data.

The I (infancy) component characterized by a rapid, nevertheless in the postnatal period already decelerating growth, is modeled by an exponential (Gompertz) function and is regulated mainly by IGF-I factor, and also by fetal hormones. The C component of the growth curve is in the Dynamic Phenotype system modeled by a logistic function and is characteristic by a slow, steady growth over long period of time. This phase of growth is under control of the growth hormone.

The third component, P (puberty) takes over, or is superimposed on the C component at the time of the beginning of adolescence. This phase of growth is characterized by a period of quickly accelerating growth during pubertal spurt, which then starts decelerating and levels as it nears the asymptote. It is controlled by the complex of sexual hormones and growth hormone. It is a phase of rapid growth as the organism under the influence of the sexual hormones grows rapidly in order to prepare for sexual maturity and reproduction.