31 - 13. Genetic influences on development.

13. Genetic influences on development.

© SPMM Course 13. Genetic influences on development.

Brain development is genetically programmed. For example, the cortical thickness in lateral prefrontal, medial prefrontal, parietal, and temporal cortices is highly heritable. Regions crucial for evolutionarily advanced functions (such as higher‐order cognition, sociality, and language) show both high heritability and an increase in heritability with age during development. This pattern is similar to the age-related increase in heritability for traits such as IQ (discussed earlier). This could be either due to an age‐dependent gene expression or due to gene‐environment interaction. Gene-environment interaction refers to the influence of the relationship between genotype and the environment in shaping a phenotype. Consider the example of phenylketonuria, a genetic condition caused by the lack of an enzyme that breaks down the amino acid phenylalanine, resulting in intellectual disabilities. However, the usual dietary environment of a child can be altered by the use of phenylalanine-free foods, which results in normal development. Waddington proposed the concept of canalization in developmental genetics. Certain behaviour traits are strongly genetically determined (canalized), so development follows these behaviours (e.g. crawling). Other behaviours are poorly canalized (e.g. cycling), so environmental factors will influence these traits. Further, some traits appear to be heavily canalized early in development (e.g., language development), but less so later on (e.g., reading ability). Gottesman put forward the notion of a range of reactions. The genetic make-up of a child does not shape any behaviour in its entirety; instead genes only set limits (or range) within which the individual variability is shaped by the environment. Scarr & McCartney proposed the concept of ‘niche-picking’ in developmental genetics (1983). The genetic make-up of a child does not contribute to skills or behaviours as such; instead genes only contribute to propensities toward certain skills and abilities; children then seek activities that are compatible with their genetic endowment Three types of G-E interaction are notable:  Passive GE‐Interaction: A child’s environment is influenced in part by parental genes which are in turn correlated with the child’s genes. Thus, the phenotype is not a direct result of the child’s genotype; but results from environmental influences that are indirectly associated with the parental genes. This kind of passive G-E will show decreasing influence over development. e.g. a boy who does not inherit the same tallness and

© SPMM Course physique as his father may still develop into a good basketball player in line with the father due to repeated exposure to basketball sessions.  Evocative GE‐Interaction: A child’s environment is influenced in part by genetically shaped behaviour. Thus, the phenotype is not a direct result of the child’s genotype; but results from environmental influences evoked by the child’s own genes. This kind of evocative G-E will show stable influence over development. e.g. a girl who inherits impulsivity may evoke an abusive parental reaction, leading to later depression, though depression itself is not inherited.  Active GE‐Interaction: A child’s environment is influenced in part by an active choice of the child to complement genetically shaped interests. Thus, the phenotype is not a direct result of the child’s genotype; but results from environmental influences that are actively associated with the child’s own genes. This kind of active G-E will show increasing influence over development. e.g. a girl who inherits impulsivity may choose to gamble, losses in which may lead to later depression though depression itself is not inherited.