Page 20 - Like me, or else... - Michelle Achterberg
P. 20

                                Chapter 1
 is biologically based or environmentally driven. The caption of this section specifically states nature and nurture, as a broad range of literature has shown that these two are strongly intertwined (Polderman et al., 2015). But to what extent nature and nurture contribute to (brain) development has received relatively little attention in developmental neuroscience. One particularly elegant way to study this is using a twin design: Monozygotic (MZ) twins share 100% of their genes, whereas dizygotic (DZ) twins share, on average, 50% of their genes. Therefore, within-twin correlations that are stronger in MZ twins compared to DZ twins indicate heritability (Figure 4). Behavioral genetic modeling, a specific structural equation model based on twin similarities, can provide estimates for this heritability (Neale et al., 2016). The ‘ACE’ model divides similarities among twin pairs into similarities due to additive genetic factors (A) and common environmental factors (C), while dissimilarities are ascribed to unique non-shared environmental influences and measurement error (E), see Figure 4. High estimates of A indicate that genetic factors play an important role, whilst C estimates indicate influences of the shared environment. If the E estimate is the highest, the variance is mostly accounted for by unique environmental factors and measurement error (Neale et al., 2016).
Figure 4. Twin design: Within-twin correlations that are stronger in monozygotic (MZ) twins compared to dizygotic (DZ) twins indicate heritability (NB: figure is based on hypothetical data). Behavioral genetic modeling can provide heritability estimates by assessing the proportion of variance explained by additive genes (A), common, shared environment (C) and unique environment and measurement error (E). In this ACE model, the correlation between factor A is set to r=1 for MZ twins and to r=0.5 for DZ twins, based on the percentage of overlapping genes. As both MZ and DZ twins share the same environment, the correlation of factor C is set to 1 for all twins. The E factor is freely estimated.
  18































































































   18   19   20   21   22