Then, while the examination of monozygotic twins reared apart may coven wide range of environmental heterogeneity, it is most unlikely that all the environmental influences that act on a whole population will be ‘sampled’ in this way particularly with two practice of adoption agencies placing foster children in similar homes to those from which they come. Furthermore, none of the twin studies can tell us anything at all about took heritability of between- population differences. The variation of a character within any out: population can be strongly heritable whilst the difference between two populations is mama’ environmental.
2. Gene-Environment Interactions:
Other major problems may arise from the phenomena of gene- environment interaction and gene-environment correlation. The approach to estimating heritability just discussed essentially assumes that the observed phenotypic variation arises from the addition of the environmental component of the variance to the genetic component i.e. VT = VG + VE. However, different genotypes may react differently to the same environment producing a non-additive or interactive situation.
Likewise, different genotypes may not be randomly distributed over the environmental variation: monozygotic twins, for example, may seek out similar environments dependent upon their genotype.
This is a case of gene- environmental correlation and tends to overestimate heritability whereas interaction tends to under estimate it. However, the magnitudes of these interrelationships are largely unease example, are handed down through families, thus generating a correlation between generations in the knowledge they contain.
3. Transmission between Generations:
A matter which particularly affects family studies is that environments as well as genes can be transmitted and inherited between generations. The mode of transmission is very different but some of the results can be similar. Books, fore
4. Difference with Singletons:
It also needs to be borne in mind that twins are in many experiences different from singletons. Thus, they share a maternal uterus at exactly the same time and are inevitably competing for resources. Twins are also uncommon; only about 1 birth in 90 in the UK is a twin, and of these only about a quarter are monozygotic.
Difficulties of these kinds have bedeviled nature/nurture analyses and some human geneticists are completely opposed to them. This is especially so when the traits being considered are behavioral and ability ones, such as IQ where the nature of the interactions are likely to be particularly complex.