Nature Vs Nurture

A central question in biology is whether observed variation in a particular trait is due to environmental factors or biological factors – sometimes expressed as the nature versus nurture debate. Heritability is a concept which summarizes how heritable a phenotype is, in particular with reference to the resemblance of offspring and parents. Heritability is both a word that is used in common speech and a technical term in genetics (Stoltenberg 1997, Visscher, Hill et al. 2008).

Heritability is formally defined as the ratio of additive genetic variance to total phenotypic variance (Falconer, Mackay et al. 1996). Observed phenotypes (P) can be partitioned into a statistical model representing the contribution of both the genotype (G) and environmental

Yang provided evidence that most of heritability is not missing but has not previous been detected because the individual effects are too small to pass stringent significance tests. He further reported a software tool called genome-wide complex trait analysis (GCTA), which was developed to focus on the estimation of the variance explained by all SNPs together and to address the “missing heritability” problem (Yang, Benyamin et al. 2010, Yang, Lee et al. 2011).

1.1.3 Applications

The importance of heritability remains central, and it enables the dissection of phenotypic variation and the interplay between genes and environment to be unraveled more clearly. The parameter of heritability is so enduring and useful because it allows the meaningful comparison of traits within and across population, it enables predictions about the response to both artificial and natural selection, it determines the efficiency of gene-mapping studies and it is a key parameter in determining the efficiency of prediction of the genetic risk of disease (Visscher, Hill et al. 2008). 1.2 Genotype-by-environment

However, detection of selective sweeps is complicated by the fact that the effect of selection on the distribution of genetic variation can be difficult to distinguish from the pattern of genetic variation that arises after certain demographic events (Akey, Zhang et al. 2002). For example, an excess of rare alleles can be caused by both selection and by population expansion or contraction (Tajima 1989, Przeworski, Hudson et al. 2000). In dairy breeds, effective population size has decreased dramatically as a result of breed formation and widespread use of artificial insemination in the dairy industry (Goddard 1992, Zenger, Khatkar et al. 2007). One source of information, which can be used to distinguish the signatures of selection from those demographic events, is the location of marker loci in the genome. While demographic events are expected to alter patterns of allele frequencies across the whole genome, selection events alter allele frequencies of markers only in close proximity to the selected mutation (Bamshad and Wooding 2003). Hence, by sampling a large number of loci throughout the genome, empirical distributions of test statistics can be constructed and provide the opportunity to disentangle the effects of demography and selection without a priori knowledge of parameters of demographic history (Kelley, Madeoy et al.