Phenotype is all the observable characteristics of an organism that result from the interaction of its genotype (total genetic inheritance) with the environment. Examples of observable characteristics include behaviour, colour, shape, and size. The phenotype may change constantly throughout the life of an individual because of environmental changes and the physiological and morphological changes associated with ageing. Different environments can influence the development of inherited traits (like size, for example, is affected by available food supply) and alter expression by similar genotypes (for example, twins maturing in dissimilar families). So, the correct answer is 'Genotype and environment interactions'.