14 Chapter 1 • Genetics and the Organism the law of dominance does not hold true for many trait differences in many species. Indeed, had Mendel studied Fower color in the sweet pea, Lathyrus odoratus, he would have observed pink-Fowered offspring from the cross between a red and a white variety, and would not have observed the existence of dominance. ±inally, many traits, even in the garden pea, do not show independent inheritance, but are linked together on chromosomes. The need for a variety of model organisms While the use of a particular model organism can reveal quite general features of inheritance and development, we cannot know how general such features are unless experiments are carried out on a variety of inherited traits in a variety of model organisms with very different patterns of reproduction and development. Model organisms have been chosen partly for their different basic biological properties, and partly for small size of individuals, short generation time, and the ease with which they can be grown and mated under simple controlled conditions. ±or the study of vertebrate genet-ics, mice are to be preferred to elephants. The need to study a wide range of biological and ge-netic traits has led to an array of model organisms from each of the basic biological groups (±igure 1-15). VIRUSES
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