In Chapter 3, we learned that each phenotype is the result of a genotype developing within a specific environment; the genotype sets the potential for development, but how the phenotype actually develops within the limits imposed by the genotype depends on environmental effects. Stated another way, each genotype may produce several different phenotypes, depending on the environmental conditions in which development occurs. For example, genotype GG may produce a plant that is 10 cm high when raised at 20°C, but the same genotype may produce a plant that is 18 cm high when raised at 25°C. The range of phenotypes produced by a genotype in different environments (in this case, plant height) is called the norm of reaction (I FIGURE 5.18).
For most of the characteristics discussed so far, the effect of the environment on the phenotype has been slight.
Genetic maternal effect Cytoplasmic inheritance
Phenotype Determined by genes located on the sex chromosome genes on autosomal chromosomes that are more readily expressed in one sex autosomal genes whose expression is limited to one sex nuclear genotype of the maternal parent cytoplasmic genes, which are usually inherited entirely from only one parent genes whose expression is affected by the sex of the transmitting parent
I 5.18 Norm of reaction is the range of phenotypes produced by a genotype in different environments. This norm of reaction is for vestigial wings in Drosophila melanogaster. (Data from M. H. Harnly, Journal of Experimental Zoology 56:363-379, 1936.)
Mendel's peas with genotype yy, for example, developed yellow endosperm regardless of the environment in which they were raised. Similarly, persons with genotype ]A]A have the A antigen on their red blood cells regardless of their diet, socioeconomic status, or family environment. For other phenotypes, however, environmental effects play a more important role.
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