lesson7 - 7 Module 1 Review We have covered a lot of new...

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7 Module 1 Review We have covered a lot of new and probably unfamiliar ground in these first six lessons, and now is the time to revisit some of them that traditionally have been most prone to confusion. In the first part of this lesson I will review several pairs of concepts that commonly get reversed or which are troublesome in some other way. Afterwards, you will find the answer key to the Study Questions, which includes explanations for what you may have done wrong if you chose incorrectly. Troublesome Concepts Types of reaction norms . Recall that a reaction norm is a description of the range of phenotypes that result when genetically similar individuals develop in different environmental conditions. Although there are many possible reaction norms, I’ve found that students often confuse the two major types, namely modulation and conversion. Modulation simply refers to a continuum of phenotypes with differences that can be very slight. To modulate something means to adjust it in increments. You modulate the volume on the television, for example, probably by pushing the volume control on the remote one or more times. Conversion refers to phenotypic differences that are a matter of kind rather than of degree. Another common expression here would be qualitative differences rather than the quantitative differences that exist in modulation. Conversion reaction norms result in two (or sometimes more) categorically distinct phenotypic outcomes, such as male or female in alligators, or queen or workers in honeybees. Reaction norms and gene-environment interaction . Students frequently make the mistake of thinking of reaction norms and gene-environment interaction as though they are different things. Actually, gene-environment interaction is strongly connected to the reaction norm concept. If the reaction norm of one group of genetically similar individuals is significantly different than the reaction norm of a different group of genetically similar individuals, then we have a case of gene-environment interaction. This gets back to the notion that many reaction norms can be described as straight lines with particular slopes. If the slopes of two lines are significantly different, then we have gene-environment interaction. If they are not significantly different, then we simply have two similar reaction norms. Gene-environment interaction can also be found in cases where the reaction norms are not straight lines. The reaction norm for one group might look like an inverted “U” but the reaction norm for another group might look like an Module 1 Review 89
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inverted “V.” We can also have gene-environment interaction involving conversion. For example, there is a fish species whose sex is determined by incubation temperature, with females developing in cooler water and males developing in warmer water. (Each fish embryo has the ability to develop into either sex.) The conversion point, that is, the temperature at which the switch between female and male development occurs, differs between two genetically different
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lesson7 - 7 Module 1 Review We have covered a lot of new...

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