QUANTITATIVE
INHERITANCE
The hairs found on the basic Brassica rapa, Fast Plants, constitute a trait that is variable, quantifiable and heritable. Scientists are not sure why plants have hairs although they have some ideas. Furthermore, very little is known about the genetics and inheritance of hairiness. The number of different genes or alleles that control the number and location of hairs is also unknown. Observing and counting the hairs on Fast Plants will challenge and sharpen students' observational skills and provide them with the opportunity to ask many questions. Students often measure the height of Fast Plants with a ruler and estimate the actual height in units such as millimeters. Determining the number of hairs is different than estimating height in that each hair is a meristic trait, a discrete unit that can be counted directly. (Photograph produced by Bruce Fall at the University of Minnesota). The expression of the hairy phenotype appears to be under the control of a number of genes and is considered to be a polygenic trait. (poly= Greek word for many). For these reasons, two Fast Plants stocks, hairless, Hir (0-1), and hairy, Hir (3-6), have been developed for teachers, students, and scientists to investigate the quantitative nature of hairy's inheritance. The hairy phenotype is described as Hir (3-6). This particular symbol, Hir, is for hirsute (after the Latin for hair). You will also note that since the specific genotype for hairiness is not yet known, the phenotype symbol is used, Hir. As with phenotypes which show a wide variation in their expression, a scale from (0-9) can be used to quantify the phenotype. On the scale from (0-9), 0 = no expression (no hair), 1-2 = low expression (few hairs) 3- 6 = intermediate expression (some hairs), 7-8 = high expression (many hairs) and 9 = very high expression (very hairy), (see Figure 2). For an excellent college level experiment working with hairs on Fast Plants, Bruce Fall, etal. at the University of Minnesota has put a complete website outlining the experiment. Evolution By Artificial Selection: A Nine-Week Classroom Investigation Using Rcb's / University of Minnesota. http://genbiol.cbs.umn.edu/activities/WFP/WFP.html
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