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PHENOTYPIC VARIATION QUALITATIVE TRAITS: Traits that show a sharp distinction between phenotypes, i.e., coat color, sex, etc. These traits are usually controlled by a relative SMALL number of genes. Environmental influence on these traits is usually very low. Involves the genetics of individuals. QUANTITATIVE TRAITS: Traits that exhibit a CONTINUOUS RANGE from one phenotypic extreme to the other. There are no sharp distinctions between phenotypes. These traits are usually influenced by several gene pairs and environment usually has a significant influence on the trait. Involves the genetics of populations. Example: Yearling Weight Genetic factors E nvironmental factors B i r t h W e i g h t S e a s o n o f b i r t h Mother’s milk production Nutrition F e e d c o n v e r s i o n H e a l t h A D G C l i m a t e S e x B i r t h t y p e ( s i n g l e , t w i n ) All of these factors combine to produce phenotypes that tend to BLEND into each other. There are individuals on either extreme, but the bulk of the population tends to fall into the mid-range. 1 0 0 x # of pigs 80 x x 6 0 4 0 2 0 x x 0 x x 25mm 26-30 31-35 36-40 41-45 46-51 51+
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THE BELL CURVE / NORMAL DISTRIBUTION: In a large population, quantitative traits tend to fall into a bell-shaped curve. There are more individuals who are “AVERAGE” than there are “EXTREMES”. Utilizing this curve allows for the DESCRIPTION of the population as it relates to a specific trait. This allows for the ranking/placing/identification of individuals within the population. Individuals ABOVE the mean are SUPERIOR, while individuals below the mean are inferior. Thus begins the basis for selection. TERMS OF DESCRIPTION Mean (average) : Mathmatical term applied to the midpoint of a population. It does NOT describe any one individual. It tells us nothing about the variation of the population. Variance: s 2 : Defined as the average of the squared deviations from the mean. This gives a measure of TOTAL variation, but it has NO DIRECTION. Standard Deviation: : The average that each individual will deviate from the mean. It provides a more accurate measurement of variation within the population. When combined with the MEAN, the
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This note was uploaded on 01/16/2012 for the course ANSCI 319 taught by Professor Donhenneke during the Fall '11 term at Tarleton.

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