BIO207H5 Lecture Notes - Quantitative Trait Locus, Quantitative Genetics, Polygene

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Published on 19 Apr 2013
Chapter 5 - Quantitative Genetics
Discontinuous traits
-exhibit few distinct phenotypes
-continuous or quantitative traits display a range of phenotypes
Continuous traits
-have a range of phenotypes
-many loci contribute to phenotype
-environmental factors influence the phenotype produced by a genotype
-can be studied by using samples of populati hygons & statistics such as
(1) Mean
(2) Variance
(3) Correlations between characters
-combined with analysis of variance & regression analysis
variation can be partitioned into genetic & environmental components
broad-sense heritability of a trait is the proportion of the phenotypic variance
resulting from genetic differences among individuals
-narrow-sense heritability is proportion of the phenotypic variance due only
to additive genetic variance
-both measures depend on particular population in certain environment
amount that a trait changes in one generation as a result of selection on the trait is
called the response to selection
-magnitude of the response to the selection depends on the selection
differential & narrow-sense heritability
genetic correlations arise when 2 traits are influence by the same genes or linked
-trait is selected, genetically correlated traits also exhibit a response to
Quantitative trait loci determine continuous traits can be identified through
marker-based mapping
-QTL mapping provides estimate of the number & relative importance of
genes influencing quantitative genetic variation
The Nature of Continuous Traits
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Most traits studied have been characterized by presence of only few distinct
-seed coats of pea plants, either grey or white, seedpods either green or
yellow & plants either tall or short
-in each trait phenotypes were markedly different & each phenotype is
easily separated from all other phenotypes
-traits with few distinct phenotypes are called discontinuous traits
Discontinuous traits
-simple relationship exists between genotype & phenotype
-when dominance occurs the same phenotype is produced by 2 different
-single genotypes can give rise to range of phenotypes as the genotype
interacts with variable environments during development to give rise to
norm of reaction
-traits exhibit only few distinct phenotypes
-described in qualitative terms
Continuous traits
-there are not many traits with phenotypes that fall into a few distinct
-traits like human birth weight, adult height, protein content in corn, &
number of eggs laid by Drosophila exhibit wide range of possible
-traits with a continuous distribution of phenotypes
-phenotypes of continuous traits must be described in quantitative measures
which are known as Quantitative Traits
-field of quantitative genetics studies inheritance of these traits
Questions Studied in Quantitative Genetics
great deal of genetic variation
-amount of variation & how its distributed determines population’s genetic
-quantitative genetics plays important role in understanding of evolution,
conservation, & complex human traits
-especially important in agricultural genetics, where traits such as crop
yield, rate of weight gain, milk production, & fat content are all studied
-psychology uses it to study IQ, learning ability, & personality
-human geneticists use it to study traits such as blood pressure, antibody
titer, fingerprint patterns & birth weight
-individuals different tin the quantity of a trait
-quantitative geneticists would ask:
(1) To what degree does the observed variation in phenotype result from
differences in genotype & to what degree does this variation reflect the
influence of different environments?
(2) How many genes determine the phenotype?
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(3) Are the contributions of the determining genes equal?
(4) Are the effects of alleles additive?
(5) When selection occurs for a particular phenotype, how rapidly can the
trait change?
(6) What is the best method for selecting & mating individuals to produce
desired phenotypes in the progeny?
The Inheritance of Continuous Traits
Francis Galton & Karl Pearson demonstrated that many traits in humans such as
weight, height, & mental traits, the phenotypes of parents & their offspring are
statistically associated
-Concluded that traits are inherited
-but not sure how genetic transmission occurs
Polygene Hypothesis for Quantitative Inheritance
trait may have range of phenotypes b/c environmental factors affect the trait
-same genotype may produce a range of phenotypes (norm of reaction) or
multiple genotypes may produce the same phenotype
-Wilhelm Johansen published study showing quantitative variation in seed
weight in bean had both environmental & genetic determinants
-He recognized that both environment & genotype influence some
quantitative traits
-These traits are called Multifactorial Traits
-Inheritance of quantitative traits can be explained as being controlled by
many genes
-This explanation is called the Polygene or Multiple-gene hypothesis for
quantitative inheritance
Polygene Hypothesis for Wheat Kernel Colour
Polygene hypothesis can be dated back to when Hermann studied the colour of
wheat kernels
-started crossing true-breeding lines of red kernel plants & white kernel
-F1 had grains that were all the same shade of intermediate colour between
red & white
-Interbred F1’s, F2 progeny showed kernels that were white & many
shades of red in a ratio of ~15 red (all shades) : 1 white kernels
-15 : 1 ratio is modified ratio of 3 : 1 for expected monohybrid cross
-saw there were 4 discrete shades of red among the progeny
-counted relative number of each class & got a ratio of 1:4:6:4:1
phenotypic ratio of plants with dark red, medium red, intermediate red,
light red, & white kernels
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