Week 6 samenvatting
Ch 19
19.1 measuring quantitative variation
A quantitative trait: any trait exhibiting complex inheritance because it is controlled by
a mix of genetic and/or environmental factors
A complex trait is any trait that does not show simple Mendelian inheritance
A complex trait can be either a categorical trait such as the presence of absence of a
disease condition, or a continuously variable trait such as height in humans
Multifactorial hypothesis: a hypothesis that explains quantitative variation by
proposing that traits are controlled by a large number of genes, each with an small
effect on the trait
Continuous trait: a trait that can take on a potentially infinite number of states over a
continuous range, such as height in humans
Categorical trait: a trait for which individuals can be sorted into discrete or
discontinuous groupings, such as tall versus short stems for Mendel’s pea plants
The field of quantitative genetics studies the inheritance of complex traits using some
basic statistical tools including the mean, variance, standard deviation, and normal
distribution
19.2 a simple genetic model for quantitative traits
X=G+E
Vx = Vg + Ve
Correlation coefficient “r” measure of association between two variables
An individual’s phenotype for a trait can be expressed in terms of its deviation from
the population mean.
The phenotypic deviation (x) of an individual is composed of the sum of its genetic
deviation (g) and its environmental deviation (e).
The phenotypic variation in a population for a trait (Vx) can be decomposed into the
genetic (Vg) and the environmental (Ve) variances
19.3 broad-sense heritability: nature versus nurture
Broad-sense heritability (H2) is the ratio of the genetic (Vg) to the phenotypic (Vx)
variance
H2 provides a measure of the extent to which differences among individuals within a
population are due to genetic versus environmental factors
Estimates of H2 apply only to the population and environment in which they were
made. H2 is not useful for interpreting differences in trait means among populations
H2 = correlation of (e.g. monozygotic twins)
H2 = genetics (Vg)/(genetics + environment (Vx or Vg + Ve))
H2 value between 0 and 1, indicating the proportion of variance that can be attributed
to genetic variation
19.4 narrow-sense heritability: predicting phenotypes
h2 = regression of offspring on parent average
h2 = Va/Vx or Va/(Va + Vd + Ve)
Va = the difference between the two homozygous classed divided by 2
Vd = Xb1b2 – ((Xb2b2 + Xb1b1)/2) (Vd = 1 means dominance)
When the trait value for the heterozygous class is midway between the two
homozygous classes, gene action is called additive
Ch 19
19.1 measuring quantitative variation
A quantitative trait: any trait exhibiting complex inheritance because it is controlled by
a mix of genetic and/or environmental factors
A complex trait is any trait that does not show simple Mendelian inheritance
A complex trait can be either a categorical trait such as the presence of absence of a
disease condition, or a continuously variable trait such as height in humans
Multifactorial hypothesis: a hypothesis that explains quantitative variation by
proposing that traits are controlled by a large number of genes, each with an small
effect on the trait
Continuous trait: a trait that can take on a potentially infinite number of states over a
continuous range, such as height in humans
Categorical trait: a trait for which individuals can be sorted into discrete or
discontinuous groupings, such as tall versus short stems for Mendel’s pea plants
The field of quantitative genetics studies the inheritance of complex traits using some
basic statistical tools including the mean, variance, standard deviation, and normal
distribution
19.2 a simple genetic model for quantitative traits
X=G+E
Vx = Vg + Ve
Correlation coefficient “r” measure of association between two variables
An individual’s phenotype for a trait can be expressed in terms of its deviation from
the population mean.
The phenotypic deviation (x) of an individual is composed of the sum of its genetic
deviation (g) and its environmental deviation (e).
The phenotypic variation in a population for a trait (Vx) can be decomposed into the
genetic (Vg) and the environmental (Ve) variances
19.3 broad-sense heritability: nature versus nurture
Broad-sense heritability (H2) is the ratio of the genetic (Vg) to the phenotypic (Vx)
variance
H2 provides a measure of the extent to which differences among individuals within a
population are due to genetic versus environmental factors
Estimates of H2 apply only to the population and environment in which they were
made. H2 is not useful for interpreting differences in trait means among populations
H2 = correlation of (e.g. monozygotic twins)
H2 = genetics (Vg)/(genetics + environment (Vx or Vg + Ve))
H2 value between 0 and 1, indicating the proportion of variance that can be attributed
to genetic variation
19.4 narrow-sense heritability: predicting phenotypes
h2 = regression of offspring on parent average
h2 = Va/Vx or Va/(Va + Vd + Ve)
Va = the difference between the two homozygous classed divided by 2
Vd = Xb1b2 – ((Xb2b2 + Xb1b1)/2) (Vd = 1 means dominance)
When the trait value for the heterozygous class is midway between the two
homozygous classes, gene action is called additive
