Download Lectures for December 5&7, 2005 (Chapter 18: The Genetic Basis of

BIO 304
Genetics
Lecture Outline
Chapter 18
The Genetic Basis of Complex Inheritance
The distribution of heights among 4,995 British
women is very close to a normal distribution
Properties of 3 normal distributions having
the same mean but different variances
A normal distribution can be described by two
variables, Mean and Standard deviation
F2 progeny from a
Trihybrid (3 gene) cross
In this example, all
three genes
quantitatively affect
the same trait. The
F2 phenotypic
ratio is
1:6:15:20:15:6:1
The distribution of phenotypes with
3 or 30 genes affecting the same phenotype
The colored bars
show the result
with 3 genes. The
hatched bars show
the result for 30
genes. Either result
closely matches a
normal distribution
shown by the black
line
The distribution of a trait
approaches a normal
distribution as the
number of genes increases
In a homozygous population,
variation is caused only by the environment
Distribution of a quantitative trait
UConn students
From Heredity (1914)
by Albert Blakeslee
Students and faculty from UConn
Remake of the famous photo
Phenotypic
Distribution of a
Quantitative Trait
Statistics in Quantitative Genetics
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Population samples (random samples)
Mean
Phenotypic variance
Standard deviation
Statistics in Quantitative Genetics
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Correlation
Correlation coefficient
Covariance of x and y
Regression analysis
The combined effect of genotypic
and environmental variation
Genotypic
variance alone
Environmental variance
for the genotype aa
Environmental variance
for the genotype Aa
Environmental variance
for the genotype AA
Combined total variance
in the population
Components of Variance
• Phenotypic variance can be divided into
genetic and environmental components.
Vp=Vg + Ve + V i
Interaction between the genotype
and the environment
Components of Variance
• Environmental variance can be estimated
from variances of inbred parents and
their F1 hybrid:
Ve=(VP1 + VP2 + VF1)/3
Components of Variance
• Genetic variance is made up of
several other variances:
Vg=Va + Vd + Vi
Vg=(VF2 - Ve )
Components of Variance
Additive variance can be calculated:
Va= 2[VF2 – (VB1 + VB2)/2]
Heritability Estimates
• Useful in predicting phenotypes of
offspring from crosses.
• Heritability is a statistical measure of
how strongly the offspring resembles the
parents.
Hypothetical ParentOffspring Regressions
Parent-Offspring
Regressions
Offspring vs. midparent
A. Migratory activity in a bird
(h2 = 0.45  0.08)
B. Shell length in a snail
(h2 = 0.36  0.17)
Half-sib Design
Note the full-sibs
nested within half-sibs
Means and variances in F1 and F2 progeny from a
cross with unlinked genes and complete additivity
Effect of selection on the distribution of phenotypes
in the progeny relative to the parents
Depression of yield in corn
due to inbreeding
The fraction of affected individuals in the parents
and the progeny of affected parents for a threshold trait
Actual risks for many congenital
abnormalities and the theoretical estimates
of risks for each type of inheritance
Narrow sense heritability (h2) = fraction of genotypic variance due to additive genes
Map locations for many quantitative trait loci
(QTLs) for 3 phenotypes in the tomato genome