Repeatability refers to the ability of a process or measurement to yield the same results when it is repeated under the same conditions.

NORMAL DISTRIBUTIONS OF PHENOTYPES
Mice
Fruit Flies
In:Introduction to Quantitative Genetics Falconer & Mackay 1996

CHARACTERIZING A NORMAL DISTRIBUTION
Mean and variance are two quantities that describe a normal
distribution.
MEAN
VARIANCE

USEFUL PARAMETERS FOR QUANTITATIVE GENETICS
 Mean: The sum of all measurements divided by the
number of measurements





 i
n
x
N
N
x
x
x
x
1
...
2
1
 Variance: The average squared deviation of the
observations from the mean
       2
2
2
2
2
1 1
 







 x
x
N
N
x
x
x
x
x
x
Variance i
n


CORRELATIONS AMONG CHARACTERS OR RELATIVES
0 + —
Covariance:
    
y
y
x
x
N
y
x
Cov j
i 

 
1
,

COMPONENTS OF PHENOTYPIC VARIATION
 The total phenotypic variance for a character (VP) is a function of:
 Genetic variance (VG)– the variance among the mean phenotypes
of different genotypes
 Environmental variance (VE)– the variance among phenotypes
expressed by replicate members of the same genotype
VP = VG + VE
 Differences between monozygotic
twins are due to environmental
factors.

IN: Lynch & Walsh. 1998. Genetics and Analysis of Quantitative Traits. Sinauer Assoc.
ENVIRONMENTAL VARIATION WITHIN PURE LINES

COMPONENTS OF GENETIC VARIATION
VG = VA + VD + VI
 The total genetic variance for a character (VG) is a function
of:
 Additive genetic variance (VA) – variation due to the
additive effects of alleles
 Dominance genetic variation (VD) – variation due to
dominance relationships among alleles
 Epistatic genetic variation (VI) – variation due to
interactions among loci

DOMINANCE VERSUS ADDITIVE GENETIC VARIANCE
 Dominance variance is due to dominance
deviations, which describe the extent to which
heterozygotes are not exactly intermediate
between the homozygotes.
 The additive genetic variance is responsible for
the resemblance between parents and
offspring.
 The additive genetic variance is the basis for
the response to selection.

RESEMBLANCE BETWEEN RELATIVES
 When there is genetic variation
for a character there will be a
resemblance between relatives.
 Relatives will have more similar
trait values to each other than
to unrelated individuals.

RESEMBLANCE BETWEEN RELATIVES DEPENDS
ON THE DEGREE OF RELATIONSHIP
Monozygotic twins
Full sibs
Parent-offspring
Half sibs
Slope of a plot of two variables (x,y) = Cov (x,y) / Var (x)
x
y

DEGREE OF RELATEDNESS AND THE COMPONENTS
OF PHENOTYPIC COVARIANCE
VA = additive genetic variance
VD = dominance genetic variance
VEs = variance due to shared environment
Relationship Phenotypic covariance
Monozygotic twins: VA + VD + VEs
Parent-offspring ½ VA
Full sibs (½ VA) +(¼ VD) +VEs
Half sibs, or
Grandparent – grandchild ¼ VA

Repeatability refers to the ability of a process or measurement to yield the same results when it is repeated under the same conditions.

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Repeatability refers to the ability of a process or measurement to yield the same results when it is repeated under the same conditions.