Genetic Variance from a Single Locus
The purpose of this Applet is to examine the ratio of additive
genetic variance to dominance genetic variance at a single locus.
To execute this Applet, enter the genetic value for the heterozygote
Aa. This value can range between -1.0 and 1.0. A value
of -1.0 implies that allele a is dominant, a value of
0.0 implies that the system is totally additive, and a value of
1.0 implies that allele A is dominant. Values between
-1.0 and 0.0 (or between 0.0 and 1.0) imply partial dominance.
The second value to enter is the frequency of allele a.
Naturally, this value must lie between 0.0 and 1.0.
Do the following exercises:
- Set the value of the heterozygote to 0.0, so that gene action
is completely additive. Now enter any value for the frequency
of allele a and click on Submit. Notice the value next
to the label "Percent Additive." This gives the percentage
of the genetic variance that is additive, i.e., the additive genetic
variance divided by the quantity (additive + dominance variance).
Enter 5 different values for the frequency of allele a
and record the Percent Additive variance. What does this tell
you about how gene action contributes to the ratio of additive
to dominance variance?
- Set the value of the heterozygote to 1.0. In this case allele
A is completely dominant to allele a. Not let the
frequency of allele a range from .10 to .90 in increments
of .10, clicking on the Submit button each time. Record the frequency
of allele a and the result of the "Percent Additive"
column. What does this tell you about the other major factor
that contributes to the ratio of additive to dominance variance?
- To reinforce the learning from the previous exercises, let
us examine the variance components from a rare recessive disorder.
Set the genetic value for the heterozygote to 1.0 and the frequency
of allele a to .01. This is roughly the case for phenylketonuria.
Click on Submit and record the Percent Additive variance?
- To further reinforce this, let's examine the situation of
Huntington's Disease, a rare autosomal dominant. Again, set the
value of the heterozygote to 1.0, but this time, let the frequency
of allele a be .99. Again record the Percent Additive Variance.
- Compare and contrast the results of exercise (3) with exercise
(4). In both cases, the gene action (i.e., the genetic value
of the heterozygote) is the same. What accounts for the discrepancy
between the figures for Percent Additive? What general lesson
does this confirm about the relationship of "biological gene
activity" to "statistical variance components?"