Download 8 - Bioclass

Polygenic Inheritance
Definition: 'A single characteristic that is controlled by two or
more genes'
Each allele of a polygenic character often contributes only a small
amount to the over all phenotype. This makes studying the individual
alleles difficult.
In addition environmental effects smooth out the genotypic variation
to give continuous distribution curves.
Polygenic characteristic more often show continuous
variation.
(a) Is the genotypic variation in the population?
(b) Phenotypic variation = genotypic variation + environmental
variation.
Vp = Vg + Ve
Example: Polygenic Inheritance/ Human skin color
This is controlled by as many as 4 genes each with its own alleles.
As the number of genes increases the amount of phenotypic variation
increases.
The alleles control the production of melanin which is a pigment that
colors skin.
In this example the calculation is performed with 2 genes each with 2
alleles. The cross is between two individuals heterozygous at both
alleles
Allele Key
*
A= add melanin
*
a= no melanin added
*
*
B= adds melanin
b= no melanin added
AaBb X AaBb
meiosis
*
The graph shows the variation in skin color and frequency. If the
calculation is performed for 4 genes the graph smoothes out the
different color types
*
The frequency of groups will be affected by the local gene pool
and this graph should not be over interpreted for skin group frequency
Example 2: Finch Beak Depth
Finches are seed eating birds that use their beaks to break open
seeds. The depth of beak is under polygenic control of three genes
with two alleles each.
*
Produce the Punnett square for the following cross between two
birds which are heterozygous at all three loci.
Allele key:
*
A= add depth
*
a= no depth added
*
*
B= add depth
b= no depth added
*
*
C= add depth
c= no depth added
cross:
Example Wheat Color
Wheat polygenics was first worked out in 1909 by Nilsson- Ehile
Each of the alleles (for the three genes) of the wheat has a small effect
on the phenotypic variation for color. The additive effect produces the
continuous variation that is found in Wheat red coloration
Cross:
*
Two plants heterozygous at each 3 gene loci
*
calculate the phenotypic ratio and draw a graph of the
phenotypic variation
Allele Key:
*
3 genes with 2 alleles each. A, B and C
*
Each 'dominant' allele produces one unit of color.
*
The homozygous dominant at all three loci produces dark red
wheat.
*
The homozygous recessive at all three loci produces pale white
color.
Example: Wild Columbine
Another example of how the height of a plant is controlled by a
polygenic system. Here there are two genes each with two alleles.
Again cross two heterozygotes at each loci.
Allele key:
*
M1= Tall plant
*
M2= Small plant
*
*
N1=Tall plant
N2=Small plant
Example Chicken Combs
Controlled by two genes this produces four phenotypes.
*
*
Using the allele key:
gene 1: P and p
gene 2: R and r
Exercise:
Produce a completed allele key and a punnett grid with genotypic and
phenotypic ratios that explain the phenotypes