Polygenes
10.3.1 Define Polygenic inheritance.
10.3.2 Polygenic inheritance contribution to continuous variation.
10.3.1 Define Polygenic inheritance
Definition: 'A single characteristic that is controlled by two or more genes'
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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. |
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10.3.2 Polygenic inheritance contribution to continuous variation.
a) Is the genotypic variation in the population. The more genes involved with the characteristic the greater the number of phenotypic classes.
(b) Phenotypic variation = genotypic variation + environmental variation. The environmental component smooth the genotypic category differences.
Example of polygenic effect is human skin colour.
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 colours 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
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A= add melanin (with a nominal value of 1 unit)
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a= no melanin added (with a nominal value of 0 units)
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B= adds melanin (with a nominal value of 1 unit)
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b= no melanin added (with a nominal value of 0 units)
Graph: Variation in skin colour
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The graph shows the variation in skin colour and frequency. If the calculation is performed for 4 genes the graph smoothes out the different colour 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
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The addition of environmental effects on melanin production will smooth out the discontinuous categories to a curve of continuous variation.
Example: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.
Allele key:
A= add depth (1 unit)
a= no depth added
B= add depth (1 unit)
b= no depth added
C= add depth (1 unit)
c= no depth added
Heterozygous cross: AaBbCc X AaBbCc
