One of the basic characters of a functioning multi gene system is that the F2 generation of a cross between two individuals would exhibit a continuous venation forming a spectrum. But in some instances, F2 individuals of a polygenic system vary in their behavior in that they do not resemble any of their parents are grandparents.
In some instances the characters of F2 exceed the parameters of the P, generation, which is normally maintained in the F In other words – suppose the parents differ in length between 2 – 6 cm, any spectrum exhibited by F2 must be within this parameter.
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Transgressive Variations:
Instances where such variations occur transgressing the limits are called transgressive variations. Punnet and Bailey have reported some instances of transgressive variations in poultry birds.
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In a cross between a large golden Hamburg chicken and small Sebright bantam variety, the F progeny was intermediate between the two parents as could be expected in a polygene system.
But surprisingly in the F2 generation (obtained by a cross between F, individuals) some of the birds were either larger than the golden Hamburg parent or (some) were smaller than the Sebright bantam parent.
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This was explained as due to the fact that the either of the parents did not have the entire dominant or all the recessive alleles with the result they did not set the parameters. In the F2 generation
There is a possibility of some of the birds getting the entire dominant or all the recessive genes, thus causing them to transgress the so called limits set by the parents. The genotype here is controlled by 3 pairs of genes with the golden Hamburg having AABBCC and Sebright bantam having aabbccDD.
Regressive Variations:
This is another form of variation in polygene in which, the phenotype of the offspring tends to exhibit the mean of the Ponulation rather than that of the parents. For instance, the height in human beings is one such trait subject to regressive variations.
The children of two tall parents are sometimes shorter than either of them because their height tends to-approach the mean height of the population, rather than mean height of the parents. In some cases short parents have tall children for the same reason.