lecture 20

Quantitative inheritance – Multiple factor hypothesis – Nilsson Ehle experiment on wheat kernel colour

MULTIPLE FACTOR HYPOTHESIS - Polygenic inheritance quantitative characters:

• It shows more or less continuous variation and are governed by a large number of genes called ' multiple gene' or 'multiple factor' or 'polymeric genes' or 'polygenes'.

Nilson -Ehle's studies on kernel colour in wheat:

• The Swedish geneticist Nilson - Ehle (1908) effected crosses between different true breeding strains of wheat with red kernels and those with white kernels.

• Careful examinations however revealed that, a red colour of the F1 was not so intense as the red colour of the parent and that in the F2.

• Some red grains were as dark as those of parent and others only as dark as those of the F1. It was possible to separate the F2 in to the following;

• It is evident that, red colour is due to two pairs of alleles. Each gene is capable of producing red colour.

• Each is in completely dominant over white and in cumulative in its effect. The intensity of red colour depends upon the number of colours producing gene present.

• From these studies, Nilson-Ehle proposed the multiple factor hypothesis for the inheritance of quantitative characters. This assumes that there is a series of independent genes for a given quantitative trait.

• Dominance is usually in complete, but these genes are cumulative or additive in their effect. Each gene adds something to the strength of expression of the character, whereas its allele does not possess any effect.

Studies on ear length in corn (Emerson and East 1913):

• The number of extreme types was small, large number of F2 being intermediate in ear length.

• Mean length - 12.89 cm (approximate intermediate between parents) equal to F1 means.

• The increase in variability in the F2 was due to genetic segregation and recombination.