Genotype and Phenotype Variations of Corolla in Plants | Genetics

In this article we will discuss about the genotype and phenotype variations of corolla in plants.

East in 1910, 1916 performed experiments on flower length in Nicotiana longiflora. He took two homozygous, pure breeding lines differing in flower length, one having 41 cm the other 93 cm for average length of corolla. The minor variations in flower length within a strain were apparently caused by the environment, but the major differences between the two strains were no doubt genetic.

East crossed the two lines and obtained an F₁ progeny with flowers intermediate in length to both parents. The plants were presumed to be genetically uniform and the minor variations observed were attributed to the environment. On selfing F₁ plants the F₂ progeny showed a wide range of phenotypic variation. The F₂ plants were also selfed to get an F₃ progeny.

Corolla lengths in F₃ could be classified into different groups; the mean corolla length in each group seemed to correspond to the plant used as F₂ parent. This indicates that differences in corolla lengths between the F₃ groups are genetically based. The variations within a group are caused by the environment.

For determining the number of genes involved in the cross the proportions of genotypes identical to the parents are estimated. If 2 pairs of genes are participating, the parental phenotypes produced by homozygous genotypes would be present in 1/16 of the progeny.

With 3 pairs of genes only 1/64 individuals would be identical to the parents; in the case of four genes only 1/256 show the parental phenotypes. Thus if we observe the extreme phenotypes of the parents occurring in 1 out of 16,1 out of 64 or 1 out of 256 off-springs, it would indicate that 2, 3 or 4 pairs of genes respectively are involved.

Beyond 4 genes it is difficult to recognise a parental phenotype in F₂ generation. This is due to the fact that with an increasing number of genes, the cross between two F₁ heterozygotes produces a very large number of different genotypes, so that the proportion of individuals homozygous for all the genes is very small and difficult to detect.

For instance, if 5 gene pairs are involved there would be 1024 different gamete combinations and 243 different genotypes in F₂ (Fig. 4.2). If 10 gene pairs participate, they would produce more than 59000 different genotypes!

If we make the assumptions that polygenes assort independently, and that they produce identical additive effects then we can say that the work of Nilsson-Ehle and East could demonstrate that quantitative traits follow the same rules of inheritance as qualitative traits. While making this statement, the influence of some genetic phenomena such as linkage, epistasis and dominance have also not been considered. The effect of environment is regarded as negligible.