Write a note on inheritance of corolla length in Nicotiana.
Answers
The inheritance due to multiple factors or polygenes will be illustrated in this section with the help of three examples : (i) kernel colour in wheat (studied by H. Nilsson-Ehle). (ii) skin colour in Humans (studied by C.B. Davenport) and (iii) corolla length in Nicotiana longiflora (studied by E.M. East).
Kernel colour in wheat
Kernel colour in wheat is a quantitative character and was studied by H. Nilsson-Ehle for the first time in 1908. It was argued that if one gene was considered or in other words, if the two parents differed due to one gene only, a 3 : 1 ratio for red and white kernels was obtained in F2 generation. However, out of three red, one was as red as one of the parents and two were lighter and were comparable to F1 individuals. This indicated that the dominant alleles had a cumulative effect. If 'K' stands for red colour and V for white, the two parents could be designated as RR and rr, the F1 could be designated as Rr and F2 would be obtained in 1RR : 2Rr : 1rr ratio. In these three classes, RR should be red, Rr should be intermediate in colour and rr should be white.
In case there were two genes involved, a 15 : 1 ratio (15 coloured : 1 white) would be obtained (Table 4.1). If different shades are taken into account, 1:4:6:4:1 ratio will be obtained, provided Rx and R2 contribute equally to the colour (Fig. 4.1). However, it is known now thatthere are three genes involved in kernel colour in wheat. Obviously if the two parents differ in all three genes, in F2 63 : 1 or 1 : 6 : 15 : 20 : 15 : 6 : 1 ratio will be obtained (Fig. 4.2). By the study of kernel colour in wheat, Nilsson-Ehle reached the conclusion that the effect of each dominant allele was cumulative and hence forwarded his multiple factor hypothesis. The hypothesis states that for a given quantitative trait there could be several genes, which were independent in their segregation, but had cumulative effect on. phenotype.
Relative frequencies (theoretical) of different grain colours in wheat in F2 population derived from two strains differing in two gene pairs
Fig. 4.1. Relative frequencies (theoretical) of different grain colours in wheat in F2 population derived from two strains differing in two gene pairs.
Relative frequencies (theoretical) of different grain colours in wheat in F2 population derived from two strains differing in three gene pairs
Fig. 4.2. Relative frequencies (theoretical) of different grain colours in wheat in F2 population derived from two strains differing in three gene pairs.