Inheritance of Kernel Colour in Wheat | Genetics

In this article we will discuss about the inheritance of kernel colour in wheat.

Nilsson-Ehle performed many crosses between varieties of wheat having red seeds and those having white seeds. In all crosses except one the F₂ generation gave a ratio of 3 red to 1 white. In the one exceptional cross he used a very old red variety from the north of Sweden.

The F₁ plants produced seeds intermediate in colour between the two parents, and the F₂ generation segregated in the ratio of 63 red: 1 white. The noteworthy feature of this experiment was the variation in the intensity of the red pigment in wheat grains produced by F₃ plants.

There were all gradations from the deep red of one parent to pure white of the other parent so that the plants could be divided into 7 different colour classes in the ratio of 1: 6: 15: 20: 15: 6: 1. Nilsson-Ehle could distinguish 6 phenotypic classes with varying intensities of red as follows: 1 deep red (like one parent), 6 dark red, 15 red, 20 medium red, 15 light red, and 6 very light red (Fig. 4.1). Only one out of 64 plants produced completely white kernels and another one of the 64 had red kernels identical to the parents in the first cross.

Nilsson-Ehle postulated three pairs of genes controlling grain colour in wheat with genes for red (ABC) dominant over genes for white (abc). It also appeared that all alleles contributed equally in the production or absence of red pigment. Each of the three gene pairs when considered singly in crosses segregated in the expected Mendelian fashion producing F₂ progeny of 3 red and 1 white.

When the genes were considered two at a time, F₂ segregated in the ratio 15 red to 1 white. All the three genes considered together produced an F₂ ratio of 63 red to 1 white, and segregated in a manner typical for a Mendelian tri-hybrid cross.

The cross is best explained by assuming that each red gene contributes a small degree of red colour to the wheat kernel resulting in the range of red phenotypes actually observed. The proportion of plants showing deep red kernels (AABBCC) like one parent, and white (aabbcc) like the other parent is very small as expected.

The intermediate phenotypes appear with greater frequency. Moreover, the intermediate degrees of red appear to suggest some kind of blending due to the continuous range of phenotypes.

Yet it can be explained by Mendelism if we assume that many genes are controlling the trait. The term polygenes was used by Mather for genes with small effects on a character that supplement each other to produce noticeable quantitative changes. These are also called additive effects.

The small quantitative effects of polygenes can also result from environmental fluctuations. But the two can be distinguished by performing controlled breeding tests. The more difficult task is to determine the number of genes involved in a cross. This can be estimated from the frequency of F₂ genotypes which are identical to the parents.

Continuous variations in quantitative characters are due to increased numbers of segregating gene pairs in the F₁ heterozygote. The resulting distribution of phenotypes in F₂ can be plotted on a graph. The parental phenotypes and the intermediate types can be read off from the graphs as shown in Fig. 4.2. However, as will be explained later, this method is applicable only when 2, 3 or 4 gene pairs are involved but not more.

The term polygene is often used interchangeably with multifactorial or multiple factor inheritance. But strictly speaking polygenes should refer to conditions determined by a large number of genes each with a small effect acting additively.

Multifactorial inheritance is determined by a combination of genetic and environmental factors. Mostly it is difficult to ascertain whether or not environmental factors are involved, or whether all the genes controlling a trait have small and additive effects.