3. Gene complex

A pair of genes controlling the production of a single trait is true in some cases only. Most characteristics are determined by the interaction of two or more pairs of genes. Such interacting genes are said to form a “gene complex”. There are several types of gene interactions.

Complementary genes

Definition. Two independent pairs of genes, which interact to produce a trait together, but each dominant gene alone does not show its effect, are called complementary genes.

Several varieties of white-flowered sweet peas (Lathyrus odoratus) are known and a cross between most of these varieties yields only white-flowered offspring. However, a cross between two particular white-flowered varieties produces plants with purple flowers. F1 plants on inbreeding or self-fertilization give F2 generation in a ratio of 9 purple flowered to 7 white-flowered plants.

Explanation. Two pairs of genes located in different chromosomes regulate the flower colour in these varieties of sweet peas. One dominant gene, say C, controls the synthesis of the raw material for the purple pigment; and the other dominant gene, say E, controls the formation of an enzyme that converts the raw material into the purple pigment. At least one C gene and one E gene must be present to produce purple flowers. The absence of any one or both results in white-flowers. Of the above two white flowered varieties, one is genotypically ccEE, and the other Ccee. The former lacks the gene for the formation of the raw material, and the latter lacks the gene for the formation of enzyme needed to change the raw material into the final pigment. Hence, both are white-flowered. A cross between CCee and ccEE gives F1 generation, all the plants of which are genotypically CcEe and phenotypically purple-flowered, as they carry a gene for the raw material as well as a gene for the enzyme required to convert the raw material into the pigment.

F2 generation derived from a cross between two purple-flowered plants of F1 generation. The cross gives a ratio of 9 purple to 7 white flowered plants instead of the Mendelian dihybrid F2 ratio of 9 : 3 : 3 : 1.

A pure variety of purple - flowered sweet peas can be produced by self – fertilization of a plant with the genotype CCEE.

The extracts of white flowers of the two parents having different dominant genes appear colourless when kept separately but when mixed together, purple colour develops. This shows that the products of gene C and E are able to interact complementarily even in the test tube. This proves that the genes C and E produce anthocyanin (the pigment) in two biochemical reactions. The end product of first reaction forms the substrate of the second reaction.

Definition. Two independent pairs of genes, which interact to produce a new trait together, but each dominant gene alone produces its own trait, are called supplementary genes.

Example. The following cross illustrates this kind of interaction of genes –

The various breeds of chickens (domestic fowls) have some characteristic types of comb. The white Leghorn has single comb, which consists of a large upright blade. The Wyandotte has rose comb, which is elongated but low and bears papillae on the base. The Brahma has pea comb, which is quite small and bears 3 longitudinal ridges. All these three types are true breeding, and the pea comb and the rose comb are dominant over the single comb. When the pea-combed and the rose-combed chickens are crossed, all the offspring in F1 generation have an entirely new type of comb that resembles half a walnut kernel and is called the walnut comb. When the 2 walnut-combed F1 individuals are mated, offspring of F2 generation show a ratio of 9 walnut-combed : 3 pea-combed : 1 single-combed. The walnut comb is, thus, not a unit character, but merely a pheonotype of pea and rose when they act together.

Explanation. Two pairs of genes located in different chromosomes determine the comb type, and each displays a simple dominance. If P represents the gene for the pea comb and p its recessive allele, and if R represents the gene for the rose comb and r its recessive allele, the gene equipment for the pea comb would be PPrr, and that for the rose comb ppRR. Any individual with at least one dominant gene of each of the above two comb types, i.e., one P and one R, would have a walnut comb. Single comb would result from the lack of both dominant gene (pprr).

The above cross indicates that -

a) combinations of the genes P and R give walnut phenotypes due to their interaction that meaifies the trait;

b) combinations of the gene P give pea phenotypes due to its own simple dominance ;

c) combinations of the gene R give rose phenotypes due to its own simple dominance;

d) lack of both P and R genes gives single phenotype.