Examples of Multiple Alleles

 Another example of multiple alleles is the eye color in Drosophila. The wild eye color in Drosophila is red. There is a wide variation in eye color in Drosophila. The first white-eyed mutant was discovered and later on, several other colors of the eye were reported. The main eye colors include wild, white, cherry, blood, eosin, apricot, ivory, and cream. The wild color is dominant over all other eye colors and exhibits a 3:1 ratio in the F2 generation. The cross between individuals of other eye colors exhibits intermediate expression in F1 and true expression only in homozygous conditions.

This variation in eye color was initially considered to be due to multiple alleles of the same gene. However, later on, it was found to be due to pseudo-alleles.

4. Self-Incompatibility alleles in plants

The most common example of multiple alleles in plants is the series of self-incompatibility alleles. Such alleles were reported in Nicotiana and later on, they were found in several other plant species like Brassica, radish, tomato, potato, etc. In these species, self-incompatibility is governed by a single gene S which has multiple alleles, viz., S1, S2, S3, S4, and so on. Now cases of digenic and transgenic self-incompatibility have also been reported. In evening primrose 37 and in red clover 41 alleles of self-incompatibility have been reported.

Crosses between individuals having self-incompatibility alleles will lead to three types of situations as given below

a. Fully sterile

When both males and females have similar alleles, viz., S1S2 x S1S2 the cross will be incompatible and there will be no seed set.

b. Partially fertile

Such crosses are obtained when male and female plants differ for one allele, viz., S1S2 x S1S3. This cross will produce, S1S3 and S2S3progeny. In other words, half of the progeny will be fertile.

c. Fully fertile

The fully fertile crosses are obtained when male and female plants differ in respect of both alleles, viz., S1S2 x S3S4. This cross will produce four fertile genotypes, viz., S1S3, S1S4, S2S3, and S2S4. Thus, plants which have self-incompatibility alleles are always heterozygous for this gene.

The genes causing self-sterility in plants probably produce their effects by controlling the growth rate of the pollen tubes. Incompatible combinations, the pollen tube grows more and more rapidly as it approaches the ovule, but in non-suitable ones, the growth of the pollen tube slows down considerably, so that the flower withers away before fertilization can take place.

5. Blood groups in man

Several genes in man produce multiple allelic series which affect an interesting and important physiological characteristic of the human red blood cells. The blood is fluid connective tissue containing two principal components; cell (red, white, and platelets) and liquid (plasma), The plasma is composed of clotting protein fibrinogen and serum. The red blood cells have special antigens properties by which they respond to certain specific components (antibodies) of the blood serum. An antibody is a molecule that is synthesized by animals in response to a foreign substance. The antigen is a molecule that can cause the formation of antibodies. Antibodies are generally proteins called immunoglobulins whereas the antigen can be of protein, glycoprotein, or glycolipid in nature. The antigen-antibody relationship is one of the great specificity like that between lock and key. It was found that all persons could be classified into four groups with regard to the antigen property of the blood cells. The blood group is determined by the type of antigen which is present on the surface of red blood cells; the blood in which RBC carry antigen A on their surface is called the A-type blood group. Similarly, the RBC of blood group B will be having antigen B on its surface. Both antigens A and B will be present on the cell surface of RBC in the case of the AB blood group. However, no antigen is present on the RBC of blood group O. The antibodies in the blood plasma will be of a type other than the antigen type on RBC; for example, blood groups A, B, O, and AB will be having B, A, both AB and no antibody in its plasma respectively.

Each antigen and its associated antibody has a peculiar chemical configuration. Landsteiner discovered in 1900 that when the red cells of one person are placed in the blood serum of another person, the cells become clumped or agglutinated. If blood transfusions were made between persons of two such incompatible blood groups, the transfused cells are likely to clump and shut out the capillaries in the recipient, sometimes resulting in death. However, such reactions occurred only when the cells of certain individuals were placed in serum from certain other persons.

The agglutination reaction happens between the antibodies (in serum) of the recipient and the antigen (on RBC) of the donor. The antibodies present in the donor’s serum will be diluted and generally not react with the antigens of the recipient’s RBC. When an individual with blood group B (having antibody A in serum) receives the blood of group A (with A antigen), the agglutination of the donor’s RBCs will occur because of the presence of A antibodies in the recipient serum. A similar reaction will happen when a person with blood group A receives group B blood. Since the AB blood group lacks these antibodies in its plasma, it can receive the blood of A, B, AB, and O type, and hence the individual is called a universal recipient. As the individual with blood group O contains antibodies of both A and B in his plasma, he can receive the blood of only O type but can donate to A, B, and AB type recipient and hence called as a universal donor.

The ABO blood type is controlled by a single gene I with three types of alleles inferred from classical genetics: i, IA, and IB. The I stands for isoagglutinogen, another term for antigen. The gene is located on the long arm of the ninth chromosome (9q34). The IA allele gives type A, IB gives type B, and i gives type O. Individuals with IAIA or IAi have to type A blood, and individuals with IBIB or IBi have type B. IAIB people have both phenotypes because A and B express a special dominance relationship i.e. codominance. As both IA and IB are dominant over i, only ii people have type O blood.