Multiple Alleles Blood Type

Multiple Alleles
According to Mendelism, a genetical character is controlled by the combined action of two genes or factors in the same loci of two homologus chromosomes. It means that each gene has two alternative forms or alleles and their expression are knwon as allelomorphs. Of these two, one is dominant and the other recessive. However, some genetical characters are determined by several forms of an allele known as multiple alleles. There are many examples for multiple allelism.

1. Skin colour in rabbits Rabbits have five kinds of skin colour, coloured (agouti), chinchilla,
himalayan albino and albino, light grey. Of these, the chinchilla variety is lighter in colour. Himalayan albino will have pink eyes, white coat colour and black colour in tips like nose, tail and feet.

A crossing of a homozygous coloured rabbit with an albino resulted in following F1 and F2 generation. In chinchilla, coat colour is lighter than the coloured (agouti). The coloured character is dominant over chinchilla.

 However F1 hybrids between chinchilla and himalayan albino (Cchch) or between chinchilla and albino (cchca) show light grey skin colour. The occurrence of light grey colour is due to partial expression of the gene for chinchilla in a heterozygous state.

Genotype                Phenotype

2. ABO blood groups in human beings The ABO blood group system in human beings was established by K. Landsteiner. It is based on the presence or absence of certain antigens. There can be two anitigens A or B in the blood, resulting in four blood groups, namely A,B, AB and O. These are called ABO blood groups or Landsteiner blood groups. The inheritance of ABO system illustrates a new principle in genetic control of phenotypes. The blood of a person having A group will have the antigen A and a person having B group will have the antigen B. With these antigens A and B there are certain naturally occurrring antibodies in the serum of the blood. The antibodies in a particular individual will be found only for those antigens which are absent in blood of this individual. The presence of antigens and antibodies occur as follows

Antibodies in the blood of ‘group A’ will agglutinize red blood corpuscles of the blood group B. Similarly the antibody in blood ‘group B’ will agglutinize red blood corpuscles of the blood group A. Since no antibody is found in ‘group AB’ blood, it will not agglutinize any other group . ‘Group O’ will have antibodies for ‘group A’ and B. Hence ‘group O’ will agglitinize group A and B. Compatibility of donor blood to that of the recipient will occur as follows

From the table provided it is obvious that ‘group AB’ is universal recipient. ‘Group O’ is universal donor. The gene for ABO system is conventionally represented by the sym bol ‘I’. Alleles IA and IB code for the enzymes involved in the formation of the antigens A and B respectively and the allele ‘Io’ for no known protein. Thus the genes can result in 6 possible genotypes but four possible phenotypes as found in the table

The alleles IA and IB jointly express themselves in the individual, they are codminant. Both IA and IB are dominant to the recessive allele Io. Disputed parentage and blood groups The identification of blood group may help to decide in cases concerned with parentage issues. By knowing the blood groups of parents, it is possible to determine the possible blood groups in the children. The impossibility of a particular blood group in the progeny can also be pointed out.

Rh blood group   Rh factor in blood was discovered by Landsteiner and Wiener in 1940. It was initially discovered in rabbits, immunized with the blood of Rhesus monkey. The human beings whose blood will get agglutinated with rabbit serum were designated as Rh+, and whose blood if not agglutinated were designated as Rh-. Wrong transfusion can cause agglutination of blood in the recipient Hence before transfusion of blood, along with identification of ABO blood group, it is necessary to test compatibility of Rh factor. The presence of Rh+ child in the uterus of the Rh- mother can cause agglutination in the blood of the fetus. Even though such an unfortunate incident may not happen in the first pregnancy, it could occur in successive pregnancies. The death of the foetus in such cases is due to haemolytic anemia. This disease is called erythroblastosis fetalis.