Interaction of genes 2012

What are multiple alleles? All Mendelian characters have only two alternate expressions or have only two alleles. This may not be true in all cases, for a genes, once having undergone mutation, may mutate again to give one more altered form or allele and in their case, there will be three alleles. Theoretically a gene may mutate any number of times leading to the formation of many variant or multiple alleles

Definition Multiple allele is the series of gene mutation from wild to a number of mutated forms all of them affecting the same trait.

Salient features of multiple alleles They occupy the same gene locus on the chromosome. In diploid organisms, as there are only two homologous chromosomes, there will be only two allelic states of the multiple series. In haploid gametes multiple allelic series is represented by a single allelic state. No crossing over is known between multiple allele as they occupy the same level of the homologous chromosomes. In a cross between two alleles, the F 2 generation reveals the same combination of alleles. A single multiple allelic series affects only one trait-eye colour, shape, skin colour etc.

Activity What are the Blood groups found in human? What determines blood groups? Why can’t we have blood transfusion from any donor?

Blood Groups in Man A, B, AB and O. 1. Antigen A- Blood group A 2.Antigen B- Blood group B 3.Both Antigen A and B- Blood group AB 4.No Antigen- Blood group O Antibody called as agglutinin Antigen called as agglutinogen  The reaction between antibody and antigen is very specific and is often compared to the lock and key mechanism.

A, B, AB and O groups In this blood groups some of them contain natural antibodies against certain others. Persons of blood group A contains within the serum of their blood an antibody which agglutinates RBC of blood group B. Group B serum has antibody against A. The AB group serum however has no antibodies while O group has both antibodies for A and B.

Hence O group cannot receive blood from any group except itself. But it has no antigens and can be given to any other group it does not cause the production of antibodies in blood types.

Antibody and antigens of various blood groups. Blood groupAntigen (RBC) Antibody ( Serum) AAB BBA ABA, BNone O A,B

During blood transfusion if the donor’s blood has antigens for the recipient, it will harm the individual if the recipients blood has antibodies for the donor’s blood. For eg., A person with O group has antibodies for both A and B and cannot receive blood from either because the antigen in A and B cause the production of more antibodies and the serum of O agglutinates RBC of both A and B. O group people can receive blood only from O group. On the other hand O blood can be given to all others because it has no antigen for A and B.

Hence when it is injected to other groups, it does not cause the production of antibodies in the serum of the recipient and hence no agglutination take place. This is the reason why O group is known as the Universal donor. In the same way AB group is known as Universal recipient.

Genetic Basis of A,B, O Blood groups It is believed that O is the original gene, and two dominant mutation from this have given rise to A and B blood groups. The blood groups are determined by a series of three allelic gene viz., I A, B B and I ( Beteson 1925) The genotype of different blood groups are given in the following table. Blood groupGenotype homozygousGnotype heterozygous A IAIAIAIA I A i B IB IBIB IB I B i AB I A I B Oii

Allele I A and I B are dominant over i allele. but when they are together, they behave as an intermediate producing AB blood group. This is an example of co-dominance where both the genes have equal expression

When marriage occurs between people of belonging to different blood group Genotype of parents Blood gp. Of parents Blood gp. Of offspring Genotype of Offspring I A I A X I A I A A X AAIAIAIAIA I A I A X I A iA X AAI A I A, I A i I A i X I A iA X AA, OI A i, ii I B I B X I B I B B XBBI B I B I B XI B I B B XBBI B I B, I B I B I A I B X I A I B AB XABA, AB,BI A I A, I A I B, I B I B

Rh factor Landsteiner and Wiener (1940) discovered one more type of antigen (agglutinogen) in RBC’s of human beings. It is named after the Rhesus monkey and called Rh factor or Rh antigen. It is present in about 95% of Indians ( about 85% of white also have it). However, serum does not have the corresponding antibody in the human beings.

Person whose blood agglutinates with Rh serum of monkey are called Rh positive and no agglutination takes place they are called Rh negative. Six groups of Rh factors have been identified in human beings C, c, D, d, E and e. out of these D and d are commonest. D is a Mendelian gene hence is dominant over d. Almost all human beings (85%) who are Rh positive belong to D group while Rh negative people belong to d group.

Importance of Rh factors Assume that a Rh negative individual gets a blood transfusion from a Rh positive individual. At first instance nothing will happen as there are no natural antibodies in the serum of Rh negative individual. But the presence of Rh blood will induce antibody production. Nothing will happen however as there are no sufficient antibodies. Should the same Rh negative require another transfusion from Rh positive blood, donor blood will show agglutination due to the antibody already present. This may be harmful to the recipient.

A Rh negative woman marries a Rh positive man. The child of this couple obviously belongs to Rh positive as the gene D is dominant over d. In the first pregnancy however the child is not harmed even though the maternal blood and the child’s blood mix. Once again the mother’s Rh negative blood will not produce enough antibodies to haemolyse the child’s blood.

But with each succeeding conception the antibodies in the mother’s blood accumulates, sometimes the third or even the second child itself may have its blood ( Rh positive) haemolysed leading to erythroblast from the bone marrow enter the blood ( to compensate for the haemolysed RBC). This condition is called Erythroblastosis foetalis. Such a baby is generally still born or even when born highly anemic. It can be saved with total transfusion of Rh negative blood.