When the mother of a child names a person as to be the father of the child and when the man denies this, it remains to be proved whether the putative father is the actual father of the child. Apart from morphological features, examination of blood group factors comes for consideration to settle the dispute.
The basis of blood group tests in such cases is that 1. a blood group antigen will be present in a child, only if at least one of the parents has it; and that, 2. if the blood group antigen in one parent is homozygous then it must appear in the child.
Thus, from 1 above we get that if the blood cells of a child contain A-antigen then at least one of its parents have A-antigen in his or her blood cells.
From 2 we get that if the father or mother have homozygous A-antigen (AA), then it must be present in the blood cells of the child.
The only chance of exception of the above two theories is the chance of mutation which may occur in 1 in 50,000 new-borns.
On the basis of the above two theories, if a group antigen is present in the blood of the child which is neither present in the mother nor in the putative father, the antigen must have come from another person. Having no dispute about the identity of the mother, it can be said that if the antigen is absent in the mother, then it must have come from the father and as such the putative father is not the actual father of the child because the actual father must possess that antigen which has been inherited by the child from one parent and which is absent in the mother.
Considering ABO group system, the A, B and O antigens are inherited by 3 allelomorphic genes and every person has two of these three types of genes, one from each parent. Hence with ABO group system, we get 6 types of individuals depending on the gene combinations of the persons e.g., AA, AB, AO BB, BO, and OO. These are the genotypes of A B O group systems. Group genotypes represent the group antigens inherited from both the parents and the group phenotype represents the effective group factor of an individual resulting from the genotype combination in him.
AA genotype is presented as A phenotype.
AO genotype is also presented as A phenotype as the O genotype does not represent any antigen and is silent.
Similarly, BB and BO genotypes are presented as B phenotype AB genotype is presented as AB phenotype as both A and B antigens maintain their characters in the individual.
OO genotype is presented as O phenotype.
With their spermatozoon and ovum, the parents contribute one genotype each out of two present in him or her. Thus, in the child there will be two genotype antigens again. Mating between above genotypes/phenotypes will produce children having genotypes and phenotypes.
MN Group System
When it is not possible to exclude the putative father from being the actual father, by ABO grouping test, MN group system is tried.
In MN grouping system, a single pair of gene possesses either M and M antigen or M and N antigen or N and N antigen. Thus with MN system the combinations shown in are possible. Both M and N antigens are dominant.
|Parents||Possible children||Impossible children|
|Phenotype Genotype||Genotype Phenotype||Genotype Phenotype|
|M+M MM+MM||MM M||MN, NN MN, N|
|M+N MM+NN||MN MN||MM, NN M, N|
|N+N NN+NN||NN N||MM, MN MN, M|
A weak N2 has been recognised, which is a weak antigen. Being weak it evades detection. Hence, its presence can be assumed as in the following case —
As the N2 antigen in the above case may go undetected because it is a weak antigen, it may be wrongly taken that the phenotype and genotype of the child is M and MM respectively, though actually the phenotype and genotype of the child is MN2 and MN2. Similarly, the N2 being weak will go undetected in the putative father. It will hence appear that the phenotype and the genotype of the putative father are N and NN respectively. In this case as the child’s blood is wrongly taken to have MM genotype, one M factor will be taken as must have come from each parent. But the putative father showing no evidence of presence of M factor may be wrongly excluded from the fathership of the child, though he is actually the father of the child.
Distribution of the MN system —
Gr. M = 28%, Gr. N = 22% and Gr. MN = 50% of the whole population.
Test for MN system —
The MN system can be studied by tube or tile method as in case of ABO group system.
Ss GROUP SYSTEM
Along with M, N and MN antigens, one of the two other antigens, namely S or s remains closely linked. Distribution of S antigen is about 55% and that of s antigen is about 45%. S is usually linked with M antigen (75% of M cells have S factor in them).
As MN and Ss systems are closely linked, taking both the systems together the following phenotypes and genotypes are possible. S antigen is dominant. Anti-S is available. But anti-Ss is very very rare.
|MNS||MSNS, MSNs, MsNS|
Presence of Ss factors in blood can be studied either by tube method or by tile method.