The Multiple Alleles.

Some of the genetic traits are controlled by more than one pair of allelomorphic genes, and so called multiple alleles. However, each individual carries only two of these allelic genes in a specific locus on a homologous pair of chromosomes. Here is an example for these cases:

1. Blood groups in Man:

After many bad incidents due to blood transfusion, scientists discovered that Human blood differs in some respects from one person to another. As a results of their studies, Human blood was classified into four main groups: (A), (B), (AB), and (O). It was also discovered that these blood groups are inherited as multiple alleles.

There are three allelic genes (A), (B), and (O) from which six genotypes are formed:

(AA), (BB), (OO), (AB), (AO), (BO).

But since the allele (O) is recessive to both (A), and (B) alleles, and there is a case of lack of dominance between (A), and (B) alleles, there are only four phenotypes, and these are the blood groups (A), (B), (AB), and (O). The genotypes can be determined by following up the phenotypes of the parents, and their children. This helps in judging the paternity of disputed children or referring mixed children to their real parents.

The following example illustrates the case of two newly born children that were mixed up in the hospital. Each of the two children belongs to one of the following families:

The 1^st.family: The blood groups of the parents are (A) and (AB), and the blood group of the baby is (O).

The 2^nd.family: The blood groups of the parents are (B) and (O), and the blood group of the baby is (B).

By knowing how the blood groups are inherited, the first family cannot produce a child with blood group (O), while the second family can have babies with blood groups (B) and (O). Thus, the first child (O) cannot belong to the first family. Though this cannot be proven that this child or the other (B) belongs to any of the two families. Despite that, the blood group analysis proved to which family the two children belong. Blood group analysis is helpful only in denying the paternity of a man to a certain child. Blood group analysis helps also in Human race taxonomy, and is used in blood transfusion.

Blood transfusion:

Blood can be transfused between persons of the same blood groups, or between persons of other blood groups according to a definite system. This is due to the presence of:

- Antegins on the surfaces of red blood cells.

- Antibodies in the blood plasma.

. In blood group (A): there is antigen (A) on surfaces of red blood cells, and antibody (B) in the plasma.

. In blood group (B): there is antigen (B) on surfaces of red blood cells, and antibody (A) in the plasma.

. In blood group (AB): there are antigen (A), and antigen (B) on surfaces of red blood cells, and there are no antibodies in the blood plasma.

. In blood group (O): there are no antigens on surfaces of red blood cells, and there are antibodies (A) and (B) in the blood plasma.

The following table summarizes the information about blood groups, and their relation to blood transfusion:

N.B.: