Incomplete dominance (1:2:1 ratio) : After Mendel, several cases were recorded where F1 hybrids were not related to either of the parents but exhibited a blending of characters of two parents. This is called incomplete dominance or blending inheritance.
Example : In 4-O�clock plant, (Mirabilis jalapa), when plants with red flowers (RR) are crossed with plants having white flowers (rr) the hybrid F1 plants (Rr) bear pink flowers. When these F1 plants with pink flowers are self pollinated they develop red (RR), pink (Rr) and white (rr) flowered plants in the ratio of 1:2:1 (F2 generation).
Example : In Snapdragon or dog flower (Antirrhinum majus) the dominant character of leaf (Broadness) and flower (Red) shows incomplete dominance over recessive characters (Narrowness and white) in dihybrid cross.

Codominance (1:2:1 ratio) : In codominance, both the genes of an allelomorphic pair express themselves equally in F1 hybrids. 1:2:1 ratio both genotypically as well as phenotypically in F2 generation.
Example : Codominance of coat colour in cattle.
In cattle gene R stands for red coat colour and gene r stands for white coat colour. When red cattle (RR) are crossed with white cattle (rr), the F1 hybrids have roan coloured skin (not the intermediate pink). The roan colour is actually expressed by a mixture of red and white hairs, which develop side by side in the heterozygous F1 hybrid. In F2 generation red, roan and white appear in the ratio of 1 : 2 : 1. The phenotypic ratio equal to genotypic ratio RR, Rr, rr (1 : 2 : 1).
Example : Codominance in andalusian fowl
In andalusian fowl a cross between pure black and pure white varieties results in blue hybrids.
Example : Codominance of blood alleles in man
(a) MN blood type in man is an example of codominance. The persons with MN genotype produce both antigen M and N and not some intermediate product indicating that both the genes are functional at the same time.
(b) In ABO blood group system gene A and B responsible for blood group A and B are codominant. The hybrid has AB blood group.

Multiple allele :- More than 2 alternative forms of same gene called as multiple allele. Multiple allele is formed due to mutation.
# Multiple alleles located on same locus of homologous chromosomes.
# A diploid individual contains two alleles and gamete contains one allele for a character.

# (a) Blood proteins : According to Karl landsteiner (1900) a Nobel prize winner, blood contains two types of proteinous substances due to which agglutinations occurs.
(1) Agglutinogen or antigen : It is a protein found on the cell membrane of RBC�s.
(2) Agglutinin or antibody : This the other proteinous substance, found in the plasma of the blood.

Whenever the blood of a person receives the foreign proteins (antigen) his blood plasma starts forming the antibodies in order to neutralize the foreign antigens.

# (b) Agglutinations : Two types of antigens are found on the surface of red blood corpuscles of man, antigen A and B. To react against these antigens two types of antibodies are found in the blood plasma which are accordingly known as antibody � anti-A or a and anti-B or b. Agglutination takes place only when antigen A and antibody a occur together or antigen B and antibody b are present in the blood. Under such condition antibody a reacts with antigen A and makes it highly sticky. Similarly antigen B in presence of antibody b become highly sticky with the result RBC�s containing these antigens clump to form a bunch causing blockage of the capillaries. Agglutination in blood is therefore antigen-antibody reaction.

(1) ABO blood group : Landsteiner divided human population into four groups based on the presence of antigens found in their red blood corpuscles. Each group represented a blood group. Thus there are four types of blood groups viz. A, B, AB and O. He observed that there was a reciprocal relationship between antigen and antibody according to which a person has antibodies for those antigens which he does not possess. For example a person of blood group B does not possess antigen A but his blood plasma has antibody �a� due to which agglutination with the blood of a person with blood group A occurs. Similarly persons with blood group AB possess both the antigens A and B but their blood plasma does not possess any of the antibodies. In the same way person having blood group A does not possess antigen B but antibody �b� is found in his blood plasma. Persons with blood group O possess none of the antigens and that is why their blood possesses both the antibodies �a� and �b�.

Blood transfusion : Blood transfusion is best done in the persons of same blood group. At the same time it is possible to know in which different blood groups the blood transfusion can be made possible.
Persons with blood group AB are called universal recipients because both antigens A and B are found in their blood and the two antibodies �a� and �b� are absent. Therefore, such persons can receive blood of all the blood groups. In the same way persons who have blood group O� are universal donors as they lack both the antigens and Rh� person can donate to Rh+ person as well as Rh� person but Rh+ person cannot donate blood to Rh� person. But at the same time such persons can not be given the blood of any other blood group except blood group O because their blood possesses both the antibodies �a� and �b�. Persons belonging to blood group A and B contain only one antigen and one antibody against it, in their blood. Such persons can therefore receive blood either of the blood group of their own or the blood group O.

A place where blood of different blood groups is safely stored in bottles for emergency use, is called blood bank. Blood after proper testing is stored in a sealed bottle at a definite temperature (4�-6�c) to be preserved for a definite time period.
Artificial anticoagulants are used to prevent blood clotting in the blood banks. These anticoagulants are added to the blood preserved in bottle. Such anticoagulants include sodium citrate, double oxalates (sodium and ammonium), dicumarol and EDTA (ethylene diamine tetra acetic acid). The whole blood in this way can be stored for a maximum period of 21 days.

Inheritance of blood groups : Blood groups in human are inheritable trait and are inherited from parents to offsprings on the basis of Mendel�s Laws. Blood group inheritance depends on genes received from parents. Genes controlling blood group in man are three instead of two and are called multiple alleles. All these three genes or alleles are located on the same locus on homologous chromosomes. A person can have only two of these three genes at a time which may be either similar or dissimilar in nature. These genes control the production of blood group/antigens in the offspring. The gene which produces antigen A is denoted by Ia, gene for antigen B by Ib and the gene for the absence of both antigens by Io. it is customary to use the letter I (Isohaemagglutinogen) as a basic symbol for the gene at a locus. Based on this, six genotypes are possible for four blood groups in human population.

(1) Rh factor : Landsteiner and Weiner (1940) discovered a different type of protein in the blood of Rhesus monkey. They called it Rh antigen or Rh factor after Rhesus monkey. When injected the blood of these monkeys into the blood of guinea pigs they noticed the formation of antibodies against the Rh antigen in the blood of guinea pigs. Formation of Rh antigen is controlled by dominant gene (R) and its absence by recipient gene (r). People having this antigen with genotype (RR or Rr) are called Rh positive (Rh+) and those whose blood is devoid of it with genotype (rr) are Rh negative (Rh�). About 85% human beings in Europe and 97% in India are Rh+.
(2) Importance of Rh factor : Generally human blood is devoid of Rh antibodies. But it has been noticed that on transfusion of blood of a Rh+ person to Rh� person, the recepient develops Rh antibodies in its blood plasma. If Rh+ blood is transfused for the second times it causes agglutination and leads to the death of Rh� person.
(3) Erythroblastosis foetalis : This disease is related to the birth of a child related with Rh factor. It causes the death of the foetus within the womb or just after birth. It was studies by Levine together with Landsteiner and Wiener. The father of Rh affected foetus is Rh+ and the mother is Rh�. The child inherits the Rh+ trait from the father. A few Rh+ red blood corpuscles of foetus in the womb enter in the blood of the mother where they develop Rh antibodies. As mother�s blood is Rh� i.e. devoid of Rh antigen, it causes no harm to her. These Rh antibodies alongwith the mother�s blood on reaching the foetal circulation cause clamping of foetal RBCs or agglutination reaction. The first child is some how born normal because by that time the number of antibodies in mother�s blood remain lesser but they increase with successive pregnancies. Thus the foetus following the first child dies either within the womb or just after its birth. This condition is known as erythroblastosis foetalis. So a marriage between Rh+ boy and Rh� girl is considered biologically incompatible.

# However, there is no danger if both parents are Rh� or mother is Rh+ and father is Rh�. Rh factor serum has been developed which when given to the Rh� mother after each child birth saves the next child. This serum contains Rh antibodies which destroy the Rh antigens of foetus before they can initiate formation of Rh antibodies in the mother.