INCOMPLETE DOMINANCE:-

    1) The law of dominance is not always found to be correct because in many cases complete dominance is absent.
2) Incomplete dominance some traits of F, phenotype is intermediate between parental traits.
3) In this case, both the genes shows partial expression of the character from an allelomorph.
4) One gene cannot suppress the expression of other completely i.e. a pair is not one dominant one recessive.
5) It can be explained with the help of following example:
In Mirabilis jalapa or four O'clock plant two pure verities one with red flower (RR) and other with white flowers (rr) when crossed F, hybrids with pink flowers (Rs) are produced.
Selfing of F, hybrids for F, generation produces three type of flowers such as red (RR), pink (Rr) and white (rr).
6) It means that phenotypic and genotypic ratio are same i.e. 1:2:1. (d) It explains segregation of factors but not the mixing of factors.
7) Incomplete dominance is the main reason for the shade produced.
8) Incomplete dominance does not favour the blending theory of inheritance, though in F, all are pink; both the potential traits, red and white reappear, each in F, generation.

        9) Other similar example is snapdragon (Antirrhinum majus) 

Co-dominance.

1. In Co-dominance there is equal expression of genes from allelic pair in F, hybrids. Such alleles which are able to express themselves independently even if present together in heterozygote are called as co-dominant alleles.
2. It can be explained by taking an example of coat color in cattle.
3. The two cattle one with red coat i.e. skin with red colour hair (RR) and other with white coat i.e. skin with white colour hair are used F, hybrids with roan colour are produced with the genotype (RW).
4. For F, generation, when Roan are crossed red (RR), roan (RW) and white (WW) are produced in the ratio (1:2:1).
5. In case of co-dominance genotypic and phenotypic ration obtained are same i.e. similar to codominance.
6. The main difference in incomplete dominance and co-dominance is that in case of incomplete dominance the phenotype of hybrid is intermediate between the phenotypes of parent while in co-dominance both the genes are expressed equally.

CALCULATE RATIO FROM ABOVE CROSS

C)      Multiple alleles :

1) It indicates the presence of two or more alleles of a gene.

2) Repeated mutation of the same gene in different directions results in development of multiple allele.

3) Despite of the presence of several alleles of the  same gene in a population, an individual can have only two alleles.

4) Examples of multiple alleles are as follows :

a) The four different colour coat in Rabbits are regulated by multiple alleles.

b) Self compatibility in some plants.

5) Characteristics of multiple alleles :

a) There are more than two alleles of the same gene.

b) All the multiple alleles occur on the same gene locus of the same chromosome or its homologue.

c) A chromosome possesses only one allele of the group.

d) An individual possesses only two alleles while the gametes carry single allele.

e) Different alternatives of the same character are expressed by multiple allele.

f) Possibility of codominance, dominance-recessiveness or intermediate dominance is also present.

g) Show Mendelian inheritance pattern.

 

ABO blood groups in Humans :

a) ABO blood group system in human beings is an example of codominant, dominant-recessive and multiple alleles.

2) Humans show six genotypes and four blood groups or their phenotypic expression as A, B, AB and O.

3) The antigen A and B which decide the blood group of individual are present over surface coating of erythrocytes or RBCs.

4) The antigens occur on oligosaccharide rich head of a glycophorin.

5) Blood group A persons have antigen A, group B have antigen B, AB have both antigens while blood group O persons do not carry any antigen on the surface of their RBCs.

6) The alleles which are able to express themselves in the presence each other are called codominant.

     Thus, blood group alleles show both codominant and dominant-recessive relationships (IA = IB > i)

7) It can be explained with the help of following charts :

 Biochemical genetics of blood groups

1) ABO blood groups are controlled by the gene ‘I’ also called as ‘L, located on 9th chromosome that has 3 multiple alleles, out of which any two are found in a person.

2) The blood groups show/exhibit Mendelian inheritance (Bernstein 1924).

3) The IA and IB alleles produce an enzyme called glycotransferase for the synthesis of sugars.

4) These sugars are attached to lipid and produce glycolipids.

5) These glycolipids then associate with RBC membrane toform blood group antigens.

6) Allele i does not produce enzyme/antigen.

7) Blood group being a hereditary character the knowledge of blood groups of the parents can give information about the possible blood groups of children and vice versa.

D)      Pleiotropy [Pleiotropic genes]

1) The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy.

2) The gene having a multiple phenotypic effect because of its ability to control expression of two or more characters is called pleiotropic gene.

3) It is not essential that all the traits are equally influenced. If it is more evident then it is a major impact and less evident then it is secondary effect.

4) Gene is responsible for causing number of related changes which are together called as syndrome.

5) In human beings pleiotropy is exhibited by syndrome called as sickle cell anemia.