Answer:
(a) Codominance (1 : 2 : 1) - It is the
phenomenon of two alleles (different forms of a Mendelian factor present on the
same gene locus on homologous chromosomes) lacking dominant-recessive
relationship and both expressing themselves in the organism.
Example - AB blood
group : Alleles for blood group 
and
blood group 
are
codominant so that when they come together in an individual, they produce blood
group AB. It is characterised by the presence of both antigen A (from 
)
and antigen B (from 
)
over the surface of erythrocytes.
Incomplete dominance (1
: 2 : 1) - It is the phenomenon of neither of the two alleles being dominant so
that expression in the hybrid is intermediate between the expressions of the
two alleles in homozygous state. 
phenotypic
ratio is 1 : 2 : 1, similar to genotypic ratio.
Example - In Mirabilis
jalapa (four o'clock) and Antirrhinum majus (snapdragon or dog
flower), there are two types of flower colour in pure state: red and white. When
the two types of plants are crossed, the hybrid or plants of 
generation
have pink flowers. If the latter are selfed, the plants of generation
are of three types-red, pink and white flowered in the ratio of 1 : 2 : 1. The
pink colour apparently appears due to mixing of red and white colours
(incomplete dominance).
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