(a) Variation: These are morphological; and physiological differences; which occur within a species which could be hereditary or acquired from the environment.
(b) Advantages of variation: - Appearance of new varieties - variation allows survival of the fittest - competition arises - nature selects the most adapted varieties - that are more resistant to diseases - and better adapted to changes in the environment.
(c) Differences between continuous and discontinuous variation

continuous	Discontinuous
caused by co-dominance	caused by complete dominance
Intermediates are present	No intermediate characteristic
Affected by the environment	not affected by the environment
They are controlled by genes /polygenes	Not polygenic
Follow the normal distribution curve/frequency of characteristics evenly distributed	Not evenly distributed

(d)(i) Sex-linked characters: These are characteristics associated with the sex of an individual; the gene for the expression of the character; is borne on one of the sex/the X chromosomes.
(ii) Co-dominance: This is a condition in a heterozygote; in which both alleles have equal dominance; and the phenotype is a mixture of the opposing characters.
(iii). Hybrid vigour: This is the production of new varieties within a species; which are healthier/have greater resistance to diseases/are better adapted to the environment; as a result of out breeding/crossing non related members of a species.