Q.(a) Explain Incomplete Dominance with an example. OR

(a) Incomplete dominance blends both parental phenotypes in the heterozygote, illustrated by pink Mirabilis jalapa F1 flowers and a 1:2:1 F2 ratio; (b) different classes of plant secondary metabolites (alkaloids, terpenoids, phenolics, glycosides) come from specific plants and have specific commercial/medicinal uses.

(a) Incomplete dominance, with the Mirabilis jalapa example

In a typical Mendelian cross, one allele is completely dominant over the other, so a heterozygote looks exactly like the dominant homozygote. Incomplete dominance is an exception to this: neither allele of a gene pair is fully dominant over the other, so the heterozygous individual displays a phenotype that is INTERMEDIATE between (a blend of) the two homozygous parental phenotypes, rather than resembling either parent exactly.

The classic textbook example is flower colour in the four-o'clock plant, Mirabilis jalapa:

• Let R = allele for red flower colour, r = allele for white flower colour.
• P generation: red-flowered plant (RR) x white-flowered plant (rr).
• F1 generation: all offspring are pink-flowered (Rr) -- neither fully red nor fully white, but a genuine intermediate blend, because R is not completely dominant over r.
• F2 generation (from selfing the pink F1, Rr x Rr): the offspring appear in the ratio 1 Red (RR) : 2 Pink (Rr) : 1 White (rr).

The key significance of this example is that, in incomplete dominance, the F2 phenotypic ratio (1:2:1) is identical to the underlying F2 genotypic ratio (1 RR : 2 Rr : 1 rr) -- unlike in complete dominance, where the phenotypic ratio (3:1) differs from the genotypic ratio (1:2:1), because the heterozygote's distinct intermediate phenotype (pink) reveals its genotype directly.

(b) Secondary metabolites, their plant sources, and uses