Question

Write short notes on following
(i)Incomplete dominance
(ii)Struggle for existence

Answer 1

SHORT NOTES:

(i) Incomplete Dominance:

When F1 hybrids exhibit a mixture or blending of characters of two parents, the case is considered as incomplete or blending inheritance or incomplete dominance.

It simply means that the two genes of the allelomorphic pair are not related as dominant or recessive, but each of them express itself partially. Some examples are:

(a) In the case of 4 O' clock plant (Mirabilis jalapa), when plants with red flower (homozygous RR) are crossed with plants having white flower (homozygous rr), the heterozygous F1 hybrid (Rr) bear pink flowers. When these pink flowers are self pollinated , they develop red (RR), pink (Rr) and white (rr) flowers in the ratio 1:2:1, respectively.

(b) Similar case of incomplete dominance is seen in snapdragon (Antirrhinum).

One parent is homozygous for red flowers (AA) and the other is homozygous for white flowers (aa). No dominance is present and the heterozygous F1 flowers are pink. The 1:2:1 ratio of red, pink and white flowers in the F2 progeny is shown by appropriate colouring.

(ii) Struggle for existence:

As a result of struggle for existence, only those organisms could survive, which have favourable variations to adapt with the environmental conditions or with so many variations in populations. In other words, the struggle for existence results in survival of the fittest. Survival of the fittest refers to the idea that the natural selection tends to favour those organisms that are most fit to survive with the reproductive age in a particular environment and to produce offsprings.

Example, according to the theory of Darwin, giraffe exhibited great variations in the length of their neck and legs.

He stated that both long neck and legs and short neck and legs containing giraffes were present, but due to the scarcity of grass on land, these had to feed on the leaves of tall trees. Now giraffe with long neck and longer legs could get food more easily and had better chances of survival. As a result, long neck and legs containing giraffes became abundant and giraffes with short neck and legs became extinct.