NCERT Class 12 Biology: Chapter –5 Principle of Inheritance and Variation Part 7

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Question 13:

What is artificial selection? Do you think it affects the process of natural selection? How?


Artificial selection is a type of selective breeding in which humans select desired traits or a combination of traits for exploiting the variations among species. It can affect natural selection. For example when Mendel selected and crossed only the true breeding varieties which were created by continuous self-pollination; the process of natural selection get arrested and promoted the growth of desired traits only in plants. This can prevent the plant from getting better adapted by eliminating harmful and promoting useful genes through natural selection. Artificial selection is fast process, it cannot bring the evolutionary change brought by natural selection in population, and can eventually lead to decreased yield and growth.

Question 14:

With the help of an example differentiate between incomplete dominance and co-dominance.


Incomplete dominance and co-dominance

Incomplete dominance


(i) Phenotypes from both the parents are partially manifested in generation

(i) Phenotypes from both the parents are completely manifested in generation

(ii) Example: When snapdragon plants with red flowers were crossed with flowers, the generation produced pink flowers

(ii) Example: ABO blood grouping in humans shows co-dominance.

Question 15:

It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle cell anaemia is persisting in human population. Why?


Sickle cell Anaemia is persisting in human population. It is a good disease example of a balancing selection, with affected individuals carrying mutations in both the parental and maternal inherited hemoglobin gene. As a result their red blood cells are less efficient in carrying oxygen throughout the body. But there is a biological advantage associated with sickle cell Anaemia that the patients are better protected against malaria. This resistance offered against malaria disease could be the reason for the harmful alleles of sickle cell Anaemia to persist in human population despite of the elimination of harmful genes over time.

Long Answer Type Questions

Question 1:

In a plant tallness is dominant over dwarfness and red flower is dominant over white. Starting with the parents work out a dihybrid cross. What is standard dihybrid ratio? Do you think the values would deviate if the two genes in question are interacting with each other?


The following punnett square shows cross between tall plant with red flowers (TTRR) and dwarf plant with white flowers (ttrr). All plants in generation will be tall and will produce red flowers.

Red flowers (TTRR) and dwarf plant with white flowers (ttrr) …

Red Flowers (TTRR) Dwarf Plant with White Flowers (Ttrr)

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When plants of generation are allowed to self-pollinated, phenotype of plants in generation can be shown by following Punnett Square.

  • In this case, the standard dihybrid ratio which can be shown as follows:

    • Tall plant red flower

    • Tall plant white flower

    • Dwarf plant red flower

    • Dwarf plant White flower

  • The standard dihybrid ratio works only when the genes for contrasting characters are different chromosomes. If characters are on the same chromosome, they may interact with each other. In that situation, the dihybrid ratio would show variation from the standard dihybrid ratio.