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heredity and evolution

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
What are Mendel’s laws of inheritance?

Mendel's Laws of inheritance are-:
i. Law of dominance: When two homozygous individuals with one or more sets of contrasting characters are crossed the characters that appear in the F1 hybrids are dominant characters.

ii. Law of segregation: Contrasting characters brought together in a hybrid remain together without being contaminated and when gametes are formed from the hybrid the two separate  from each other and only one enters each gamete.

iii. Law of independent assortment: In inheritance of more than one pair of contrasting characters simultaneously, the factors for each pair of characters assort independently of other pairs.


How is the sex of the child determined in human beings?

In human, 23 pairs (46) chromosomes are present. The 23rd pair of chromosomes are called sex chromosomes. Women are homogametic (XX) and have similar pair of sex chromosomes, that is both are X chromosomes. Men are heterogametic (XY) and have different pair of chromosomes—one is a normal size X chromosome and a short one called Y chromosome.
Children inherit an X chromosome from their mother regardless of whether they are boys or girls. The child who inherits an X chromosome from father will be girl, and one who inherits a Y chromosome from father will be a boy.


If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

Trait B may have arisen earlier as it is present in more members of the population. It must have arisen earlier and has now spread to 60% of the population. Whereas trait A must be new as it has spread to only 10% of the population.

How does the creation of variations in a species promote survival?

The variations ensure that the survival of the species as they allow the members of population to adapt to the changing environment and conditions. The species are more suitable to the changed environment . Thus continuity of the species is maintained.
For example, Bacteria variants which can withstand tolerate heat will survive better in a heat wave than variant which cannot heat wave. It proves that creation of variations in a species promote survival.


In evolutionary terms, we have more in common with
  • a Chinese school-boy
  • a chimpanzee
  • a spider
  • a bacterium


a Chinese school-boy

a Chinese school boy as he is also of the same species (Homo sapiens) as we are.


a Chinese school boy as he is also of the same species (Homo sapiens) as we are.



An example of homologous organs is
  • our arm and a dog’s fore-leg
  • our teeth and an elephant’s tusks
  • potato and runners of grass 
  • all of the above


all of the above
all of the above. Because all the pairs are similar in structure, and origin.
all of the above. Because all the pairs are similar in structure, and origin.