Heredity and Evolution Class 10 Notes Science Chapter 9


1. Heredity: The transmission of characters from parent to their off springs is known as heredity.

  • (i) Genetics: The study of heredity and variations is known as genetics.
  • (ii) Clones: Clones are those organisms which are the carbon copies of one another.
  • (iii) Variation: Variation in sexually reproducing organisms are caused due to the following factors like environment, crossing over and recombination of genes and mutation.

2. Mendel’s laws of inheritance: The first study of inheritance was done by Gregor Mendel on garden pea. Mendel’s studies yielded three laws of inheritence given as:

  • (i) Law of dominance: It states that in a cross of parents that are pure for contrasting traits, only one form of the trait will be appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.
  • (ii) Law of segregation (Law of purity of gametes): This law states that during gamete formation, the alletes for each gene segregate from each other so that each gamete carries only one allele for each gene.
  • (iii) Law of independent assortment: This law states that genes of different traits can segregate independently during the formation of gametes.

3. Genotype and phenotype: Genotype is the composition of genes present in an organism and the characteristic which is visible in an organism is called its phenotype.

4. First filial and second filial generation: When two parents cross (or breed)
to produce progeny (or offspring), then their progeny is called F1-generation (first filial generation) and when the first generation progeny cross among themselves to produce second progeny, then this progeny is called F2- generation or second filial generation.

5. Mendel’s experiment: Mendel conducted his famous experiments on garden pea (Pisum sativum). He used a number of contrasting characters like round/ wrinkled seeds, tall/ short plants, white/ violet flowers and so on.
(i) During monohybrid cross:

  • When tall pea plants are crossed with short pea plants then in F generation only tall plants were obtained.
  • F2 progeny of FI tall plants are not all tall but one quarter of them are short indicating that both tallness and shortness traits were inherited in FI but only tallness trait was expressed due to dominance.

(ii) During dihybrid cross:
In dihybrid cross two pairs of contrasting characters were considered. Tall plant with round seeds were crossed with short plant with wrinkled seeds. In FI tall plants with round seeds were obtained. On selfing these F1 plants with F2 produced tall plants with round seeds, short plant with wrinkled seeds and some new combinations (tall plant with wrinkled seeds and short plant with rounds seeds) were also obtained. The tall/short trait and round wrinkled traits are independently inherited.

6. Gene: DNA is the source of making protein in a cell. The section of DNA that provides information for one protein is called gene.

7. Sex determination: It is the process by which the sex of a person is determined. All human chromosomes are not paired, 22 pairs are called autosomes. Women have a perfect pair of sex chromosomes XX. But men have a mismatched pair XY

8. Evolution: It is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced.
(i) Evidences of evolution

  • (a) Homologous organs: Those organs having similar basic structure but have been modified to perform different functions, e.g., forelimbs of frog, lizard, bird and human.
  • (b) Analogous organs: Those organs which are different in basic structure but perform same function, e.g., wings of birds and wings of bat.
  • (c) Fossils: A fossil is the naturally preserved remains or traces of animals or plants that lived in the geologic past.

(ii) Darwin’s theory of evolution: Charles Robert Darwin (1809-1882) explained the evolutionary principle in his famous book “The origin of species”. The theory proposed by him is popularly known as theory of natural selection or Darwinsim.

The main features of the theory of natural selection are as follows:

  • Over production
  • Limited food and space
  • Struggle for existence
  • Variations
  • Natural selection or survival of the fittest

9. Speciation: The process by which new species develop from the existing species is known as speciation.
The factors which leads to speciation are : Geographical isolation, Genetic drift Variations

10. Types of evolution

  • (i) Convergent evolution: It occurs when species have different an central origins but have developed similar features.
  • (ii) Divergent evolution: It occurs when two separate species evolve differently from a common ancestor.
  • (iii) Parallel evolution: It is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.

11. Evolution by stages: Evolution of complex organs have taken place bit-by-bit over generations. e.g., eye, feathers of birds have evolved because of survival advantage of intermediate stages. Thus, changes in DNA during reproduction are the main cause of evolution.

12. Human evolution: All human beings belong to single speceisHomo sapiens, although there were many races of humans. They have originated in Africa, some ancestors left Africa and migrated to West Asia, Central Asia, Eurasia South Asia, East Asia, Indonesia, Australia, America, while others stayed there.
Excavating, time-dating, studying fossils, determining DNA sequences have been used for studying human evolution

Related Articles: