Heredity And Evolution Class 10 Notes Biology Science Chapter 9

What are Heredity and Evolution?

  • Genetics is the process of transmission of body features from parents to offsprings and the laws related to transmission. In other words we can say Genetics deals with study of both heredity and variations.
  • The word “ Genetics “ was coined by William Bateson in 1906.
  • Variations are mainly seen during sexual reproduction due to the following reasons :
  • (i) Crossing over during meiosis process.
  • (ii) Alterations in genetic material due to mutations.
  • (iii) Mixing of female and male gametes that come from two different individuals i.e., father and mother
  • Gregor Johann Mendel is considered as “Father of Genetics.” He had formulated the Laws of Inheritance by performing hybridisation experiments on Pisum sativum – Garden pea plant.
  • He had studied seven contrasting pairs of characters in pea plants.
(i) Height of the plant – Tall or short (ii) Colour of flower – Purple or white
(iii) Shape of seed – Round or wrinkled (iv) Colour of seed – Yellow or green
(v) Colour of pod – Green or yellow (vi) Shape of pod – Inflated or constricted
(vii) Position of flower –Axial or terminal
  • The cross between the two pure breeding varieties of an organism taking into account only a single character at a time is called monohybrid cross.
  • Example of monohybrid cross : A pure tall pea plant is crossed with pure dwarf pea plant. In F1 generation tall pea plants are produced with heterozygous condition i.e., they carry both dominant and recessive alleles. When these plants are crossed they give tall and dwarf plants in the ratio 3 : 1 which is also called phenotypic ratio and their genotypic ratio is 1 : 2 : 1.
  • The cross between the two pure breeding varieties of an organism taking into account two characters at a time is called dihybrid cross.
  • When a pure pea plant with round yellow seeds is crossed with pure pea plant having green wrinkled seeds, in F1 generation hybrid plants with round yellow seeds are produced. But again when these plants are crossed they produce round-yellow, round-green, wrinkled-yellow, wrinkled-green in the ratio 9 : 3 : 3 : 1, which is the phenotypic ratio.
  • Based on Mendel’s breeding experiments three Laws were deduced.
  • Law of Dominance : The phenomenon of appearance of only one of the two contrasting traits in F1 generation is called dominance. The other character remains suppressed which is known as recessive character
  • Law of Segregation : At the time of reproduction when gametes are formed the factors segregate so that each gamete receives only factor of each character. This is called Law of purity of gametes.
  • Law of independent assortment : At the time of reproduction, two pairs of factors of each of the two traits in a dihybrid cross segregated independently during gamete formation and randomly formed combinations in F2 generation. Inheritance of factors controlling a particular trait in an organism is independent of the other.
  • The mechanism by which sex of an individual is determined when it begins its life is called sex determination.
  • In human beings, sex is determined by XX-XY mechanism. Males have XY sex chromosomes whereas females have XX sex chromosomes.
  • In some reptiles sex is determined by environmental factors.
  • Gene is a fragment of DNA molecule that has a particular nucleotide sequence which encodes for a particular protein.
  • Evolution is the constant process of gradual change occurring in an organism since the origin of life which gives rise to variety of complex organisms on the surface of earth.
1. Heredity: Transmission of genetical characters from one generation to the next generation is called heredity. 5. Monohybrid cross: A breeding experiment which involves the alternative traits of one single character is called monohybrid cross.
2. Gene: It is a specific segment of DNA on a chromosome occupying specific position and determines the hereditary characters. 6. Dihybrid cross: A cross between two pure breeding individuals taking into consideration alternative traits of two different characters is called a dihybrid cross.
3. Traits: The alternative forms of a character are called traits. 7. Variation: It is the differences in the traits shown by the individuals of a species and also by the offsprings of the same parents are called variations.
4. Genetics: It is the branch of biology that deals with the study of heredity and variations. 8. Sex determination: The mechanism by which sex of an individual is determined when it begins its life.