• Heredity :- The process of transfer of characters or traits from the parents to their offsprings is called heredity.
  • Variations :- The differences between the characters or traits among the individuals of the same species are called variations.
  • For example human height is a trait which shows different variations like tall, short, medium etc.

  • Another example is earlobe, it shows two variations free earlobe and attached earlobe.
  • The variation is necessary for evolution The great advantage of variation is that it increases the chance of survival in a changing environment

Accumulation of variations during reproduction

  • The reproduction produces variations in offsprings .
  • The  minor variations are seen  due to inaccuracies in DNA copying.
  • These variations are less in asexual reproduction and more in sexual reproduction.
  • Some variations are useful variations and they help the organism to adjust to the changes in the environment.
  • Some variations do not help the organism to adjust to the changes in the environment and they may die and become extinct

Transfer of characters

  • Characters are transferred through DNA molecules present in the gene on the chromosomes which are present in the nucleus of the cell.
  • The inheritance of characters is due to the fact that both the father and mother contributes equal amount of genetic material to the child.
  • So for each trait there are two factors one from the father and one from the mother
The structure of the chromosome

CHROMOSOME: A thread like structure in the nucleus of cell formed of DNA and protein
which carries the genes.
GENE: A unit of DNA or a segment of DNA which controls specific characteristic of an
ALLELES: They are various forms of a gene which occur at the same particular position or
locus over the chromosomes.
DOMINANT GENE: The gene which decide the expression of a character even in the presence of an alternative gene.
RECESSIVE GENE: The gene which decides the expression of an organism in the presence of another identical gene.

  • The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the same letter in small.
  • For example the dominant gene for tallness is represented by T and the recessive gene for dwarfness is represented by t.
  • Genotype:-  The genotype is the description of genes present in an organism.
  • Phenotype:- The characteristic or trait which is visible in an organism Is called phenotype. Ex- Tall or dwarf.

Rules for the Inheritance of Traits – Mendel’s Contributions

Mono-Hybrid Crossing 

  • Gregor Johann Mendel conducted experiments with garden pea plants and determined the rules for the inheritance of traits
  • Mendel selected pea plants having one pair of character – a tall pea plant and a
    short pea plant.
  • He selected pure tall (TT) and pure short (tt) pea plants and cross pollinated them.
  • He obtained all tall plants (Tt) in the first generation (F1 )
  • When the first generation plants were self pollinated, he obtained tall and dwarf
    plants in the ratio 3:1 in the second generation. (F2)
  • The ratio of pure tall (TT), hybrid tall (Tt) and pure dwarf (tt) was in the ratio 1:2:1
  • The trait that is expressed in the F1 generation is called the dominant trait and the trait that is suppressed in the F1 is called the recessive trait.

Dihybrid cross

  • When plants having two pairs of characters (Eg:- shape and colour of seeds) were crossed (Dihybrid cross)
  • Mendel selected pea plants having two pairs of characters – shape and colour
    of seed.
  • He selected plants having round yellow seeds (RRYY) and wrinkled green seeds (rryy) and cross pollinated them.
  • He obtained all plants with round yellow seeds (RrYy) in the F1 generation.
  • When these plants were self pollinated in the F2 generation out of 16 plants, 9 had round yellow (RrYy), 3 had round green (Rryy), 3 had wrinkled yellow (rrYy) and 1 had wrinkled green (rryy) seed  In the ratio 9:3:3:1.Heredity and Evolution NCERT Solutions For Class 10th SCIENCE