Genetics 

Genetics: It is the branch of science that deals with the principles of inheritance and its practices.

  • Mendel was the first to carry out the study on the transmission of characteristics from parents to offspring.
  • Mendel proposed that heredity is controlled by genes.

Mendel’s law of inheritance

  • Mendel experimented on garden pea plant (Pisum sativum) having many visible contrasting characters.
  • He used seven contrasting pairs of characters or traits in garden pea.
Trait Dominant Trait Recessive Trait
Seed shape Round Wrinkled
Seed colour yellow Green
Flower colour violet White
Pod Shape Inflated Constricted
Flower position Axial Terminal
Stem Height Tall Dwarf
Pod Colour Green Yellow

  • Mendel crossed pea plants having these seven pairs of contrasting characters/traits and produced offspring from them.
  • His experiments included three steps –
  1. Selection of true breeding plants
  2. Obtaining F 1 plants by cross-pollination
  3. Self-pollination of F 1 plants to obtain F2 generation

Important terms:

  • Genes: Functional unit of heredity
  • Alleles: Alternative forms of the same gene; for example, the gene for plant height in pea plant has two alleles T or t
  • Phenotype: It is the physical expression of character; for example, tall and dwarf plants.
  • Genotype: It is the genetic constitution of an organism; for example, TT or Tt is the genotype for tall plants while tt is the genotype for dwarf plants.
  • Dominant: It is the character/trait that is able to express itself over another contrasting trait; for example, tall plant is dominant over the dwarf plant.
  • Recessive: It is the character/trait that is unable to express itself over another contrasting trait.
  • Test cross: It is a cross between organisms with unknown genotypes and recessive parents. This cross is used for determining whether the given individual has a homozygous or heterozygous genotype.
  • Monohybrid cross: It is a cross between two parents that have one pair of contrasting characters; for example, a cross between tall (TT or Tt) and dwarf (tt) plants. The phenotypic ratio obtained in monohybrid cross is 3:1 while genotypic ratio is 1:2:1.
  • Based on observations on monohybrid crosses, two laws were proposed –
    1)Law of Dominance
    2) Law of segregation
  • Dihybrid cross: It is the cross between two parents that have two pairs of contrasting characters; for example, the cross between round yellow seed and wrinkled green seeds. The phenotypic ratio obtained in dihybrid cross is 9:3:3:1.  
  • On the basis of observation of dihybrid cross, the law of independent assortment was proposed.
    3) Law of Independent assortment
  • Law of dominance: It states that only one parental trait gets expressed in the F1 generation while both the traits get expressed in the F2 generation.
  • Law of segregation: It states that two alleles segregate from each other when characters are transferred from parents to offspring during reproduction.
  • Law of independent assortment: It states that the members of different pairs of alleles assort independently into gametes.

 


For more notes click here: Cell cycle Notes