Unit – I B – 02

The terminology of Genetics (Contd.)

Phenotype:

  • The external appearance of an individual for any trait is called phenotype for that trait.
  • In garden pea plant Height of a stem is a character. Its two variants are tall (T) and dwarf (t). Thus tall and dwarf are phenotypes.
  • If we consider seed of garden pea plant, then smoothness of coat of seed can be considered as a character. Thus smooth and wrinkled are phenotypes.

Genotype:

  • It is a representation of the genetic constitution of an individual with respect to a single character or a set of characters.
  • In garden pea plant genotype for a tall plant is TT or Tt. For a dwarf plant, the genotype is tt.
  • If we consider seed of garden pea plant, then smoothness of coat of seed can be considered as a character. Thus for smooth seed plant genotype is (SS) or (ss) and for wrinkled seed plant,  is (ss).

Homozygous:

  • An individual possessing similar alleles for a particular trait is called homozygous or pure for that trait. They produce only one type of gamete.
  • e.g. Parental tall with TT and dwarf plant with tt are homozygous.

Heterozygous:

  • An individual possessing dissimilar alleles for a particular trait is called heterozygous or hybrid for that trait. They produce two types of gamete.
  • e.g. F1 Generation of hybrid Tt.

Pure Line:

  • An individual or a group of individuals (population) that is homozygous or true breeding for one or more trait.

Hybrid:

  • It is heterozygous individual produced from any cross involving pure parents having one or more contrasting traits.

Monohybrid:

  • It is heterozygous for one trait and produced in a cross between two pure parents differing in single pair of contrasting character.
  • e.g. Hybrid tall produced in a cross between pure tall and dwarf parent.

Dihybrid:

  • It is heterozygous for two traits and produced in a cross between two pure parents differing in two pairs of contrasting character.
  • e.g. Hybrid tall produced in a cross between pure tall bearing white flowers and dwarf parent bearing purple flowers.

F1 Generation:

  • The progeny (offspring) produced from a cross is called first filial or F1 generation.
  • It shows uniform expression.

F2 Generation:

  • The second generation (progeny or offspring) produced from selfing (interbreeding) of F1 generation offspring is called second filial or F2 Generation.
  • It shows two or more types of individual in particular

Punnett Square (Checker Board):

  • It is a diagram that is used to show possibilities of combinations in particular cross or breeding experiment.
  • It helps us to know possible genotypes and phenotypes of offspring produced in the cross

Terminology of Genetics

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