03 Inheritance and Variation - part 02 - Genetic Terminology

03 Inheritance and Variation - part 02 - Genetic Terminology


Genetic Terminology :

Character : 
  •  It is a specific feature of an organism e.g.height of stem. 
Trait :
  •  An inherited character and its detectable variant e.g. Tall or dwarf. 
Factor : 
  •  It is a unit of heredity, a particle present in the organism which is responsible for the inheritance and expression of a character. 
  • Factor is passed from one generation to the next through gametes. 
  • Factor determines a genetical (biological) character of an organism. 
Gene : 

  • It is a particular segment of DNA which is responsible for the inheritance and expression of that character. 
Alleles or Allelomorphs : 

  • The two or more alternative forms of a given gene (factor) are called alleles of each other.
  •  They occupy identical loci (positions) on homologous chromosomes. 
  • Allele is a short form of Allelomorph. 
Dominant : 

  • It is an allele that expresses its trait even in the presence of an alternative allele i.e. in heterozygous condition only. 
  • Alternatively, the allele that expresses in F1 is called dominant. (It is an allele of a pair that masks the expression of other allele in F1 generation.) 
Recessive : 

  • This allele is not expressed in the presence of an alternative allele (in heterozygous condition). 
  • It expresses only in the presence of another identical allele. 
  • It is an allele that does not express in F1 hybrid.

Phenotype : 

  • The external apperance of an individual for any trait is called phenotype for that trait. 
  • It is observable and is determined by different combinations of alleles. 
  • e.g. In pea, for the height of stem (plant) tall and dwarf are the two phenotypes (Tall is determined by TT or Tt and dwarf by tt). 
Genotype : 

  • Genetic constitution or genetic make up of an organism with respect to a particular trait. 
  • It is representation of the genetic constitution of an individual with respect to a single character or a set of characters. 
  • e.g. pea tall plants can have genotype TT or Ttand dwarf has tt. 
Homozygous (pure) : 

  • An individual possessing identical allels for a particular trait, is called homozygous or pure for that trait. 
  • Homozygous breeds true to the trait and produces only one type of gametes 
  • e.g., tall with TT and dwarf with tt.
Heterozygous : 

  • An individual possessing contrasting alleles for a particular trait, is called heterozygous. 
  • Heterozygous does not breed true for that trait and produces two types of gametes e
  • .g. F1 generation hybrids (Tt). Heterozygous individual is also called hybrid. 
Pure line : 
  • An individual or a group of individuals (population) which is homozygous or true breeding for one or more traits, constitutes pure line i.e. plant which breeds true for a particular character. 
  • It is a descendent of a single homozygous parent produced after self fertilization. 
Monohybrid : 

  • It is heterozygous for one trait and is produced from a cross between two pure parents differing in single pair of contrasting characters 
  • e.g. Hybrid tall produced in a cross between pure tall and pure dwarf parents. 
  • It is a heterozygote for a single pair of alleles. 
F1 generation : 

  • It refers to the first filial generation. 
  • It consists of all off-springs produced from a parental cross. 
  • Alternatively, it is first generation from a given mating between pure parents having contrasting characters.

F2 generation : 

  • The second generation (progeny) produced by selfing (inbreeding) of F1 generation offsprings is called second filial generation. 
  • e.g. Progeny produced from a cross between two F1 individuals (e.g. Tt ×Tt). 
Punnett square/checker board : 
  • It is a probability table representing different permutations and combination of fertilization between gametes of the opposite mating types. 
  • In short, it is a diagrammatic representation of a particular cross to predict the progeny of a cross.
Homologous Chromosomes : 
  • The morphologically, genetically and structurally essentially identical chromosomes present in a diploid cell, are called homologous chromosomes. 
  • Such chromosomes synapse during meiosis.

 Back cross : 

  • It is a cross of F1 progeny with any of the parents (e.g. F1 tall ×pure tall; F1 tall ×pure dwarf i.e. Tt ×TT/tt).

Test cross :

  • It is a cross of F1 progeny with cross of pure tall, round seeded plant with homozygous recessive parent 
  • e.g. F1 tall × dwarf, wrinkled seeded plant. Mendel also pure dwarf i.e. Tt × tt. 
  • It is used to test the performed the dihybrid cross between pea homozygous/ heterozygous nature of hybrid. 
  • It   is a kind of back cross.

Phenotypic ratio : 

  • It is the ratio of the offsprings produced in F2and subsequent generation with respect to their physical appearence 
  • e.g. 3Tall : 1 dwarf, is F2 `Phenotypic ratio' in monohybrid cross.


 Genotypic ratio : 


  • It is the ratio of the offsprings produced in the F2 and subsequent generation with respect to their genetic make up 
  • e.g. 1 TT : 2Tt : 1 tt, is F2 genotypic ratio in monohybrid cross.

Monohybrid cross : 

  • A cross between parents differeing in only one heritable trait is called monohybrid cross. 
  • e.g. cross of pure tall and pure dwarf plants. 
  • Mendel performed the monohybrid cross between two pea plants with only one pair of contransting character.

Dihybrid cross : 


  • A cross between parents differing in two heritable traits, is called dihybrid cross.
  • e.g. cross of pure tall ,round seeded plant with dwarf,wrinkled seeded plant . 
  • Mendel also performed the dihybrid cross between pea plants that differed in two pairs of contrasting charactares. 

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