Terminology of Genetics 02

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Genotype:

  • 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. The genotype of pure smooth seeded parent pea plant is SS and it will always breed true for the smooth-seeded character, but plants having Ss on selfing would give rise to a population represented by 3: 1 ratio for smooth seeded plants and wrinkled seeded plants.
  • Characteristics of Genotype

  • It is a gene complement of an organism with respect to one or more characters.
  • Individuals with different genotype can have the same or different phenotype.
  • It is not influenced by the phenotype.
  • It is not affected by the environment or age.
  • The genotype of an individual can be obtained by performing specific experiments.

Phenotype:

  • The external appearance (morphological or physiological characters) of an individual for any trait is called phenotype for that trait. e.g. Smooth-seeded shape or wrinkled shape of seeds represent two different phenotypes. The phenotype is the external manifestation of the genotype of an organism. It is an interaction between the genotype and environment. Phenotype changes to some extent with environment and age.
  • 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.
  • Characteristics of Phenotype

  • It is the external appearance (morphological or physiological characters) of an individual for any trait
  • Individuals with different phenotype generally have the different genotype.
  • It is the expression of the genotype.
  • It is affected by the environment or age.
  • The phenotype of an individual can be obtained from direct observations.

alleles

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.
  • Characteristics of Homozygous:

  • The alleles of a pair are identical in a homozygous individual.
  • They possess either both dominant (TT) or both recessive alleles (tt).
  • Homozygous individuals may be homozygous dominant or homozygous recessive.
  • They are pure for the trait.
  • Gametes produced by the homozygous individuals are identical. Thus only one type of gametes is produced.
  • On self-breeding, they produce only one type of offspring.

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.
  • Characteristics of Heterozygous:

  • The alleles of a pair are different in a heterozygous individual.
  • They possess one dominant allele and one recessive allele.
  • Heterozygous individuals show only of one type.
  • They are not pure for the trait.
  • Two types of gametes are produced.
  • On self-breeding, they produce three types of genotypes: homozygous dominant, homozygous recessive and heterozygous.

Pure Line:

  • An individual or a group of individuals (population) that is homozygous or true breeding for one or more trait. The pure line is obtained by repetitive self-fertilization or breeding between homozygous identical ancestors. Offspring of pure line are exactly identical. The term pure line was coined by Johannsen.

Hybridisation:

  • Crossing organisms belonging to different species for getting desirable qualities in the offspring is called hybridization..

Hybrid:

  • It is heterozygous individual produced from any cross involving pure parents having one or more contrasting traits. If pure tall plant (TT) is crossed with a pure dwarf plant (tt), progeny is hybrid tall (Tt).

Monohybrid:

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

Monohybrid Cross:

  • It is a cross that involves the study of inheritance of only one pair of contrasting character at a time. The inheritance of tall and dwarf characters is an example of a monohybrid cross.

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.

Dihybrid Cross:

  • It is a cross that involves the study of inheritance of two pairs of contrasting characters at a time. The inheritance of yellow round seed character and green wrinkled character is an example of a dihybrid cross.

Poly-hybrid Cross:

  • It is a cross that involves the study of inheritance of more than two pairs of contrasting characters at a time..

F1 Generation:

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

FGeneration:

  • 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.

Back Cross:

  • When F1 hybrids are crossed with either of the parents or parental type, then such a cross between the offspring and the parents is known as back cross.

Test Cross:

  • The test cross is a cross between a heterozygous F1 hybrid and double recessive homozygous. The test cross is used to determine whether the individuals exhibiting dominant character are homozygous or heterozygous.

Genome:

  • Genome is the total number of genes present in the haploid set of chromosomes. A gamete is haploid and contains only one set of the genome.

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
Science > Biology > Genetic Basis of InheritanceYou are Here
Physics Chemistry  Biology  Mathematics

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