Terminology of Genetics – 01

Heredity:

  • The transmission of characters from one generation to the next, that is from parents to offsprings (progeny) is known as heredity.
  • These hereditary characters are present on the chromosomes in the form of genes. These gene combinations express characters which may be more similar to one of its two parents. The differences in characters of offspring mainly depend upon a unique process of crossing over that occurs during meiosis. This is one of the main reasons for producing recombination.

Inheritance:

  • Inheritance is the process by which characters are passed on from parent to progeny. it is the basis of heredity. Inheritance studies both the similarities and variations.

Variations:

  • Variation means differences between parents and their offsprings or between offsprings of same parents or between members of the same population (same species).
  • Above photograph shows variation species elephant but two variations (Asian elephant and African elephant). They are same species but there is variation in their size, structure etc. due to their adaptations to their habitats.
  • Causes of Variations:
    • It arises due to mutation or sudden change in the genes.
    • At fertilization, there is random mixing of paternal and maternal chromosomes with different gene combinations. Such a source of variation which is most common is called genetic recombination. The variation arises because genes get shifted and exchanged during meiosis at the time of formation of gametes, giving rise to new gene combinations.
    • Heritable Variations generally arise because of mutation and recombination.
  • Importance of Variation:
    • It has survival value for the population because if the environment changes, some individuals (variants) may be able to adapt to new situations and save the population from dying out (extinction).

Genetics:

  • Genetics is a branch of a biology that deals with heredity and variations. This term was coined by William Bateson in 1906. Word Genetics is derived from Greek word “Genesis”, which means ‘to grow into’.

Clones:

  • Organisms produced by asexual reproduction are exact copies of their parents. They are carbon copies of each other and of parents. They are called ramets. The group of identical individuals produced from the single parent is known as a clone. Animals produced by asexual reproduction or plants (budding, fission, spore formation, grafting, and layering) produced by vegetative propagation are identical to their parents and hence are clones. They can be made in the lab.
  • Clones are organisms that are exact genetic copies of each other because every single bit of their DNA is identical.
  • Characteristics of Clone:

  • It is formed by asexual reproduction
  • It is produced from a single parent. Hence it is monoparental.
  • It is formed by mitotic cell division.
  • As meiosis is absent, recombination of genes does not occur.
  • They are carbon copies of each other and of parent both in genotype and phenotype.
  • Members of a clone are genetically identical.

Offspring:

  • Organisms produced by sexual reproduction are called offspring and they are not identical to either of their parents but inherits some of the characteristics of both the parents.
  • Characteristics of Offspring:

  • They are produced by sexual reproduction
  • It is produced from two parents (a male and a female). Hence it is biparental.
  • It is formed by the fusion of the male and the female gametes produced by meiosis.
  • Due to meiosis, recombination of genes takes place.
  • Offspring differ from each other and also from parents.
  • Offspring have differences in their genotype.

Factor:

  • It is a unit of heredity. This concept was given by Mendel.
  • The unit of inheritance and expression of a particular character is controlled by inheritable units called factor (gene) which are present in pairs in parental cells and singly in the gametes.
  • It is responsible for the inheritance and expression of character.

Gene:

  • It is a particular segment of DNA which is responsible for the inheritance and expression of that character. Johannsen used the term gene for the first time. Each gene has information for the synthesis of a particular polypeptide.

Character:

  • It is a feature of the organism. e.g. Height of stem or height of a person, flower colour, seed shape, eye colour, skin colour, etc.
  • A character is a feature of the organism or external appearance of the organism,

Trait:

  • The trait is the morphologically or physiologically visible character, e.g. colour of flower, and shape of the seed.
  • It is inherited character and its detectable variant. e.g. Tall or dwarf
  • The trait is inherited character and it’s detectable variant
  • For e.g consider a height of a plant, it is external appearance hence height his character or feature. Now it has two variants, tall or dwarf, then tall and dwarf are their traits. Traits are in multiple forms and environmentally determined.

Alleles or Allelomorphs:

  • The two (or more) alternative forms of a gene (factor) are called alleles of each other. They occupy identical positions on homologous chromosomes.
  • For example in pea plant, the gene for producing seed shape may occur in two alternative forms: smooth (S) and wrinkled (s). Genes for smooth wrinkled seeds are alleles of each other and occupy the same locus on homologous chromosomes.

Homologous and Heterologous Chromosomes:

  • The term homozygous and heterozygous were coined by Bateson and Saunders (1902) for the types of symbolised gene combinations.
  • The morphologically and structurally similar chromosomes present in a diploid cell are called homologous chromosomes or homologues. The chromosomes of true breeding tall (TT) and true breeding dwarf (tt) pea plants are homozygous.
  • If the two members of an allelic pair are not the same, then the individual chromosome is called heterozygous or heterologous. The chromosomes of next generation of crossing between the true breeding tall (TT) and true breeding swarf (tt) pea plants can be tall (Tt) which is heterozygous.

Dominant of Allele or Dominant Trait:

  • It is an allele that expresses its trait even in the presence of an alternative allele.
  • Out of the two alleles or allelomorphs of a trait, the one which expresses itself in a heterozygous organism in the F1 hybrid is called the dominant trait (dominant allele).
  • If the allelic combination in an organism is Tt, and T (tallness) expresses itself but t (dwarfness) cannot, so T is the dominant allele, and tallness is dominant on dwarfness.

Recessive of Allele Recessive Trait:

  • It is an allele which is not expressed in the presence of an alternative allele. i.e. in the heterozygous condition.
  • Out of the two alleles or allelomorphs of a trait, the one which remains masked in the F1 individual but gets expressed in the next generation (F2), is called recessive.
  • If the allelic combination in an organism is Tt (heterozygous), and T (tallness) expresses itself but t (dwarfness) being recessive cannot express itself, so T is the dominant allele, and tallness is dominant on dwarfness. Recessive allele does express itself only in the homozygous state (e.g. tt).

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