Unit – I B – 01
Terminology of Genetics
- Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity. Variation is the degree by which progeny differ from their parents.
- Inheritance studies both the similarities and variations.
- Heredity is the transfer of character from parents to their offspring.
- 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.
- Variations are the differences between the parents and the offspring of the same parents and among the individuals of the same species.
- Genetics is a branch of a biology that deals with heredity and variations.
- Word Genetics is derived from Greek word “Genesis”, which means ‘to grow into’. This term was coined by William Bateson in 1906.
- Animals produced by asexual reproduction or plants produced by vegetative propagation are identical to their parents and can be closed as clones.
- 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.
- Sometimes they may show intermediate characters.
- It is a feature of the organism. e.g. Height of stem or height of person
- An inherited character and its detectable variant. e.g. Tall or dwarf
- A character is a feature of the organism or external appearance of the organism, while trait is inherited character and it’s detectable variant e.g consider one character as Height of a plant, it is external appearance or feature while, tall and dwarf are their trait . Traits are in multiple forms and environmentally determined.
- It is a unit of heredity. A particle present in the organism which is responsible for the inheritance and expression of character.
- 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 gene (factor) are called alleles of each other. They occupy identical positions on homologous chromosomes.
Homologous and Heterologous Chromosomes:
- The term homozygous and heterzygous 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 true breeding red (RR) and true breeding white (rr) plants are homozygotes.
- If two members of an allelic pair are not same, then the individual chromosome is called heterozygous or heterologues. The next generation of crossing between the true breeding red (RR) and true breeding white (rr) plants is red (Rr) is heterozygous.
Dominant of Allele:
It is an allele that expresses its trait even in the presence of an alternative allele. i.e. in the heterozygous condition.
Recessive of Allele:
It is an allele which is not expressed in the presence of an alternative allele. i.e. in the heterozygous condition.