Introduction to Genetics: Genetics Terminologies
(Concept of Genetics: Definition of Terminologies in Genetics)
What is genetics?
Ø Genetics: Genetics is the study of Heredity and Variation of Inherited Characters.
Ø Heredity: The tendency offspring to resemble their parents is called heredity.
Ø Variation: The tendency of offspring to vary from their parents is called variation.
Ø The term ‘Genetics’ was coined by William Bateson in 1905
Ø Genetics is a relatively young branch of biological science.
Ø The study of genetics started with the work of Gregor Johan Mendel (Father of Modern Genetics)
Ø Today, many modern branches of genetics are there such as Cytogenetics, Molecular Genetics, Phylogenetics, Developmental Genetics and Behavioral Genetics.
Contribution of Mendel in Genetics
Ø Gregor Johan Mendel (1822 – 1884), an Austrian Monk, is known as the “Father of Modern Genetics”.
Ø The Modern Concepts of Genetics took birth from his pioneering work on Pisum sativum (Garden Pea).
Ø Mendel published his results in the annual Proceedings of the Natural History Society of Brunn in 1866.
Ø The title of his publication: Experiments in Plant Hybridization (German).
Ø Mendel died as an unrecognized man; His studies remain in dark for about 34 years.
Rediscovery of Mendel’s original work
Ø In 1900, three scientists independently rediscovered Mendel’s work.
@. Carl Corens (Germany)
@. Hugo deVries (Holland)
@. Erich von Tschermak (Austia)
Ø Mendel’s findings were now known as Mendelism or Mendelian Lows of Inheritance.
Terminologies in Genetics
Ø The term ‘Gene’ was coined by Johanson in 1909.
Ø Definition: Gene is the hereditary determining factor and it consists of a continuous segment of DNA.
Ø In eukaryotes, the gene occupies in a specific position on the chromosome called locus (plural loci).
Ø Alleles are also called as allelomorphs.
Ø Definition: Alleles are alternating forms of a gene which occupy identical loci on the homologous chromosome.
Ø The allele controls the contrasting characters of the same trait.
Ø Usually, the alleles exist in TWO different forms: (1) Dominant allele and (2) Recessive allele
(3). Dominant and Recessive Alleles
Ø The Dominant allele will always express phenotypically.
Ø The Recessive alleles will express only in the absence of a dominant allele.
Ø The dominant alleles masks or suppress the expression of the recessive alleles.
Ø Dominant alleles are classically symbolized with English capital letters (Example: Tall – T).
Ø Recessive alleles are symbolized with small letters (Example: Dwarf – t).
(4). Genotype and Phenotype
Ø Genotype: Genotype is the genetic makeup (constitution) of an organism.
Ø Phenotype: Phenotype is the physical features/appearance of an organism.
Ø The phenotype is the expression of genotype in an organism.
Ø The phenotype is produced not only by the genotype but also by the interaction between the genotype and environmental factors. (Example: If a pea plant with genotype TT will only be tall if the soil is sufficiently rich to provide nutrients and water).
Ø Trait: Height
Ø Phenotype : Tall and Dwarf
Ø Genotype: TT or Tt and tt
Ø Homozygous is a condition in which both the members of an allelic pair in the homologous chromosome are identical (either dominant or recessive allele).
o Tall : TT
o Dwarf : tt
Ø Homozygous individuals are pure or true-breeding. They produce only one type of gamete with specific to particular gene.
Ø Heterozygous is a condition in which the members of an allelic pair in the homologous chromosome are NOT identical (one dominant and one recessive allele).
o Example: Tall : Tt
Ø Heterozygous individuals are the progenies of hybridization.
Ø They cannot be tree-breeding. They produce different types of gametes with specific to particular gene.
Ø Hemizygous is a condition when the gene is present only in one copy.
Ø The hemizygous condition is observed usually in male individuals.
o Genes on the X chromosome of a male are hemizygous since males have only one X chromosome)
o Similarly, the genes on Y the chromosome in a male are also hemizygous (only one Y chromosome in males).
Ø Dominance is the ability of an allele to express itself phenotypically both in homozygous (TT) and in heterozygous (Tt) conditions.
Ø Recessiveness is the inability of an allele to manifest its phenotype in heterozygous (Tt) condition.
Ø In the example (Tt), ‘t’ is recessive since it fails to express its phenotype in the presence of a dominant gene ‘T’.
(10). Hybridization and Hybrid
Ø Hybridization is the process of crossing of two genetically different individuals.
Ø Hybrid: The progeny of hybridization is called the hybrid.
Ø A monohybrid is an organism which is heterozygous with respect to only ONE pair of allele at a locus under study.
Ø Example: Tall (TT) X Dwarf (tt)
Ø A dihybrid is an organism which is heterozygous with respect to TWO pairs of alleles at two loci under study.
Ø Example: Yellow Round (YYRR) X Green Wrinkled (yyrr)
(13). Monohybrid Cross
Ø Monohybrid cross is a cross between two individuals which differ from each other with respect to ONE pair of allele under study
Ø Example: Tall (TT) X Dwarf (tt) = Tall (Tt)
(14). Dihybrid Cross
Ø A dihybrid cross is a cross between two individuals which differ from each other with respect to TWO pairs of allele under study.
Ø Yellow Round (YYRR) X Green Wrinkled (yyrr) = Yellow Round (YyRr)
(15). F1 and F2 Generation
Ø The ‘F’ stands for Filial meaning son.
Ø F1 generation is the FIRST generation progeny of hybridization.
Ø F2 generation is the progeny of hybrid (F1) when it is selfed or crossed with its siblings.
(16). Reciprocal Cross
Ø Reciprocal cross means two reverse crosses in which the sexes of the parents are interchanged.
Ø If the traits are autosomal, the reciprocal cross always yields same result.
Ø If the traits are on sex chromosomes, the reciprocal cross gives different results.
Ø Backcross is the cross (hybridization) of F1 progeny with one of its parents.
Ø If the F1 is crossed with the dominant parent, all the progenies (F2) will be with dominant phenotype.
Ø If the F1 is crossed with the recessive parent, individuals with both phenotypes (dominant and recessive) will appear in equal proportions.
Ø The ratio of progenies produced during the back cross is called back cross ratio.
(18). Test Cross
Ø A test cross is a type of backcross in which the F1 progeny is crossed with its double recessive parent.
Ø A test cross is used to determine whether the individuals of the F1 exhibiting dominant character are homozygous or heterozygous
Ø In other words, a test cross is performed to detect the genotype of F1progeny.