Genetics Terminologies

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

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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

(1). Gene

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).

(2). Allele

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

(5). Homozygous

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).

Ø  Example:

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.

(6). Heterozygous:

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.

(7). Hemizygous

Ø  Hemizygous is a condition when the gene is present only in one copy.

Ø  The hemizygous condition is observed usually in male individuals.

Ø  Example:

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).

(8). Dominance

Ø  Dominance is the ability of an allele to express itself phenotypically both in homozygous (TT) and in heterozygous (Tt) conditions.

(9). Recessiveness:

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.

(11). Monohybrid

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)

(12). Dihybrid

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)

Learn more: Monohybrid Inheritance (Law of Dominance and Law of Segregation)

(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.

(17). Backcross

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.

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