Introduction to Genetics: Glossary of Genetics Terminologies (Short Notes with PPT)

Introduction to Genetics PPT

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.

Define heredity and Variation

Contributions of Bateson in GeneticsContribution 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.

Scientists who rediscovered Mendels Work
Terminologies in Genetics

What is Allele

(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).

Define genotype

(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.

Define hybridization

(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.

Genetics reciprocal cross(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.

Why we do test cross in genetics

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