Mendel’s Laws of Inheritance – Terminology and Symbols

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Character is a well defined morphological or physiological feature of an organism.
Trait is the distinguishing feature of a character.
Gene is the inherited factor that determine the biological character of an organism.
• A pair of contrasting characters is called allelomorph or allele.
•  Term allele was given by W. Bateson for alternative forms of same gene, e.g., T and t, Y and y, R and r are pair of alleles.
•  Dominant allele is one of the factor of an alleles pair which can express itself whether present in homozygous or heterozygous state, e.g., T (tallness in pea), R (round seed in pea).
Recessive allele is the factor of an allele pair which is unable to express its effect in the presence of its contrasting factor in a heterozygote, e.g., t in Tt.
• The effect of recessive factor is expressed only when it is present in the pure or homozygous state, e.g., tt in dwarf pea plant.
•  Wild allele is the one which was originally present in the population and is dominant and widespread.
• The recessive allele is less common and might have formed through mutation of wild allele. It is thus called mutant allele.
• The diploid condition in which the alleles at a given locus are identical is called homozygous or pure.
• In homozygous condition, organism have two similar genes or alleles for a particular character in a homologous pair of chromosomes, e.g., TT or tt.
• Organisms containing two different alleles or individual containing both dominant and recessive genes of an allelic pair, e.g., Tt, is known as heterozygous or hybrid.
• When only one allelic pair is considered in cross breeding it is called monohybrid cross.
• When two allelic pairs are used for crossing it is called dihybrid cross.
•  Involvement of more than two allelic pairs in a cross is called polyhybrid cross.
• Second stage of Mendel’s experiment is called F, generation or first filial generation.
• Third stage of Mendel’s experiment is called second filial or F2 generation.
• Complete penetrance is 100% ability of an allelic combination to produce expected phenotype.
•  Incomplete penetrance is failure of an allelic combination to provide cent percent phenotypic expression, e.g., polydactyly, diabetes mellitus.
Hybrid vigour or heterosis is the superiority of hybrid over either of its parents in one or more traits.
• Genotype is the sum total of heredity or genetic make up.
• Phenotype is the external feature of organism.
• The portion or region on chromosome representing a single gene is called gene locus.
• The alleles of a gene are present on the same gene locus on the homologous chromosomes.
• All the genotypes of all organisms in a population form the gene pool.
• Pure line or pure breeding line is a strain of individuals homozygous for all genes considered. The term was coined by Johannsen.
 Punnett square is a checker board which was devised by R.C. Punnett and used to show the result of a cross between two organisms.
• Genome is a constitution of all the genes contained in a single set of chromosomes, i.e., in a haploid nucleus. Each parent, through its reproductive cells, contributes its genome to its offspring.
• A single genome is present in haploid cells, two in diploid cells and many in polyploid cells.

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