Genetic Terminology - Part 2 - Principles of inheritance and variations

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Genetic Terms

• Hybrid. The heterozygous organism produced after crossing two genetically differ ent individuals is called hybrid.
• The process of obtaining hybrids or hybridisation is employed to improve the quality of economically important plants and animals as it combines the useful traits of different varieties.
• The harmful effect of recessive traits disppears as the dominant traits of all the characters of the parents often come together in the hybrid. As a result the hybrid possesses qualities better than either of the parents.
• It is called hybrid vigour or heterosis.
• Depending upon the number of characters in which parents differ from each other , there may be monohybrids (one character), dihybrids (two characters), trihybri polyhybrids. etc.
• F1 Generation. For first filial (filus - son, filia - daughter, Bateson, 1905) generation is the generation of hybrids produced from a cross between the genetically different individuals called parents. For example, Tt individuals are produced in F1 generation from a cross between TT and tt parents.
• F2Generation ( Bateson , 1905 ). For second filial generation is the generation of individu als which arises as a result of inbreeding or interbreeding amongst individuals of F1 generation
• Genotype (Gk. genos - race ; typos - image ; Johannsen, 1911). It is the gene complement or genetic constitution of an individual with regard to one or more characters irrespective of whether the genes are expressed or not. For example, the genotype of hybrid tall Pea plant is Tt, pure tall TT and dwarf tt.

• Phenotype ( Gk . phainein - to appear , typos - image : Johannsen, 1911).
• It is observable or measurable distinctive structural or functional characteristic of an individual with regard to one or more characters which is a result of gene products brought to expression in a given environment.
• The characteristic may be visible to eye (e.g., height of a plant, colour of flower) or may require special test for its identification (e.g. , serological test for blood group).
• For recessive genes, the phenotype is similar to genotype. For dominant genes, the phenotypic expression can be due to its homozygous genotype or heterozygous genotype. For example, phenotypic tall Pea plant can be genotypically TT or Tt .
• Reciprocal Cross . A second cross of the same genotypes in which the sexes of the parental generation are reversed , is called reciprocal cross.
• The cross AA ( female) x aa ( male ) is the reciprocal of the cross aa (female) x AA (male).
• Punnett Square. It is a checkerboard or square divided into smaller squares as shown, It was developed by a British geneticist Punnett (1927) and is known after his name as Punnett square.
• It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
• The possible gametes are written on two sides, the top row (horizontal row) and left column (vertical column).
• Usually male gametes are written in top row and female gametes in left column.
• All possible combinations are represented in boxes below in the squares . Thus various types of phenotypes and genotypes are obtained.

• Pollen. The young male gametophyte of a plant , surrounded by the microspore wall.
• Stamens. These are male reproductive organs of a flower. Stamen consists of two parts filament and anther.
• Pistils. These are female reproductive organs of a flower. Pistil consists of three parts stigma, style and ovary. Stigma receives pollen grains during pollination.
• Emasculation. The removal of stamens from bisexual flowers in order to avoid sell - polination in these flowers during hybridization.
• Syndrome. A group of symptoms that occur together and represent a particular disease.
• Back Cross. The cross of an F , hybrid with one of the two parents is called backcross . In such cases there are two possibilities.
• In one possibility, there is cross between F1 hybrid ( Tt ) and dominant parent ( TT ). In such a cross, plants will be 100 % tall
• In second possibility, there is cross between F1 hybrid ( Tt ) and recessive parent ( tt ). In such cross, plants will be 50 % tall and 50 % dwarf.
• Use of Back Cross. In plant breeding back cross is performed a few times in order to increase the traits of that parent. For example, a crop plant is crossed with a wild variety in order to obtain its disease resistance. In the process most good traits of the crop plant get diluted. The hybrid is, therefore, repeatedly crossed with parent crop plant in order to transfer the good traits back into it.
• Test cross. A cross between an individual of unknown genotype and recessive parent is called test cross.
• There are two possibilities of such a cross .
• ( A ) When pure dominant (TT) of unknown genotype plant is crossed with a pure recessive parent plant (tt), in such a cross, 100 % plants will be tall as in the normal monohybrid cross.
• ( B ) When hybrid dominant (Tt) plant of unknown genotype is crossed with a pure recessive (tt) parent plant, in such cross 50 % plants will be tall and 50 % plants will be dwarf like backcross (B)
• Use of Test Cross. It is called testcross because it is used to test whether an individual is homozygous (pure) or heterozygous (hybrid).
• It is also used as test for linkage.
• Types of Testcross. Testcross is of two types -Monohybrid and Dihybrid. A monohybrid test cross deals with single trait and gives F1 phenotypic ratio of 1 : 1 , i.e. , 50 % each. A dihybrid testcross deals with two traits at a time and gives F1 phenotypic ratio of 1 : 1 : 1 : 1 , 1.e. , 25,% each
• Monohybrid Cross. It is a cross between two organisms of a species which is made to study the inheritance of a single pair of alleles or factors of a character.
• Monohybrid Ratio.
• It is a ratio which is obtained in F2 generation when a monohybrid cross is made and the offspring of F1 generation are selfbred.
• Monohybrid ratio is usually 3 : 1 (phenotypic ratio) or 1 : 2 : 1 (genotypic ratio) in which 25 % of the individuals carry the recessive trait, 25 % pure dominant and 50 % have hybrid dominant trait.

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