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COURSE Principles of inheritance and variations Lesson 4 Important terms used in genetics 2 Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR NET (JRF). SRF, GATE 21 Years of Pre-medical teaching experience
4. Alleles The genes, which occupy similar positions of homologous chromosomes are called alleles. . In a diploid cell, there are two copies of a gene, one on each homologous chromosome. Both these genes govern the same character but may be different in their allelomorphic nature. These genes are called alleles of each other. 5. Genotype and phenotype .The genetic configuration of an organism (in relation to the inheritance of characters) is called genotype, whereas the outcome of the inheritance, which is visible externally or morphologically, is called phenotype. For example In Mendel's experiments the genotype of tall plants were TT or Tt. Here TT and Tt are the genotypes, while the tall appearance is the phenotype. 6. Homozygous and heterozygous .When (in genotype of a diploid cell), both alleles of a character are exactly similar, then such a condition is called homozygous condition. Contrary to this when two alleles of a character are not similar (and represent different allelomorphic character) then such a condition is called heterozygous condition
7. Cross or hybridization It is an act of sexual reproduction between two desired organism. . Though it occurs naturally, it can be carried out artificially also for experimental purposes In genetics there are many types of crosses such as monohybrid cross, dihybrid cross, polyhybrid cross, back cross and test cross etc. 8. Monohybrid cross . In this cross, only one character is considered at a time for analysis. If we consider only height of the plant (as a character) during a cross, then it would be called as monohybrid cross. 9. Dihybrid cross . When two characters are considered simultaneously in a cross, then the cross is called as dihybrid cross. Suppose if we consider height along with the shape of the seed, then it would be a dihybrid cross. 10. Polyhybrid cross . In polyhybrid crosses, many characters are considered simultaneously, for their analysis, in a single cross. (Note - The mechanism of all the crosses is exactly the same. The difference is only of consideration and analytic method)
11. Reciprocal crosses These involve two crosses that consider similar characters but with reversed sexes. Suppose in first cross A is male and B is female then its reciprocal cross would include A as female and B as male. 12. F1 and F2 generations . When a fresh cross is made, the progeny so obtained is called F1 (first filial) generation. The selfing of F1 (cross between the male and female members of the progeny) again gives rise to next generation. This generation is called F2 (second filial) generation . Similarly, subsequent generations are called F3, F4, ...Fn respectively. 13. Back cross .When a cross is made between a parent and its progeny, then it is called a back cross, i.e. a back cross involves the cross of F1 offspring with either of the two parents. 14. Test cross A test cross is a back cross, which involves the cross between F1 offspring with homozygous recessive (double recessive) parent The test cross is used to determine the homozygous or heterozygous nature of the F1 offspring.
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