Question

A true breeding tall plant is crossed with true breeding dwarf plant.F1 progeny is 100% tall and F2 has tall:dwarf ratio 3:1 i)Explain why F1 shows only one type of parental phenotype ii) name the patterns in which the ratio deviates from above.Also mention the deviated phenotypic ratio

Answer 1

Law of Dominance::-As per this law,the Gene causing tallness T masks the expression of Gene causing dwarfness t.The T is dominant on t.So the Genotypes for which plants will be tall are::-(1) TT pure tall (2) Tt heterozygous For dwarf plants the Genotype can only be 'tt' Result of Cross of pure tall to dwarf(always pure) ::-F1 TT x Tt :- all plants will be of Genotype Tt(heterozygous) Results of F2:- TtxTt we will get TT:Tt:tt--in ratio of1:2:1(Genotypic ratio) But as TT and Tt will by tall. So Phenotypic ratio of F2 is 3:1 for tall and dwarf plants. The Genotypic ratio is different from Phenotypic ratio.Dear,this is all because of Law of Dominance and Law of Segregation.Such Ratio explained above can never deviate as no mutation has yet been noticed by Geneticists as far as tallness and dwarfness character of pea plants is concerned. Hope it helps.