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Genetics The study of inherited characteristics Monohybrid inheritance Let the allele for round seeds be: Let the allele for wrinkled seeds be: Parents phenotype genotype Gametes R (dominant allele) r (recessive allele) round seeds RR R x wrinkled seeds rr R r r F1 generation gametes gametes R R r Rr Rr r Rr Rr F1 phenotypes 100% plants producing round seeds F1 genotypes 100% heterozygotes Rr • F1 intercross • Parents phenotype genotype • Gametes • F2 generation round seeds x round seeds Rr Rr R r R r gametes gametes R r R RR Rr r Rr rr Phenotype 75% plants producing round seeds 25% plants producing wrinkled seeds Genotype 25% RR 50% Rr 25% rr Ratio 3:1 Round seeds: wrinkled seeds Does the observed ratio match the theoretical ratio? • The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled • Mendel’s observed ratio was 2.96:1 • The discrepancy is due to statistical error • The observed ratio is very rarely the same as the expected ratio • The larger the sample the more nearly the results approximate to the theoretical ratio • Statistical tests such as the chi-squared test can be used to test the closeness of fit of the observed and theoretical ratios What if the parents are not homozygous? • The use of the term F1 generation is limited to the offspring of two homozygous parents • The use of the term F2 is limited to the offspring of intercrossing the F1 generation • In all other cases the terms offspring (1) and offspring (2) should be used • The complete set of headings will be: Parents phenotypes Parents genotypes Gametes Offspring (1) genotypes Offspring (1) phenotypes Gametes Offspring (2) phenotypes Offspring (2) genotypes Backcross To test whether a plant producing round seeds is homozygous RR or heterozygous Rr it can be crossed with a homozygous rr plant If plant is homozygous dominant RR If plant is heterozygous Rr Parents phenotype round x wrinkled genotype RR rr gametes R R r Parents phenotype genotype gametes r Offspring round x wrinkled Rr rr R r r r Offspring gametes gametes R R r Rr Rr r Rr Rr Offspring gametes gametes R r r Rr rr r Rr rr Offspring phenotype 100% round phenotype 50% round 50% wrinkled Genotype 100% Rr genotype 50% Rr 50% rr This powerpoint was kindly donated to www.worldofteaching.com http://www.worldofteaching.com is home to over a thousand powerpoints submitted by teachers. This is a completely free site and requires no registration. Please visit and I hope it will help in your teaching.
