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Unformatted text preview: ng that the recessive trait reappears, but there are 3 tall and 1 short. Leading him to come up with a hypothesis...(next slide) • Mendel coined the terms dominant and recessive to describe this phenomenon. -Which is dominant and which is recessive? The dominant characteristic is provided by the phenotype of the F1 individuals. Monohybrid Crosses • Followed all crosses to the F2 generation and quantified progeny: Results of All Mendel’s Crosses in Which Parents Differed in One Character • In all cases, one parental phenotype disappeared in the F1 and reappeared in one-fourth of the F2 Mendel Set- Up Experiments to Answer… • Why do hybrids revert to parental phenotypes? • Can one predict the frequency of specific phenotypes in each generation? • Can these model studies provide foundation for explaining all heredity? Monohybrid Crosses…3:1 Ratios How could one explain the 3:1 ratios observed in monohybrid crosses? • Lets cross 2 pure-breeding pea plants: Yellow seeds X Green seeds: P yellow X green F1 all yellow (dominant) (self) 3/4 yellow (519) 1/4 green (170) 689 total • Mendel further tested the members of the F2 generation. F2 Monohybrid Crosses…T esting the F2’s F2 3/4 yellow (519) 1/4 green (170) • Green F2 plants were grown and selfed → All green F3 peas -What does this tell us about the F2 green pea plants? • Next Mendel took a sample of 519 yellow F2 peas and grew plants from he do that was different from other scientists? 3:1 ratio is a phenotypic ratio, meaning that you observe (in this case) 3 them. what did of one of them is however saw something different: when he observes 3:1 ratio, them are yellow, andhe self-crossed green. Mendel,he saw that some were pure breeding and impure breeding. the yellow, they are not the same, F2 generation -These 519 yellow-pea F2 plants were selfed individually to produce F3: a) 166 of the yellow F2 plants bore only yellow peas (Pure breeding) b) 353 of the yellow F2 plants bore a mixture of yellow and green peas in the F3, again in a 3:1 ratio. (Similar to the original F1 generation) the ratio is actually 1:2:1; he saw 1/3 of them were pure-breeding and the 2/3 were impure T esting the F2’s: Summary of Results • Of the F2 yellow peas: 1/3 (166/519) were like the pure breeding yellow parent 2/3 (353/519) were like the F1 yellow peas…producing yellow and green seeds again in a 3:1 ratio • All F2 green peas appeared pure-breeding (like green parental line) -Therefore, the underlying 3:1 phenotypic ratio in the F2 generation was a more fundamental 1:2:1 ratio: 3/4 yellow 1/4 pure-breeding yellow 2/4 “impure” yellow F2 1/4 green 1/4 pure-breeding green What we learned from Mendel’s monohybrid crosses • Mendel’s Model: 1) The existence of genes: Traits are determined by “discrete heritable factors.” (Genes) 2) Genes are in pairs: In the adult pea plant, each type...
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