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Why is the penotypic and genotypic ratios of a monohybrid cross between heterozygotes 9331?

Tara Andrews

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Roger Moore on January 28, 2020

The proportion of phenotypic expected in a monohybrid cross betweenheterozygotes is 3:1 (assuming complete dominance), with thegenotypic the ratio 1:2:1. So, using tall = T, short = t and R = red, r = white as an example. A monohybrid cross of Tt X Tt is expected to produce 3 tallplants and 1 short plant (i.e. the proportion of phenotypic 3:1), that would be 1TT, 2 Tt and 1 tt (ratio genotypic 1:2:1). A dihybrid cross of heterozygotes are expected to produce aphenotypic ratio of 9:3:3:1. So the cross of TtRr X TtRr would be epected: 9 tall red, 3 tall white, 3 red and 1 white short (phenotypicratio) This is because each parent has 4 possible combinations of gametes(TR, Tr, tR and tr). Therefore, there are 16 combinations ofgametes, providing a 9:3:3:1 phenotypic ratio. Both are probably the best way to visualize the use of a punnett square(see link below).


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