ShmoopTube

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Here’s your shmoop du jour, brought to you by Gregor Mendel…

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… the father of genetics… [A picture of Gregor Mendel and a DNA helix]

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…and noted pea lover.

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A 1 to 2 to 1 phenotypic ratio is the hallmark of this type of cross.

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And here are the potential answers...

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Let’s use a Punnett Square to figure it out. [A punnett square]

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Honestly, we use a Punnett Square to figure everything out.

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Although…they’re really not the best for advanced calculus…[Boy attempting to use a punnett square to solve a calculus problem]

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Okay, so…keep in mind that incomplete dominance means that a phenotype with a mixed pairing

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of genes will display neither the dominant nor recessive trait, but its own unique trait. [Mendel and Punnett stood together discussing the punnett square]

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All right…how’s A lookin’?

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“Monohybrid cross with simple dominance”.

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Well…that cross yields a 3 to 1 ratio.

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In other words, any phenotype containing a dominant “A” gene will display that trait. [Mendel crossing out Punnett and replacing it with Mendel]

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Which is what makes it “simple dominance,” and that’s not what we’re looking for.

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Option C…a Dihybrid cross…examines the phenotypic outcomes of 2 traits, not 1, which

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is what a monohybrid cross does.

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This yields 16 combinations, not 4…the sum of 1 to 2 to 1.

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Way too many combos for our liking, so that’s not the answer.

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Buh-bye, C.

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How about D, a test cross? [A punnett square with question marks]

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A test cross can be used to determine the genotype of a plant or animal. [baby deers in a field]

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Think of it as working backwards to see what an organism’s Punnett Square would look

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like.

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Very useful…and fun, if you’re bored on a train…but it’s not the kind of cross

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we’re after.

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The correct answer is B, a monohybrid cross with incomplete dominance. [A monohybrid cross punnett square]

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Think about the example of human hair. [A woman brushing her hair]

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In humans, "S" is for straight hair, while "C" is for curly hair. [A woman with curly hair and a woman with straight her in a hairdressers store]

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However, a mixed "SC" will have wavy hair.

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Anyway, this is what makes it an example of incomplete dominance, as opposed to simple

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dominance…

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So, this type of cross yields a 1 to 2 to 1 phenotypic ratio.

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Two out of four possible offspring of a monohybrid cross with incomplete dominance will have [B answer circled in green]

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a mixed phenotype.

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Let’s give it up for Punnett Squares.

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Have you ever had more fun in your life? [Punnett telling Mendel to forget about the naming of Punnett Squares]

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