Test Cross: Uncover Genotypes – Beyond Gregor Mendel

by Georgina Cornwall, PhD

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    00:00 So here I merely have a question, we are moving on to a different topic, but if you have a purple flower and you are just looking at it, how do you know if it is heterozygous or homozygous? There is a great way to test it out. It is called the test cross. We are asking a question with the test cross, we are testing for the answer. We have our purple flower. We know it could be PP or it could be Pp, but before we cross it and get a whole lot of progeny and count all the numbers, we probably should know that it is true breeding or not true breeding. So how do we tell? We are going to ask the question is this a big PP plant? In order to find out we can cross it with a homozygous recessive white flower plant and look at what the offspring are. Take a moment to think if you crossed with a homozygous white plant, you could only get the recessive allele and if you paired that with a homozygous dominant plant, what might the outcomes be? Let us take a look. Here you can see with just a recessive allele and a dominant set of alleles, you would only have purple progeny.

    01:23 But what if we had a P and a p, a heterozygous parent flower? If we have the heterozygous parent flower, then we could have a P or a p crossed with only a p because we test cross with a white homozygous recessive flower. We will look at the results. You'll noticed that we end up with 50-50. Fifty percent of the offspring would be purple and 50 percent would be white. Again that is because the white allele has maintained its integrity and is going to show up in the offspring. Again a test cross alllows us to test for heterozygosity or homozygosity in a dominant phenotype organism or plant. Here in this case, the purple flowers.

    About the Lecture

    The lecture Test Cross: Uncover Genotypes – Beyond Gregor Mendel by Georgina Cornwall, PhD is from the course Understanding Genetics.

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    Author of lecture Test Cross: Uncover Genotypes – Beyond Gregor Mendel

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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