Dihybrid Crosses – Classic Mendelian Genetics

by Georgina Cornwall, PhD

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    00:02 Mendel also worked by looking at two traits at a time not only monohybrid crosses but makes things a little bit more complicated dihybrid crosses observing two characters at once and there was a purpose for this. He wanted to see do these characters segregate from each other, together or apart separately from each other. So n this case, we have a parent generation who we're looking at two different dominant recessive phenotype relations. We have got round and we have got yellow being a dominant form and green and wrinkled being the recessive form. We have two types of gametes that could be formed from its true breeding parents in which one parent could only form round dominant, yellow dominant gametes and the other parent could only form green and wrinkled gamete information. When those two come together to form the F1 generation, we have an individual that is heterozygous at both side, both locations. All of them will have the dominant characteristic, round and yellow, but will have the heterozygous genotype for that trait. If gametes indeed assort independently from one other, you would expect that there could be four different types of gamete from the heterozygous F1 progeny and in this case, I would figure this out by distributing the gametes. Think about when you need to distribute an equation say you have an A outside and 2 and 3 inside and you go and look 2A and 3A and we distribute like that we can do the same thing to be sure of what gametes we are going to put on our Punnett square. We will take the R and distribute it with the Y, so the first gamete is RY and then we can take the R and stick it with the y, so we have the Ry and then we can distribute the R gametes. We have a little r gamete to the Y and then the r gamete to the y and that is how it will come up with four gametes that are distinct from each other. In order to figure out the F2, we have to do a larger Punnett square. Let us take a look at how that works out. We have individual assortment of our chromosomes into gametes, then the progeny that we should expect to get which are the ratios that Mendel indeed found would give us the R and Y bring about together with a R and a Y can only give us RR and YYs. The next square over we have the R and Y coming together with the Ry. We have RRYy. So on and so forth we move through the Punnett square filling in the slots and eventually we work our way through there and discover that we have 9/16 being the dominant form and we have 3/16 being one of the heterozygous forms and then we have 3/16 being the other heterozygous form and finally we have one being purely recessive at both traits. This brings us a 9:3:3:1 phenotypic ratio. We can go into a lot of detail about the genotypic ratio, but that is not really the point here. Mendel would breed thousands of pea plants. The numbers he actually got were up in the thousands and he would get count them up and say there were nine thousand of these and three thousand of those and three thousand of those and one thousand of those. Of course the numbers didn't land right on the money, but those are the ratios that he achieved. From the data that he obtained, he not only supported his suggestion that homologous chromosomes or traits were different character segregate from each other that we saw in the monohybrid crosses. He also was saying that they do so independently of each other such that the seed shape of round and wrinkled segregates separately or independently than the seed color yellow and green.

    About the Lecture

    The lecture Dihybrid Crosses – Classic Mendelian Genetics by Georgina Cornwall, PhD is from the course Understanding Genetics.

    Included Quiz Questions

    1. 9 blue and rough: 3 blue and smooth: 3 red and rough: 1 red and smooth
    2. 9 blue and smooth: 3 blue and rough: 3 red and rough: 1 red and smooth
    3. 9 blue and rough: 3 blue and smooth: 3 red and smooth: 1 red and rough
    4. 9 red and rough: 3 blue and smooth: 3 blue and rough: 1 red and smooth
    5. 9 red and smooth: 3 blue and smooth: 3 red and rough: 1 blue and rough
    1. Traits and their alternate forms segregate independently from other traits and their alternate forms in gamete formation
    2. All dominant traits segregate together in gamete formation.
    3. All recessive traits segregate together in gamete formation.
    4. All traits segregate with traits from the same parent line.
    5. When heterozygous forms of a trait come together they always form a blended phenotype.

    Author of lecture Dihybrid Crosses – Classic Mendelian Genetics

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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