Monohybrid Crosses – Classic Mendelian Genetics

by Georgina Cornwall, PhD

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    00:01 So let us introduce a couple of terms before we move on. I have mentioned phenotype already.

    00:07 The phenotype is what the individual looks like whereas genotype is what the individual’s genes say. Now you have to keep it mind that Mendel had no idea about meiosis or chromosomes.

    00:22 It is pretty astounding that he came up with these principles when he had no idea what was really happening in gamete formation. So in the F2 phenotypic ratio, he would always see that there were three of one form and one of the other forms in this case purple versus white. Mendel proposed that there were three different genotypes although he didn't call them genotypes because he didn't know about genes, but in this case, there would be one that was true breeding and two that were not breeding and then one that was carrying the recessive or hidden trait. Mendel came to these conclusions about the two factors and things moving separately during gamete formation because when he took his F2 progeny or F2 offspring and cross them, he found that when he crossed the true breeding plants and self-fertilized them, they would produce all true breeding plants. When he crossed some of them with themselves, self-fertilized he would find that the white would reemerge and when he crossed just the recessive phenotype, the one that reappeared in the F2 generation, he would find only white flowers. So again this is how he came to the conclusions that things must have been maintaining their integrity and something was happening during the formation of the pollen and egg. Of course keep in mind, he has no idea about meiosis and the separation of chromosomes at this point. So there was a French guy then named Punnett that came along and found a really nice way to show Mendel's mathematical predictions using his Punnett square. We often use the Punnett square because it makes it easy to predict the outcomes of crosses. Here we are assuming that there are two possible gametes from each parent again based on Mendel's ideas. Let us say we were looking at the purple flowers again. The male parent may have in the heterozygote may have one big P, the dominant allele, and one p. He can only make gametes of those sorts. The heterozygous female contribution would be the same. She could only make and ovule with either a P, the dominant form of purple or p, the recessive form of purple. And in Punnett square, you put the two gametes together in the top corner and we see that we have a P and a P and then we will put the gametes in the next part of the grid P and a p, and the next part a P and a p, and a p and p in the final corner. So this is a nice way to predict the phenotypic as well as the genotypic outcomes of monohybrid crosses.

    03:34 We will also see them when we move on into dihybrid crosses looking at two different characters but for now, let us look at this and see that we have two possible phenotypes.

    03:44 We have got purple and we have got white. But the genotypic ratio is 1:2:1, 1 homozygous, 2 heterozygous and 1 homozygous individual. When we look at these, we then see the phenotypic ratio of 3 purple to 1 white and so Punnett means of predicting the outcomes really exhibited Mendel's mathematics in terms of gametes.

    About the Lecture

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

    Included Quiz Questions

    1. 3 dominant:1 recessive
    2. 1 dominant:2 hybrid: 1 recessive
    3. 2 dominant:2 recessive
    4. 1 dominant:3 recessive
    5. 4 hybrid phenotype
    1. 1 TT : 2 Tt : 1 tt
    2. 4 TT
    3. 4 tt
    4. 2 TT : 1 Tt : 1 tt
    5. 2 TT : 2 tt

    Author of lecture Monohybrid Crosses – Classic Mendelian Genetics

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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    By Tanya y. on 12. September 2020 for Monohybrid Crosses – Classic Mendelian Genetics

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