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Mendel's Principles and Meiosis – Classic Mendelian Genetics

by Georgina Cornwall, PhD
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    00:01 Mendel's mathematics in terms of gametes. Gamete formation involves segregation of homologous chromosomes and we know that. So we pull the chromosomes apart and put one from each parent into each gamete and Mendel just to keep the language straight, you have to remember that he didn't know about meiosis or chromosomes. He called these factors. So he proposed that there were two transmissible factors and when we look at meiosis, the idea of Punnett square really lines up quite well where we have the formation of gametes, meiosis 1, we separate the homologous chromosomes into separate gametes and meiosis 2 simply the sister chromatids separate. So keeping it in mind Mendel didn't know about meiosis, we can say that the factors and we do, we can say that the factors are chromosomes or alleles on chromosomes, genes on chromosomes and it all matches up quite nicely. Mendel gained a number of different insights from his work with monohybrid crosses that is looking at just one trait. First of all, he assessed that an individual must be diploid as it must carry two alleles for the same trait and that gametes probably only carry one of those traits. He also said that there are various forms of a trait or various alleles for the trait. In this case, solid or stripe. There could be more than two alleles, but in his traits, he was looking at, he just had two different alleles and that is what made understanding things quite a bit much more simple, then had he picked multiple alleles. We will look at those later. He also came up with the idea of homozygous versus heterozygous. That is an individual could carry two of the same alleles, homozygous or heterozygous, he could carry variant alleles. One dominant and one recessive allele for example. He also gained the insight of dominant versus recessive such that one trait, the dominant trait would be the one that you saw, for example, purple in the purple flowers versus the recessive trait, which is the one that was not seen in the F1 generation. Finally, Mendel suggested that these alleles maintain themselves discretely in the formation of gametes, which is precisely what led him to the principle of segregation. The principle of segregations states that alleles segregate from each other during gamete formation. I‘ve kind of translated it a little bit into the language of chromosomes and meiosis, but if you recall in our discussion of meiosis, we have replicated homologous chromosomes that happen during S phase and in meiosis, they pair up during meiosis I on the metaphase plate and they segregate from each other during meiosis I.

    03:19 During meiosis II, simply the sister chromatids separate from each other. But this piece is the segregation that is explained by meiosis in terms of Mendel's factors. So alternate alleles for a particular character segregate from each other during gamete formation really means that homologous chromosomes segregate from each other during meiosis. Now humans also


    About the Lecture

    The lecture Mendel's Principles and Meiosis – Classic Mendelian Genetics by Georgina Cornwall, PhD is from the course Understanding Genetics.


    Included Quiz Questions

    1. …the presence of two transmissible factors/ alleles for each gene in a diploid organism.
    2. …the presence of one transmissible factor or allele for each gene in a haploid organism.
    3. …the presence of three transmissible factor for each gene in a haploid organism.
    4. …the presence of four transmissible factor for each gene in a haploid organism.
    5. …the presence of infinite transmissible factor for each gene in a haploid organism.
    1. Mendelian factors ----- Centromeres of non-homologous chromosomes
    2. Diploid organism ----- Two sets of each chromosome
    3. Homozygous ----- Purebred
    4. Gregor Mendel ----- Laws of Mendelian inheritance
    5. Heterozygous ----- Different alleles at the same locus on the homologous chromosomes

    Author of lecture Mendel's Principles and Meiosis – Classic Mendelian Genetics

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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