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Recombination of Chromosomes – Gene Mapping

by Georgina Cornwall, PhD
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    00:00 So now that we know that Mendel's factors are on chromosomes and we know about chromosomal theory, let us start to explore how we understand where things are on those chromosomes.

    00:14 In this lecture, we will be exploring genetic mapping. By the end of the lecture, you should be able to describe the nature of linkage groups as well as differentiate between linked and unlinked genes. In addition, you will be able to calculate distances by frequency of recombination. First it is really important for us to recall what recombination is if you remember meiosis I. We had crossing over during prophase or synapsis. To review that process, we have a set of alleles here, we will look at A and B.

    00:53 The F1 generation ends up being a heterozygote as we have seen previously. If we look at meiosis in our F1 cross produced the F2 gametes, we see if there is no crossing over then all we end up with is the parental gametes. However, if there is a crossing over event during prophase I.

    01:20 After meiosis II, we end up with some recombinant gametes. So parenteral gametes looks just like the parental types and those there had a crossing over event or synapses, will be the recombinant chromosomes. Those will go on to form the next generation. So really important to understand what recombinant chromosomes are so that we can get into gene mapping.

    01:47 Now each chromosome is a linkage group. All the genes that are on one chromosome are linked together in that group. If we have recombination, then we will see they become unlinked and so we can use those linkage groups to establish mapped distances. And I will show you how shortly. First of all, we know crossing over and synapses allow genetic recombination and we have to establish first that this involves a physical exchange of information between chromosomes as well as a phenotypic recombinations. Before we start to being able to map, we had to take chromosomes and say "Yes, they actually do swap pieces", and when they swap pieces, we see this recombinant phenotype. We have to put those two things together first. In this experiment, we have a parental type in which one of the chromosomes of the homologous pair over on the side, we can see has some markers on it. So it is a physically different looking chromosome although it is truely the homologue to the one on the left. It has this green knobby area on one end and on the other end, a yellow sort of extension on that tail of the chromosome. And so physically if these chromosomes experience a crossing over event, then we can see the difference under visualization techniques or maybe even under the microscope. What happens here? The crossing over occurs. We end up with a varied outcome in the gametes and when those gametes are crossed, we can see some recombination in phenotypes also. Here is our friend, the test cross. When we do a test cross between recombinant gametes and the original recessive parental gamete, then we end up with some recombinant phenotypes. In addition to being able to visualize the chromosomes have switched pieces, we can also see recombinant phenotypes in the form of colorless and waxy and colorless and starchy where the original parent phenotypes were colorless and starchy and colored and waxy. Recombination occurs on these linkage groups when there is a crossing over event, we can see it on the chromosomes and we can see evidence of it in the phenotype.


    About the Lecture

    The lecture Recombination of Chromosomes – Gene Mapping by Georgina Cornwall, PhD is from the course Understanding Genetics.


    Included Quiz Questions

    1. When the alleles are on different chromosomes.
    2. When the alleles are next to each other on the same chromosome.
    3. When the alleles are on the same chromosome, too close together for crossing over to occur.
    4. When the genes are associated with the same trait.
    1. Prophase
    2. Metaphase
    3. Anaphase
    4. Telophase
    1. The backcross between the pure dominant parent and recombinant gametes ends up with same recombinant phenotypes.
    2. Crossing over and synapsis lead to genetic recombinations.
    3. The physical exchange of genetic material due to crossing over during gamete formation leads to phenotypic recombinations in the next generation.
    4. Each chromosome acts as a linkage group which is comprised of all the genes present on it.
    5. If there is no crossing over or synapsis during gamete formation in a heterozygous individual, it leads to the formation of parental gametes.

    Author of lecture Recombination of Chromosomes – Gene Mapping

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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