Three Point Test Crosses
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Three Point Test Crosses

Three Point Test Crosses are allows geneticists to analyze at the gene order of loci, distances between the loci, and their linkage relationship. This article is an explanation of how to use and analyze three point testcrosses of trihybrid heterozygous crosses and how to analyze and utilize the information about recombination.

Three point testcrosses are very important in genetic analysis. We use them to see if three genes, also called trihybrid heterozygotes, are linked and find out the distance between the genes using map distances. All this allows us to see recombinants in loci of a chromosome. The first step to do is to determine and classify the parental genotypes and the recombinant genotypes. To classify the parental strands you must look at the given alleles, and you will notice that the parental genotypes are homozygous. Thus, when you look at their gametes of both parents before you cross the three genotypes, you can take one of each of their alleles in the three genotypes that are given. After use those genotypes to test cross them and obtain the F1 generation, which is also the parent’s first progeny. To find out how many gamete genotypes can be formed by any trihybrid, on must simply know that any trihybrid there are eight possible gamete genotypes possible. Where only two of each of the three genotypes gives us a total of eight possible combinations of the genotypes.

If you were given a sample, you would be given the sample size of the progeny, and then form table to separate and organize the data. Split the table into five different columns, and eight different rows. In the columns label the gamete genotypes, the number of progeny produced of that genotype, and the possible recombinants of the genotypes analyzed two at a time. The rows consist of the eight possible gametes, and you must note that the first two rows are the parental gametes. The parental gamete genotypes are used as reference to analyze if recombination occurred in the other six-gamete genotypes. After analyzing each of the gamete genotypes and determining if recombination took place you place an R, noting recombination occurred under the specific gamete genotype and the two loci it occurred in. When you are finished you may add up each column's possible recombinants using the data. This should tell you the number of recombinants that occurred between two loci in the given gamete genotypes.

To figure out the distances between the recombinants that are defined by the parental gamete genotypes, you must determine the map units of each gene. In order to do this you need to use the number of recombinants from each column that is specified for two loci in the data table, and divide them individually by the total number of the sample progeny. You should have a total of three different values that are defined by the parental gamete genotypes, giving us the frequency of recombination. Once you determine the calculated values you convert those values into percentages and this will give you the distances between them in map units. Now that you know the map units we may draw out a linear map and use the values we calculated and determine the distances between the loci. The higher the value of the map unit the farther away the loci are from each other. Once the loci are mapped, you have determined the gene order of the genotypes of the progeny and determined their linkage relationship. If for some reason the map unit distances do not add up, there must have been a double recombination occurrence. That is when you must go back to the original table that you made and look at the recombinants for the loci with the smallest value to the given loci, which did not match up with the distances in the map. You must do this since the loci are closely related in distance, therfore being an acceptable action. After determining the new values of total recombinants and map units as before you can map the new values. Then you can finally determine the new gene order of the genotypes.

This is how three point testcrosses are done and we can see its importance in genetics, as it aids geneticists determine gene order, distances between genes, and the linkage relationship of the genes. Three point crosses give us a lot of information about the genetic data of organisms. They allow us to see points of recombinants and predict possible outcomes of the offspring.

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