Homozygous Recessive, Why is It Always Used for the Test Cross?

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This article is about the use of homozygous recessive individuals for test crosses. Test crosses are used to see the recombination that occured in linked traits. Linked traits are alleles that are located on the same chromosome, sometimes very close toget

Genetics-the study of genes. Many of us who know of or have taken a course on the subject know that it is a quite complicated science that is just as important now as it was when it was first discovered. One of the most complicated parts about genetics (especially for those learning it) has to do with the crosses and test crosses. Usually used with capital and lower cases, and familiar to those who know Punnett squares, crosses are a tool geneticists use to visually map where and how genes are being crossed. Such crosses range from size and type, with dihybrid to trihybrid, and all for specific reasons. One particular example, which is what I am going to explain today, is the use of a homozygous recessive as a test cross in linked recombination.

When it comes to three traits that are being crossed, the math and charts can get messy. When linked traits are involved, it gets even messier. Linked traits are known to have a certain amount of interference with each other (which can actually be mathematically measured). So how do we study these traits, and more importantly, why do we care? Linked traits are studied with what geneticists call test crosses. A test cross uses a heterozygous individual first generation, and crosses it with a homozygous recessive. Now, why would this aspect be important? Why does it matter what kind of which we cross here? When it comes to linked traits, the recombinants that happen during mitosis and meiosis become tricky to track. However, with the use of a homozygous recessive, we will immediately know which gamete was received from the heterozygous individual since we know that it is the only variant in the equation. Scientists tend to like limiting their variables, and this is the best way geneticists can figure how to do so. The homozygosity of the crossing individual showcases the recombination that happened in the heterozygous individual, but there is a catch. When it comes to double recombinants (which are rather rare), the trait may not have appeared to change, but it does not appear so because it was essentially “switched” twice. This is why scientists (and those learning genetics) tend to utilize three trait linked alleles, although the field may do more.

Now we know why homozygous recessive crosses are used. Is it important? Well, it sure makes our jobs a lot easier. Linked traits are important for us to understand, since they effect each other so much. So next time you’re spending that 20 minutes or so finishing up the test cross problem, remember that they applications are worth the work.