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2. Multiple Gene Inheritance.pdf

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McGill University
Biology (Sci)
BIOL 202
Tamara Western

Multiple Gene Inheritance: This brings us to Mendel’s second law; independent assortment, which says that, is two characteristics follow independent assortment, then they can be treated as two separate crosses  This is useful in genetic assembly of specialized crops which have been “mixed and matched” in a genetic sense to have specific resistances to certain diseases, even drought o Ancient crops have genes for drought tolerance while some specialized crops are diseases resistant  Through specific crosses, it is possible to create modern crops which contain both of these phenotypes  These characteristics don’t have to, and often don’t lie on the same chromosome o They can be on completely separate parts of the genome Going back to Mendel’s original pea plant example, we can look at two difference characters, in this case color and shape, and see whether or not the sort independently  Two parental pea plants are used: one being round and green, RR.yy and the other being wrinkled and yellow, rr.YY o A dot is used between alleles if we are not sure whether or not they lie on the same chromosome o If they lay on different chromosomes, we use a color (RR:YY)  The F1 generation with independent assortment should be all R/r.Y.y (complete heterozygocity)  These F1s are then crossed to produce the results seen to the right o If they sort independently, which they do, they should sort into a 9:3:3:1. This is known as a dihybrid cross (two different loci involved)  The procedure can be illustrated with a Punnet square, though more complex than the ones we have looked at before o This Punnet square has 16 squares, and summing all the identical squares up produces the final ratio This phenomenon is not limited to two different loci; a trihybrid cross involves 3, tetra 4, etc. However, Punnet squares become unreasonable after a dihybrid cross, so we begin using branch diagrams or even probability calculations for 4 or more  The following branch diagram shows the results of a trihybrid cross involving parental types Dd/Gg/Ww X Dd/Gg/Ww o Since these characteristics segregate independently, they can each be treated as separate event o So, applying each “filter”, or trait, one at a time, we can start with the height, then apply segregation of color and texture  A test cross involves the crossing of a plant of usually unknown phenotype with a completely recessive plant o Again, a branch diagram will be used for a trihybrid testcross, but this time we expect EQUAL proportions of all phenotypes o This can be used as a test for heterozygocity or independent assortment, depending on what we need To understand the chromosomal basis of independent assortment, it is necessary to understand what happens at each stage of meiosis  Meiosis takes place within a cell called a meicoyte, which will eventually produce 4 haploid gametes (they meicoyte itself is diploid during interphase)  At interphase, the chromosomes are all unpaired; alleles are located at various loci (locus in singular) all over the genome  Prophase is the time when chromosomes and centromeres replicate, but the centromeres do not split o During late prophase, the homologs synapse (centromeres join together) o It is during synapsing that recombination could occur; crossing over between sister chromatids is the key for evolution and creating genetic variation  During anaphase, the centromeres attach to the spindle and are pulled to the poles (periphery) of the cell o From prophase to anaphase, there are two equally probably event which could occur; looking at the example, the A’s and B’s could be aligned, or the A’s and b’s can be aligned, producing two different outcomes  In telophase, the membrane separates and two separate cells are produced o Depending on the pathway followed, different cells are produced  Next, a second anaphase takes place in which the new spindles form, and the centromeres finally divide  At the end of meiosis II, 4 gametes are produced, each being haploid o Depending on the pathway, these gametes with either be AB and ab, or Ab and aB Independent assortment produces 50% recombinants; thus the progeny will be roughly in equal proportions of “parental type” and recombinants,  The parents are A/A:B/B and a/a:b/b, so the parental gametes would be AB or ab and the recombinants would be Ab or aB  As can be seen in the diagram, the F1 generation of the parental cross would produce a meicoyte of type A/a:B/b (heterozygote, like we’ve seen before)  Upon a testcross of the F1 against a recessive meicoyte, it can be seen that equal proportions of AB, ab, Ab and aB gametes would be produced, which shows independent assortment o Recombinants visually are simply the ones that don’t look like the parents (i.e. Ab or aB) Contrary to Mendel’s belief that genetic variance was sharp and absolute, Darwin’
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