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Purdue BIOL 10200 - Focus Questions #2 (Linkage)

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12. What is linkage and how do we define linkage? How does this relate to the behavior ofchromosomes during meiosis?Linkage refers to how “linked” two different traits on the same chromosome are. Since they’reon the same chromosome, they can’t assort independently of one-another. We analyze themwith test crosses.● If two traits are completely linked, their loci are very close together. They cannot assortindependently, so their offspring only have the parental gametes.● If two traits are incompletely linked, they are on the same chromosome but farther apart,so they can cross over and independently assort some of the time. In a test cross, theyhave the parental gametes and recombinant gametes, but not in equal proportions.● If two traits are unlinked, they are on entirely different chromosomes. Their offspringcan have both the parental gametes and recombinant gametes in equal proportion.In dihybrid crosses, there are four possible phenotypes. Parental gametes refer to thephenotypic combinations of the parents. Recombinant gametes refer to the other twophenotypic combinations.13. How can two genes on the same chromosome ever lead to recombinant gametes? How canthis “partial linkage” be related to genetic map distances? What is the unit of genetic maps?If two genes are on the same chromosome, recombinant gametes can occur via crossing over.The farther apart they are, the more recombinant offspring can occur. We measure this by takingthe proportion of recombinant (least likely) offspring to parental (most likely) offspring, thenmultiplying by 100 to get the recombinant offspring percent. Each percent corresponds to onecentimorgan of distance.14. Can genes on the same chromosome ever assort independently? How can this occur andwhat does it say about events during meiosis?Normally, if they are far enough, a single crossover leads to recombinant offspring. Twocrossovers reverts things almost back to normal, leading to parental offspring. If you repeat thisenough times, it results in relatively equal proportions of all four possible phenotypes, whichresembles independent assortment.15. What are genetic markers in humans? What are anonymous markers and why are theyuseful?A genetic marker is anything in the genetic code that “marks” a physical change in theorganism. Thus, we can use genetic markers to determine the roots of illnesses and conditions.An anonymous marker is a type of genetic marker that does not produce a phenotypic change,but is often correlated with a phenotypic change. So even though it is not the cause of acondition, it can be used to identify one, regardless.If the F0 parents are (aa / b+b+) and (a+a+ / bb), we’ll have F1 offspring that are (a+a / b+b). Ifwe cross that with an organism that is homozygous recessive for both (aa / bb), we should getF2 offspring that are:● Majority: a+a / b+b● Majority: aa / bb● Minority: a+a / bb● Minority: aa / b+bIf the F0 parents are (aa / bb) and (a+a+ / b+b+), we’ll have F1 offspring that are (a+a / b+b). Ifwe cross that with an organism that is homozygous recessive for both (aa / bb), we should getF2 offspring that are:● Minority: a+a / b+b● Minority: aa / bb● Majority: a+a / bb● Majority: aa /


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