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4.24.2007

# 4.24.2007 - 4.24.2007 Mapping Techniques Determining the...

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4.24.2007 Mapping Techniques Determining the genetic distance between linked genes: theory Maternal is gray, paternal is black Genes A and E: (possible gametes) AE = Ae = aE = ae = 25% o Would be in equal possibility, each ¼ if they are independent and unlinked (on separate chromsomes) When maternal and paternal homologs separate in meiosis, then half should get a dominant A and half should have a. This is also true when the smaller pair of homologs separate, half get E, and half get e. If half are A, half are E and A and E are independent then the chances of getting the combination of A and E is ½ x ½ = 1/4. Genes A and D: Ad = aD = ad (possible through crossing over) = AD (possible through crossing over) = 25% o Independent because far enough apart on the same chromosome.What are the relative proportions in these gametes? Does it depend on the rate of crossing over? They are still just as independent as A and E were but for a different reason. In this case it is because of crossing over but the numbers are still exactly the same. If they are independent, they are independent. ½ get d, ½ get D. ½ get A, ½ get a. You cannot tell from the inheritance the difference between genes that lie on different chromosomes or genes far enough apart on the same chromosome that they are also independent. Genes A and B: AB = ab = 40% Ab = aB = 10% o These are linked. The italicized genotypes are not going to happen often because crossing over must happen over a very small region, not the whole length of the chromosome like for genes A and E, or genes A and D. Parental types should be more common than crossover types. If there is no crossover then two parental types will occur, AB, and ab. You should have equal of each type. If there is a crossover, you can get Ab, and aB, they separate and you end up with these in equal numbers. So you can still set the two parental types equal to each other, and the two crossover types equal to each other but the parentals outnumber the crossovers. You have to make it less than 50% for the parentals but more than 25% for these reasons. There is a skew towards the parental genotypes from the crossover genotypes. We say the crossovers happen 10% because that is what is left. Parental are the genotypes that happen when no crossover happens Crossovers are genotypes that happen only if crossing over happens Genes A and C: Ac = aC = 30% (parentals) (crossovers)AC = ac = 20% o These are linked as well. The region is bigger, distance between gene A and Gene C is larger than distance between Gene A and Gene B, so the probability that the crossover will happen between the region between gene A and gene C is larger than the probability that a crossover will happen between gene A and Gene B. If the probability of a crossover is greater, crossovers happen a little more often but not enough to make them equal to the parentals. If genes are independent you can look at data from the offspring to see if they are independent but it doesn’t tell you if they

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4.24.2007 - 4.24.2007 Mapping Techniques Determining the...

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