12 What is linkage and how do we de ne linkage How does this relate to the behavior of chromosomes during meiosis Linkage refers to how linked two di erent traits on the same chromosome are Since they re on the same chromosome they can t assort independently of one another We analyze them with test crosses If two traits are completely linked their loci are very close together They cannot assort independently so their o spring 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 they have the parental gametes and recombinant gametes but not in equal proportions If two traits are unlinked they are on entirely di erent chromosomes Their o spring can have both the parental gametes and recombinant gametes in equal proportion In dihybrid crosses there are four possible phenotypes Parental gametes refer to the phenotypic combinations of the parents Recombinant gametes refer to the other two phenotypic combinations 13 How can two genes on the same chromosome ever lead to recombinant gametes How can this 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 o spring can occur We measure this by taking the proportion of recombinant least likely o spring to parental most likely o spring then multiplying by 100 to get the recombinant o spring percent Each percent corresponds to one centimorgan of distance 14 Can genes on the same chromosome ever assort independently How can this occur and what does it say about events during meiosis Normally if they are far enough a single crossover leads to recombinant o spring Two crossovers reverts things almost back to normal leading to parental o spring If you repeat this enough times it results in relatively equal proportions of all four possible phenotypes which resembles independent assortment 15 What are genetic markers in humans What are anonymous markers and why are they useful A genetic marker is anything in the genetic code that marks a physical change in the organism 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 a condition it can be used to identify one regardless If the F0 parents are aa b b and a a bb we ll have F1 o spring that are a a b b If we cross that with an organism that is homozygous recessive for both aa bb we should get F2 o spring that are If the F0 parents are aa bb and a a b b we ll have F1 o spring that are a a b b If we cross that with an organism that is homozygous recessive for both aa bb we should get F2 o spring that are Majority Majority Minority Minority a a b b aa bb a a bb aa b b Minority Minority Majority Majority a a b b aa bb a a bb aa b b