Data from trihybrid crosses can also yield information about map distance and gene order The following experiment outlines a common strategy for using.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Data from trihybrid crosses can also yield information about map distance and gene order
The following experiment outlines a common strategy for using trihybrid crosses to map genes In this example, we will consider fruit flies that differ in
Analysis of the F2 generation flies will allow us to map the three genes The three genes exist as two alleles each Therefore, there are 23 = 8 possible combinations of
offspring If the genes assorted independently, all eight combinations
would occur in equal proportions It is obvious that they are far from equal
In the offspring of crosses involving linked genes, Parental phenotypes occur most frequently Double crossover phenotypes occur least frequently Single crossover phenotypes occur with “intermediate”
The combination of traits in the double crossover tells us which gene is in the middle A double crossover separates the gene in the middle from
the other two genes at either end
In the double crossover categories, the recessive purple eye color is separated from the other two recessive alleles Thus, the gene for eye color lies between the genes for
Alternatively, the distance between b and vg can be obtained by simply adding the map distances between b and pr, and between pr and vg Map distance = 6.1 + 12.3 = 18.4 mu
Note that in the first method (grouping in pairs), the distance between b and vg was found to be 17.8 mu. This slightly lower value was a small underestimate
because the first method does not consider the double crossovers in the calculation
The product rule allows us to predict the likelihood of a double crossover from the individual probabilities of each single crossover
Therefore, we would expect seven or eight offspring to be produced as a result of a double crossover
However, the observed number was only three! Two with gray bodies, purple eyes, and normal sings One with black body, red eyes, and vestigial wings
This lower-than-expected value is due to a common genetic phenomenon, termed positive interference The first crossover decreases the probability that a second crossover will occur nearby