Discussion Assignment #5

Betwixt and Between

This assignment explores some kinds of puzzles related to linkage mapping.

Linkage mapping is based upon the (demonstrated) premise that if two genes are located on the same chromosome, the further apart they are the greater the amount of crossing over that will occur between them. In other words, the further apart they are, the more they will behave like they are independent of each other.

One linkage map unit (LMU or just MU) is equal to 1% recombination. Unlinked genes recombine at a 50% rate (based on chromosome behavior in meiosis), so 50 map units is the equivalent of unlinked.

Okay, on to the problem. Remember that linkage map units are an approximation of distance, and that they are not strictly additive.

For these problems, we need some data. Here are the results of some three-point crosses of a bunch of genes which are all linked together on one chromosome. The organism in question is a Jovian Glider (Ah, I've been many places in this universe... ;^) First, here are the genes we're looking at:

Antennae: Yes or No
Tail: Straight or Curled
Number of Toes: 4 or 5
Eye Shape: Round or Almond
Tooth Shape: Pointy or Blunt
Fur Color: Red or Yellow
Nose: Wet or Dry
Personality: Happy or Glum

Interesting little creatures, these Gliders.

A three point cross enables you to figure out the order of three genes at a time. So here are your results, including the map distances calculated between the pairs of genes. You'll learn how to do this part in class; here, I've done it for you.

First cross: Fur, personality and teeth. Your results tell you that the middle of these three genes is the fur gene. Your order and distances (note that you have no way yet to determine "right" vs "left." These things are relative until you start putting the genes all together, and then you just have to figure out relative positions; there is no absolute right or left):

Personality--[20 MU]--Fur--[5 MU]--Teeth

Second cross: Antennae, toes and fur. Again, your results tell you that fur is between the other two genes. Your order and distances:

Toes--[2 MU]--Fur--[10 MU]--Antennae

Third cross: Teeth, antennae and tail. The gene for antennae is in the middle. Order and distances:

Teeth--[15 MU]--Antennae--[15 MU]--Tail

Fourth cross: Eyes, noses and antennae. Antennae are also in the middle of this grouping of genes. Order and distances:

Noses--[17 MU]--Antennae--[4 MU]--Eyes

In addition, you've also determined that the genes at the ends of this string are the ones for the teeth and the noses. These two genes are approximately 30 MU apart.

First task: What is the complete gene order of these eight genes?

Figure out and fill in the map distances between each pair of genes in your chromosome. (In other words, diagram the chromosome, with all eight genes in position, and indicate the number of MU from each gene to the next one in the chromosome).

In your first cross above, you figured out that the distance between the Fur and Personality genes is 20 linkage map units. This means that you calculated 20% recombination between these two loci. Suppose that you did one more three point cross using an additional gene (not included in the eight that you just mapped)--say, a gene for round vs flat tummies. Your data indicate that this new gene is between the Fur and Personality genes, with the following distances:

Fur--[12 MU]--Tummies--[11 MU]--Personality

Thus, in this mating you've detected 23% recombination between the Fur and Personality genes. Yet you'd previously calculated only 20% recombination. Explain how both of these results can be correct.

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