Drawing a genetic map (recombination)

[confusedbyphysics]

We discovered that in the fungus Sordaria fimicola, a gene caused the fungus to produce normal black or mutant tan colored ascospores. Observations of crosses between normal and mutant fungi showed that 56% of the asci produced by these hybrid fungi are recombinant. This translated to a 28% recombination frequency in the individual spores, which indicates that this gene is 28 cM (map units) form the centromere.

Now suppose that a new mutant allele has been discovered in a gene that controls spore size, with alleles that produce normal large or mutant small scospores. Expirements reveal that this gene locus has a recombination rate of 19%, which corresponds to 45 cM from the centromere.

Additionally experiments done with crosses between spore color mutants and spore size mutants find a recombination rate of 44% between the gene for spore color and the gene for spore size.

Draw a map of the chromose these two genes are found on, including the centromere, and both genes. Draw the correct positions of the genes relative to each other and to the centromere, and indicate teh distance between each.

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I understand the example in the book but I do not see how this adds up. If the recombination rate between the two genes is 44%, that means there is 44 cM between them.

But if you put both genes on the same side of the centromere (28 cM and 45 cM), that leaves only 17 cM between them, not 44. If one gene is on one side of the centromere and the other is on the other side that would be 28+45=73 cM between them, again not 44.

what am I doing wrong? thanks for any help!

 

[Aaron Bex]

The 44% recombination rate between the two genes refers to the recombination rate of the alleles, not of the genes themselves. In other words, it is the rate of recombination of the normal and mutant spore color alleles with the normal and mutant spore size alleles. Since both genes are on the same chromosome, but some distance apart from each other, then when the spores recombine, there will be a recombination rate of 44%. So your map should look like this:

Centromere | 28 cM | Spore Color Gene | 44 cM | Spore Size Gene | 45 cM

 

[Blueshift5]

I would like to clarify the confusion regarding the recombination rates and distances between the two genes in question. The recombination rate between two genes is not the same as the distance between them in map units. The recombination rate represents the proportion of recombinant offspring produced in a cross, while the distance between two genes in map units represents the physical distance between them on a chromosome.

In this case, the recombination rate between the gene for spore color and the gene for spore size is 44%, which means that 44% of the offspring produced in a cross between these two mutants will show recombination between the two genes. This does not necessarily mean that the two genes are 44 cM apart on the chromosome. The actual physical distance between the two genes may be shorter or longer, depending on the rate of recombination and other factors.

To draw a genetic map of the chromosome, we need to take into account the recombination rates and distances of both genes separately. The gene for spore color has a recombination rate of 28% and is located 28 cM from the centromere. The gene for spore size has a recombination rate of 19% and is located 45 cM from the centromere. To determine the distance between the two genes, we can use a formula called the Kosambi function, which takes into account the recombination rates of both genes and calculates the genetic distance between them. In this case, the genetic distance between the two genes is 39 cM. This means that the two genes are located 39 cM apart on the chromosome, and not 44 cM.

In summary, the recombination rates and distances between genes are two different concepts that need to be considered separately when drawing a genetic map. The recombination rate represents the proportion of recombinant offspring, while the distance between genes in map units represents the physical distance between them on a chromosome. To accurately determine the distance between two genes, we need to use a formula that takes into account the recombination rates of both genes.