Principles of Biology : Infering
Gene Order on a Chromosome from a Three Point Cross
Three point mapping is a technique used in genetics to map genes on
chromosomes. Thomas Hunt Morgan received the Nobel Prize in 1933 for
his work on mapping Drosophila chromosomes. There are a few basic rules
that apply to three point crosses. We are dealing with 3 genes on a
single chromosome. Let's call them A, B anc C. Now imagine a cross
between a triple heterozygote (AaBbCc) and a triple homozygote
(aabbcc). We would expect 1/2 of the offspring to be AaBbCc and 1/2 to
be aabbcc. This is almost what we get, but because of crossing over,
not only do we see the parental phenotypes, we see other combinations.
Here are the rules:
Rule #1 : The most frequent genotypes are the parental genotypes (and
phenotypes).
Rule #2 : The least frequent genotypes (and phenotypes) are those that
result from two crossover events on the same chromosome (double
recombinant type).
Rule #3: The effect of double crossovers is to interchange the member
of the middle pair of alleles between the chromosomes.
Rule #4: Reciprocal crosses result in approximately equal numbers.
Because crossing over results in two "different" chromosomes, the two
genotype (and phenotype) classes are approximately equally
represented.
Consider the Prophase I of the parent with the triply heterozygous
genotype (AaBbCc). The pairing before crossover of the duplicated
chromosomes (identical chromatids being connected with a centromere)
might look like this:
Now, image a cross over occuring between unlike chromatids, as follows:
The result would be ...
Now, consider the gametes that would result from this cross over event.
Here we have numbered the chromatids that will go into different
gametes.
Gamete #1 - This is a parental gamete type (no crossover, all three
dominant allele occuring)
Gamete #2 - This is a recombinant gamete type (with two dominant
alleles and one recessive allele)
Gamete #3 - This is a recombinant gamete type (with two recessive and
one dominant allele).
Gamete #4 - This is a parental gamete type (no crossover, all three
recessive alleles occuring)
As these gametes form a zygote with a triply recessive gamete, their
phenotypes reflect their genotype and the number of recombinant events
can be counted.
Imagine these same images with crossing over between A and B. More
recombinant types,.
Imagine these diagrams with a double crossover -- one between A and B
and one between B and C.
This is the class of recombinant types that will be the rarest (Rule
#2) and will indicate which gene is in the middle. The gene that is
"out of place" in the least frequent phenotype category is situated
between the other two genes.
This is Nobel Prize stuff! Can you figure it out? Try Homework 7.
November 15,
10:30 a.m.