Crossover Activity: Understanding the random nature of crossover during meiosis.
GenScope File: SickleCell.gs
If there is a need to reconstruct this file, the blood types are as follows: Jill carries the trait B+/sc, m/M. Jack carries sc.B+, M/m.
It is possible for Jack and Jill to have a baby with two normal hemoglobin genes (the B+ allele) and two normal methemoglobin genes (the M allele), and you're going to do it for them! This puzzle requires a brand new process, called "crossover" in which genes "jump" from one homologous chromosome to the other. In this case, it's from one chromosome 11 to the other chromosome 11.
1. Open up the Sickle Cell file and take a cell from each parent organism using the cell tool.
2. Before you run meiosis, use the magnifying glass to look at the chromosomes. In the upper right-hand corner of the window, click on the "Crossover" button and then click on "Controlled"; see example below:
3. Now run meiosis. GenScope will stop in the middle and give you directions to "click on one of the chromatids." (A chromatid is one half of a pair of identical chromosomes.) Try this and LOOK VERY CAREFULLY at what happens. What is happening?
This is a simple animation which is
helpful in explaining what actually happens randomly in the cell. We allow
students to control crossover here so that they will understand the process.
It is important to help them see that the point of crossing over is not
the allele itself, but rather the points above or below it. Also, the further
the allele is from the centromere (the point where the sister chromatids
are joined), the more likely crossover will occur. Only one gamete will
result with the desired combination of alleles unless the student has done
more than one crossover.
Again, the student will run the meiosis animation, looking at the cell in the big meiosis window with "Crossover Controlled" option on. When the animation stops in the middle of the process, the student will be told to click on a chromatid. This time, s/he should search for the chromatid that has the M allele. By clicking on one chromatid with an M and on one with an m, the student will see the exchange take place that results in a gamete containing a chromosome with a B+ and an M allele. S/he should then follow the same process for the cell from the other parent. After s/he chooses and fertilizes the proper gametes, the resulting baby should be normal for both sickle cell and methemoglobin.
There are a couple of places where problems could arise in this exercise. If the student clicks on the allele itself, nothing will happen and you will hear cries of anguish. Similarly, if s/he clicks too close to the centromere nothing will happen. It usually takes only a couple of tries before his/her accuracy improves to the point where s/he never misses. Because only one gamete will contain the altered chromatid, s/he really has to make an effort to remember in which gamete the altered chromosomes ended up -- unless s/he did multiple crossovers, then there's no telling what s/he's got!
Hint: After you set it to Auto, it will be easier to see what is happening if you run the entire animation. To run the entire animation, click on the left hand side of the meiosis scroll bar in the meiosis window.
This is an example of backward reasoning.
Here, the student starts with a cell which has the alleles arranged on
the chromosomes in the "incorrect" sequence. After running meiosis, one
of the gametes shows up with the required combination of alleles on the
same chromosome. The student, in order to explain what has happened, has
to start at the end with what s/he wants, go back to meiosis and run the
animation. S/he must recognize that crossover has occurred when the chromatids
appear striped in the two colors. S/he must then recognize that usually
only one gamete will have the required altered chromosome because only
one allele was exchanged between two chromatids. Finally, s/he must understand
that in order to obtain an individual who is normal for both hemoglobin
and methemoglobin, the alleles on the chromosomes must be B+ and M. This
requires cruising through the organism, cell, chromosome and gene levels
and back again.
In this exercise the student has the
opportunity to observe the random nature of crossover, especially by running
the pedigree more than once. The cumulative effect of watching crossover
occur during meiosis controlling crossover, then observing the effect of
random crossovers in a pedigree should help the student understand the
basic facts about crossover.