Teacher Version


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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?

  When the student clicks on the chromatid, a blinking arrow appears on all the chromatids of the homologous chromosomes. The next direction tells her to click on the chromatid where she wants the exchange to occur. The two portions of the chromatids change places. Any alleles included in those portions will be exchanged also.

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.

4.       Now see if you can use this tool to create a chromosome that has a B+ allele along with an M allele. If you can do this, you can MAKE A BABY WITH TWO B+ ALLELES AND TWO M ALLELES!! When you make the baby, call your teacher over and explain how you did it.

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!

5.       Crossover happens all the time in nature, but it happens randomly. When crossover is on auto it happens randomly -just the way it does in real life. Click on the "Auto" button under Crossover to make it random. Now make a baby through meiosis and see if you can figure out what happens.

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.

When meiosis is run in the meiosis window with crossover on "Auto", the student can see crossover taking place -- rather, that it has taken place. The animated picture shows up at metaphase with striped blue and yellow chromatids. By checking the gametes, s/he can locate the altered chromosomes if the alteration has occurred and choose that gamete for fertilization. If the alteration has not occurred, s/he can run meiosis again until the correct chromosomes show up.

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.

6.       Now try it out in the Pedigree Window. Make a bunch of babies and see whether any of them come up with normal hemoglobin and no blue skin. When you see that, you'll know you've seen a crossover!
  The student crosses the two sickle cell carriers (have the trait but not the disease) and 4 offspring result. By toggling between sickle cell and methemoglobin in the menu, s/he can see which of the normal hemoglobin offspring are normal also for methemoglobin (have normal skin color). If any are present crossover has occurred. If none are present, the student should use the scissors tool to cut the pedigree line and recross the parents until a normal offspring is obtained.

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.

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