Making Babies Activity I: Understanding rules of inheritance for several traits, considering the random nature of meiosis.
GenScope file: MakingBabies.gs
If you need to create this file, Wilma's genotype is horns (hH), wings (Ww), legs (ll), tails (Tt), firebreathing (F-), and color (A-B-). Fred's genotype is horns (Hh), wings (Ww), legs (Ll), tails (Tt), firebreathing (fF), and color (aA/bB).
Remember that to make a baby you have
to start with a cell from the mother and a cell from the father (use the
cell tool - the one that looks like a little circle - and click on a dragon).
Then you run meiosis to make the gametes (the four little cells that carry
the baby's chromosomes) and drag one gamete from the mother (on the left)
and one from the father (on the right) into the middle window. Then you
run fertilization and the two gametes (an egg from the mother and a sperm
from the father) come together to make the baby.
1. Fred and Wilma desperately want a little boy. Can you give them one? Can you be sure before you make it that the baby will be a boy? Hint: Use the magnifying glasses to make the cells big, then run meiosis and watch the chromosomes carefully. Find the gamete that has the alleles you want and double click on it. The outline will become heavy. Then click on the minus sign magnifying glass to make the cell small again.
2. Run meiosis again, and this time see if you can make a girl. Hint: be careful -you want a LIVE girl!
The student opens GenScope and the organism window appears with Wilma and Fred dragons in view. S/he clicks on the cell tool and then on each of the dragons in turn. The cell window appears with a cell in each side of the window. In turn, the student clicks on each of the thumb tabs to start meiosis. Using the magnifying glass, s/he clicks on one window and checks out the alleles on the chromosomes in each of the gametes. Since it takes XX to produce a boy, s/he looks for the gametes with an X and chooses one to be dragged into the fertilization window. The other side (male or female) is treated in the same way. When both gametes have been dragged into the window, the student starts fertilization by clicking on the thumb tab. A naming window appears, the student names the dragon and can see the boy baby if s/he chose the right gametes.
The organism window starts the student off, surprisingly enough, at the organism level. By bringing up the cell window the student is going down one level to the cellular level. Now s/he is beginning to have some complex ideas. S/he has to understand that organisms are made of cells; that there are specialized cells for reproduction. When s/he uses the magnifying glass to look at the chromosomes another level is reached. Now the student has to think about the chromosomes inside the cell. These chromosomes are not all alike, and it is the sex chromosomes that we are interested in here. Males have XX and females XY. The correct gametes must be chosen (chromosome level) and used for fertilization (cell level) to produce the correct, new organism (organism level). Thus the student has traveled up and down through the hierarchy of multi-level reasoning as embodied in GenScope. The main topic here is sex determination.
This problem is like the first one. The student has to start with the organism window, click on the cell tool, then on each parent dragon in turn. The cell window appears, the thumb tab is clicked on for each side and meiosis is run. Using the big magnifying glass, the student can choose the correct gamete for fertilization -in this case, the gamete with the Y chromosome from the mother and the X chromosome from the father. There is the added complication of making sure that the male's X chromosome does not have the b allele. If both the Y chromosome and the non-b allele are chosen, the female dragon will appear in the organism window after fertilization.3. Notice that Fred and Wilma both have horns. Is it possible for them to have a baby with no horns? Try it and see if you can make a baby girl dragon with no horns. Hint: It is do-able. If you don't get the gametes you want, try running meiosis again.
Again the student is starting at the organism level. When s/he clicks on the cell tool, s/he moves down to the cell level. Clicking on the magnifying glass, s/he moves to the chromosome level. When s/he checks out the alleles on the male's X chromosomes to make sure that the b allele is not present, s/he is operating Řand reasoning -at the gene level. Throughout the process, the student has to keep a model of meiosis in mind, remembering that only one of the four gametes from each parent is used in fertilization, and that each gamete contains only 1/2 the chromosomes of the original cell. S/he must also keep in mind that sex is determined by the presence or absence of the Y chromosome, and that the presence of the b allele on the X chromosome will prove lethal to a girl baby because she receives only one X, from her father. Whew!
Now the student has to really start
thinking. In order to make an educated guess about whether Wilma and Fred
can have a hornless baby, s/he has to first recall which alleles are needed
for the hornless condition (hh). Once that is determined, s/he has to think
about what allelic combinations are possible in dragons with horns (HH,
Hh). Then, how do these alleles segregate during meiosis to make the gametes?
From there, which two gametes will produce a hornless dragon? But wait.
We want to produce a live female dragon. How do we get a female? One gamete
must contain the Y chromosome. That one comes from the mother. OK. That
means that the X chromosome will come from the father. We have to look
for the X chromosome that has the B allele, otherwise the baby girl will
be born dead. This is getting complicated. In the final analysis, the gamete
from the mother must have an h and a Y chromosome and the gamete from the
father must have an h and an X chromosome with B allele. Once the student
has gone through all of these mental gymnastics and writes his/her guess
down on paper, it is time go to the computer. Since meiosis is a random
process, the correct gametes won't always appear. The student may have
to run the process three or four times. This will serve to reinforce the
student's awareness that meiosis is a random process. It does not produce
the same results each time it is run.
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