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Making Babies Activity II: Understanding rules of inheritance for several traits, while considering alignment, crossover, sex-linkage and lethal genes.

GenScope File: MakingBabies.gs
Wilma is hH for horns, Ww for wings, ll for legs, F- for fire breathing and A-/B- for color. Fred is Hh for horns, Ww for wings, Ll for legs, fF for fire breathing, and aA/bB for color.

Start off by double-clicking on the Making Babies file. This will automatically start up GenScope with two dragons, Wilma & Fred, already showing. OK, now you're going to try to make babies of various kinds but this time you will control which way the chromosomes go.

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.       Make a boy dragon with no horns, wings, and two legs.

Hint: you'll be able to do this much more easily if you can control which way the chromosomes go. Here's how: before you run meiosis, click on one of the magnifying glasses to open up the big window. Up in the corner of the screen you'll see the word "Alignment" and under it "Auto" and "Controlled." Click on the button next to "Controlled." Next time you run meiosis it'll stop in the middle and let you decide where the chromosomes should go, so you'll be able to make the gametes you need.

The student is told ahead of time to activate control of alignment. S/he then runs meiosis, after using the cell tool to call up the cell window, and checks in the Big Meiosis window to make sure that the chromosomes have lined up with the alleles in the right position to make the desired gametes. If they are not lined up correctly, s/he may make use of the alignment tool to move the chromosomes from one side to the other of the midline, so that when meiosis proceeds they will migrate to where they are wanted. After the gametes form, the correct one is chosen and dragged into the fertilization chamber. The same procedure is followed for the dragon of the opposite sex. Fertilization is invoked and the student sees the resulting baby dragon.

The student must go back and forth among levels to ensure that the baby dragon produced has the correct traits. By looking at the parent dragons s/he can see that they both have horns, wings and one of them has two legs and the other none. Mentally, s/he has to remember that even though both parents have horns, there is always the possibility that they each have a recessive allele for horns and that in that case there is a 1 in 4 chance that their offspring will be hornless. Similarly, since both parents are wingless, there is a 1 in 4 chance that their offspring will have wings, since wings are recessive.

The student also has to think about how the chromosomes with the alleles have to sort themselves out so that the gametes will have the right alleles to produce the offspring specified by the problem. Once s/he has that mental picture firmly in mind, s/he can look at the Big Meiosis window during meiosis and follow the three genes as they go through the process. There are two chances the student has to make sure that s/he ends up with the gamete that s/he wants: alignment, when the chromosomes line up on the midplate; and anaphase, when the chromosomes are lined up in the two dividing cells just previous to gamete formation. At these points, the chromosomes may be moved from one side to another of the midplane, resulting in transfer from one gamete to another. The cells from each of the parents are treated in the same way. The student must drag the correct gamete from each of these meioses into the fertilization chamber.

This is a difficult problem for the beginning student because it involves multiple levels. S/he has to think about genes on chromosomes which are found in the cells of organisms. S/he must recall the rules of inheritance for three different traits. S/he must understand the process of meiosis and the ways that meiosis must be controlled in order to get the desired result. On top of all that, the student has to work backwards and up-and-down. S/he has to know which alleles are needed for a particular phenotype, where they are located on the chromosomes, where the chromosomes have to end up (in which particular gamete), and how to achieve that result by controlling chromosomal segregation during meiosis. Finally, s/he must remember which gametes to fertilize to achieve the desired result.

2.       Now try to make a dragon with four legs.  Hint: what genes does a dragon need to have four legs? The student can proceed to try this in very much the same way that has worked on all the other activities up to now. It appears to be simply a matter of choosing the appropriate gametes -i.e., the ones that have the alleles (LL) that will make a baby with four legs. However ...

This is a trick question! The student should realize, after checking the leg alleles on the parents, and reflecting on the fact that four legs requires the presence of an "L" from each parent, that only one parent has an "L" to contribute. The other parent has "ll", she has no legs. Thus it is impossible for these particular parents to have an offspring with four legs. However, most of the students in our experience go through the entire process of meiosis, gamete selection and offspring production before realizing that there is a problem. Sometimes they go through the process several times before they discover that they have been asked to do the impossible.

While this activity may seem a bit unusual, it is important because it shows students that the laws of genetics pose certain definite constraints on what can happen. Many students, after changing genes and watching the corresponding changes in the organisms, begin to think that anything is possible, and it is important for them to begin to revise that naive view.

3.       See if you can make a female, firebreathing dragon.

The student is told to activate the crossover tool and to experiment with it to find out what it can do. Then the cell window is activated and meiosis is run for the male dragon. The Big Meiosis window is opened and when the chromosomes stop their "dance" and line up, the student can see that the fire-breathing allele (f) is on the same chromosome as the b allele for color. This will result in a stillborn baby. By using the crossover tool (the scalpel) the student may transfer the f allele to the other X chromosome, which contains the B allele. Then gamete formation can proceed. The student then drags into the fertilization chamber and the resulting dragon, a firebreathing female, is produced.

There are three important ideas involved in this problem.

1. Sex is determined by the Y chromosome in the female, so the gamete with the Y chromosomes must be selected and dragged into the fertilization chamber.

2. Fire breathing is a recessive trait, so an "f" is required -only one allele because we asked for a female, and females only have one X chromosome.

3. The "f" allele is on the same chromosome as the "b" allele., which is lethal in females because they only have one X chromosome. In order to produce a fire breathing female, in this case, the "f" allele must change places with the "F" allele, thus pairing the "f" with the "B" allele. When this gamete is dragged into the fertilization chamber, the resulting dragon will be both female and firebreathing. So you can see, the student has to juggle all of these ideas while going back and forth between phenotype and genotype, between the two homologous X chromosomes, between lethal and non-lethal genes, and between dominant and recessive alleles for fire breathing and the B color gene.

4.       Can you make a bronze dragon with horns, no wings, and no legs?  No hints on this one. We'll just tell you this: it is possible -and if you can do it you're a very good geneticist. Congratulations!

This sounds complicated, but by using the tools that were introduced in the first three problems, it can be solved. Alignment and crossover are needed to find the solution.

The student has to juggle the inheritance rules for the four traits (color, horns, wings, and legs) asked for, as well as deal with sex-linkage, lethal genes, alignment and crossover. This means that s/he is going back and forth between phenotype and genotype, the cell, gene and organism windows, and meiosis including alignment By carefully checking out the arrangement of the alleles in the cells leading up to gamete formation, the student can produce the required dragon.

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