Name: _________________________                Date: ________________

Bronze & Gold Activity: Understanding a polygenic, sex linked trait with a lethal allele.

GenScope file: BronzeGold.gs
In the file we are distributing, Jennie, Jake, and Joe all have unobservable alleles and DNA. You may wish to allow your students to view these alleles. If so, click on "Password" under the "Options" menu command, and type "dragon" (without the quotes).  Then double-click on the organism whose setting you wish to alter and check the appropriate boxes.

If you need to remake the file, you can do so by giving Jake's X chromosomes the allele combination Aa and bB.  Joe has Aa and Bb. Jennie, who is gold and has a single X chromosome, is a-B-. The number of offspring should be set for 16.  The other genes can be anything you like as long as Jake and Joe look the same.  It is a good idea to set the file to "View by Small Picture" (under the "Organism" menu item) so that the students can view all the parents at once without enlarging the window. Also, open a pedigree window and place Jennie, Jake, and Joe in it, in that order. If you wish to make any alleles invisible to the students (to increase the difficulty of the puzzle) you must do so before saving the file. Just double click on the organism whose setting you wish to alter, check the appropriate boxes, and click "OK." If you want to prevent students from changing the settings, go to "Password" under the "Options" menu item, type in a password that you will be able to remember (!) and check the "Hide all owner's options" box.  Remember, if you forget your password, you will not be able to alter these settings!

When the file opens, you will see a pedigree window set to show color. There are three dragons in it - two males and one female. Jake and Joe are bronze and Jennie is gold.
 

1.       Circle all of the possible combinations of alleles for each of the dragons. (Hint: don't forget to use the Dragon Genome reference sheet.)
 

The student, by referring to the Dragon Genome reference sheet, circles all possible combinations for the color genes of the three dragons.

Color is particularly difficult for students to understand. Not only is it polygenic, but it is also x-linked and one of the genes has a recessive lethal allele. A triple whammy! This first exercise helps him to sort through the various combinations. Females are easy because they only have the one X chromosome, so the gold dragon is always a-/B-. (The dash "-" stands for the Y chromosome which carries no genes.) The males, however, have two X chromosomes, so the two color genes may be paired in two different ways for the color bronze: Aa/Bb or Aa/bB. How can you tell the difference? Only by running a pedigree and observing what the F2 generation looks like.
 

2.      Now use the cross tool  to mate Jake and Jennie.

You will get exactly 16 children. Approximately half will be girls and half will be boys. Circle the letter of all the statements below that are TRUE. Example: If "a. All the girls are the same color as their mother." is true, then circle the letter "a."

a. All the girls are the same color as their mother.
b. None of the boys are the same color as their father.
c. Some of the girls are the same color as their mother and some are not.
d. All of the boys are the same color as their father.
e. None of the girls are the same color as their mother.
f. Some of the boys are the same color as their father and some are not.

By inspection of the pedigree, the student can see that all of the females are gold, the same color as their mother. However, the males are about evenly divided between bronze, the same color as their father, and amethyst. So a and f are the correct answers.

The students can see the color differences, so why are we bothering to have them fill out the answers to these questions? Because we are interested in emphasizing the differences between the answers for females (all of the daughters look like their mother) and males (only about half the sons look like their father). Also, because the answers to the same set of questions will be very different when the students examine a pedigree from the second father!
 

3.       After you've circled the true statements, use the scissors tool  to cut out (remove) all of Jake and Jennie's children. (Put the tip of the scissors on the horizontal line joining the parents and click.) Now use the cross tool to mate Jennie with Joe (the male dragon on the right) and circle the TRUE statements below:

a. All the girls are the same color as their mother.
b. None of the boys are the same color as their father.
c. Some of the girls are the same color as their mother and some are not.
d. All of the boys are the same color as their father.
e. None of the girls are the same color as their mother.
f. Some of the boys are the same color as their father and some are not.

This time the students will observe that none of the offspring are the same color as their parents. All of the females are emerald and half the males are azure and the other half are topaz. So the correct answers are b and e.

What is the main difference between Jake and Jennie's children and Joe and Jennie's children?

There's a huge difference. The offspring are completely different colors in the two cases, even though the two fathers look identical!

By this time your students should be scratching their collective heads. Not only are all the dragons, male and female, different from their parents, but also these are brand new colors. If they have not been using the Dragon Genome handouts by now, suggest that they start doing so. The best way to deal with color in dragons is by writing down all the permutations available for each set of parents. This polygenic trait is a real test of the student's understanding of the inheritance patterns. (It's tough for teachers, too.)

The main difference between Jake and Jennie's children and Joe and Jennie's children is that, in the first instance, most of the kids have the same color as the parents while in the second, none of the kids do.

Based on your research so far, what do you think Jake, Jennie and Joe's color genes look like?
 

Jake	AA Bb	Aa Bb	AA BB	A- B-	a- B-
Jennie	AA Bb	Aa Bb	AA BB	A- B-	a- B-
Joe	AA Bb	Aa Bb	AA BB	A- B-	a- B-

Call your teacher over and explain your reasoning.

The student should answer that Jake is aA/Bb, Jennie is a-/B- and Joe is Aa/Bb.

The position of the aA/Bb and Aa/Bb in the two male dragons is important. Because of the way the chromosomes segregate and line up in meiosis, the color combinations change and produce different offspring in the two cases. For example, if the father is aA/Bb, he can only have gold daughters, because the A allele (which produces emerald females) is paired with the b allele (which is lethal for females). By the same token, a father who is Aa/Bb can only have emerald daughters.

In this particular problem, Jake has aA/Bb alleles to contribute to his daughters. Since the b allele is lethal, any of his daughters who receive that allele will die. The others will get the a-/B- combination and be gold, the same color as their mother, Jennie, who has a-/B- on her single X chromosome . By a similar chain of reasoning, all of Jennie's sons will receive the a and B alleles from her and either a and B or A and b from Jake. So Jake's sons will be either aa/BB (amethyst) or aA/Bb (bronze). Joe, on the other hand, is aA/bB, so his sons will be evenly split between aa/Bb (azure) and aA/BB (topaz).