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Firebreathing Activity: Understanding a sex linked, recessive trait

GenScope File: Firebreathing.gs
The file is locked, so the chromosomes of the three dragons in the window may not be viewed.  If you should need to recreate the file to run this activity, Jennie is F-, a non-firebreather, Joe is Ff, heterozygous and thus, non-firebreathing and Jane is f-, firebreathing.  The pedigree option for number of offspring is 16.

Bring up the Firebreathing.gs file by double clicking on it. When GenScope opens, you will see a pedigree window set to show fire. There are three dragons in it -- two females and one male. Note that you cannot see the genotype of any of the three.
 

1.       Your task will be to figure out how firebreathing is inherited. Let's begin with some predictions. If Joe and Jennie have kids, do you think any of them will be firebreathing? Circle your answer below.

a.  I believe that Joe and Jennie can have firebreathing children.

b.  I do not believe that Joe and Jennie can have firebreathing children.

Either answer is OK. It's just a guess, but they will probably think no, because neither parent breathes fire.

This seems, on the surface, a little silly. How can they even hazard a guess? But on second thought, it does have value. It makes the kids start to think about how traits are inherited. If these two parents don't breathe fire, then maybe the offspring won't either. On the other hand, there are hidden traits - maybe firebreathing is hidden. Could be!
 

a. I believe that Joe and Jane can have firebreathing children.

b. I do not believe that Joe and Jane can have firebreathing children.

The guess here will probably be that, yes, they can have firebreathing children. After all, Jane breathes fire.

They will be correct, of course, but they probably won't understand why. Joe is heterozygous, and Jane has one "f" so half of their daughters and half of their sons will breathe fire.
 
 

3.       OK, now try it. First, use the cross tool to mate Joe and Jennie. Then circle all of the statements below that are TRUE.
 

a. All of Joe and Jennie's kids are firebreathing.

b. Some of Joe and Jennie's kids are firebreathing.

c. All of Joe and Jennie's firebreathing kids are girls.

d. Some of Joe and Jennie's girls are firebreathing, some are not.

e. None of Joe and Jennie's girls are firebreathing.

f.  All of Joe and Jennie's boys are firebreathing.

g. Some of Joe and Jennie's boys are firebreathing and some are not.

h. None of Joe and Jennie's boys are firebreathing.

Half of their daughters breathe fire and none of their sons, so B, C, D and H are the correct answers.

Since Jennie is F- and Joe is Ff, half of their daughters will be firebreathers and half will not. All of their sons are non-firebreathers (Ff). The girls inherit their only X chromosome from their father, so are able to get either the "F" or the little "f". The boys get one X chromosome from their father and one from their mother. Since mom can only contribute the "F" allele, and two "f's" are needed for a male to be firebreathing, none of the boys will breathe fire.
 
 

a. All of Jane and Joe's kids are firebreathing.

b. Some of Jane and Joe's kids are firebreathing.

c. All of Jane and Joe's firebreathing kids are girls.

d. Some of Jane and Joe's girls are firebreathing, some are not.

e. None of Jane and Joe's girls are firebreathing.

f. All of Jane and Joe's boys are firebreathing.

g. Some of Jane and Joe's boys are firebreathing and some are not.

h. None of Jane and Joe's boys are firebreathing.

Half the sons and half of the daughters are firebreathers, so B, D and G are the correct answers.

Since Jane is a firebreather (f-) half of the boys and half of the girls will breathe fire. The fact that the girls get their X chromosome from their father and he has one of each kind of firebreathing alleles means that it is possible for the girls to inherit either type of allele. However, Jane has only the "f" allele on her one X chromosome which means that her sons can be either firebreathing or not, depending on which allele they get from their father.
 

Answers will vary, but a thinking student will say no since they didn't see any the first time that they ran the pedigree.

If students take a look at the chromosomes and they are observant, they will say something about there being only one X chromosome in the female, thus only one "F" allele and that you need two "f's" for firebreathing to show in males, since firebreathing is a recessive trait.

This exercise is an introduction to sex linked inheritance. When the student looks at the chromosomes in the offspring of the mating of Joe and Jennie, they will see that males have two X chromosomes and females only one. That's why there are more female firebreathers, since they only need one "f" allele to be firebreathers. Males need two.
 

The correct answer here is yes, if the right F1 generation dragons are chosen to be parents.

The male has to be heterozygous and the female a firebreather. The offspring of these two should produce firebreathing in half of the females and half of the males. If the student can explain the answer in these terms, then s/he has a grasp of the problem.
 
 

Were any of the grandsons in the F2 generation firebreathers?

9.       Based on the information you have gathered during this exercise, what do you think Jane, Jennie and Joe's alleles for firebreathing are? Circle all the answers in the table you think are possible.
 

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