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I. Weighing a Parent's Right Against a Child's: Huntington's Disease

Huntington's is inherited through a dominant gene, which means if someone inherits a single gene with the defect on it then they will have the disease. In the case study, the mother has the disease, which means she has the "bad" gene. She also has a non-affected gene. The father has two non-affected genes. The mother can pass on either the good gene or the bad gene to her offspring.

White chip = non-affected normal gene
Red chip = gene with Huntington's disease

1. Place one red and one white chip in the bag labeled Mother. Place two white chips in the bag labeled Father.
2. Without looking take one chip out of each bag. One red + one white = Huntington's disease is present. Two white = normal, no disease.
3. Record what you drew out in the data table and return the chips to the bag.
4. Repeat this procedure 9 more times, recording after each draw.

Trial	Mother's (W or R)	Father's (W)	Huntingdon's Present or Absent?
1		W
2		W
3		W
4		W
5		W
6		W
7		W
8		W
9		W
10		W

Questions:

1. How many times did the offspring have Huntington's disease?
2. How many times was the offspring healthy?
3. What's the percent chance of passing the defective gene to the offspring, from 10 total trials?
4. How does the percentage above compare to the percentage you get when you calculate using a Punnett Square approach? 5. Do you think that the parents have the right to have their children tested for Huntington's? Explain your answer.

II. How Does Genetic Information Affect a Family: Fragile X Syndrome

In humans, two X (XX) chromosomes means female; and one X and one Y (XY) means male. In some cases of Fragile X Syndrome, a woman can have one good, normal X-chromosome along with one affected X chromosome. Genes located along the affected X chromosome may be damaged, and so the processes/traits they control may not function properly.

If a woman has one normal X and one fragile X, she does not have the disorder--for her one good X can compensate, or mask the effect of the bad X--but she is considered to be a carrier for the disease.

If a male gets an X with the defect, then he will have the disorder, because he does not have another good X to mask the damaged one.

Red chip = affected X chromosome
White chip = normal X chromosome
Blue chip = Y chromosome

1. Place one red chip and one white chip in the Mother's bag (she's a carrier) and one white and one blue chip in the Father's bag.
2. Without looking, remove one chip from each bag. Two white chips = normal female; one red, one white = carrier female; one white, one blue = normal male; one red, one blue= affected male.
3. Record what you drew out in the data table and return the chips to the bag.
4. Repeat this procedure 9 more times, recording after each draw.

Trial	Mother (Red or White)	Father (Blue or White)	Offspring Gender (Male or Female)	Fragile X? (Carrier, Normal, or Affected)
1
2
3
4
5
6
7
8
9
10

Questions:

1. How many trials resulted in females?
2. How many of those females were carriers?
3. How many trials resulted in males?
4. How many of those males were affected?
5. What's the percent chance of having a Fragile X affected male?
6. If you were a female in the mother's family, would you agree to be tested for Fragile X? Why or why not?

III. When Current Technology Falls Short: Cystic Fibrosis

Cystic Fibrosis is a recessive disease. This means it is inherited only when someone receives two genes for the disease, one from each parent. If a person has one defective gene and one normal gene, that person does not have the disease, but is considered a carrier for the disease. In this case study, assume that both parents are carriers for cystic fibrosis.

White chip = normal non-affected gene
Red chip = gene with CF

1. Place one red and one white chip in the bag labeled Mother. Place one red and one white chip in the bag labeled Father.
2. Without looking, remove one chip from each bag. Two white chips = normal; one red, one white = carrier; two red chips = cystic fibrosis.
3. Record your results in the data table and return the chips to the bag.
4. Repeat this procedure 9 more times, recording after each draw.

Trial	Mother's (W or R)	Father's (W or R)	Cystic Fibrosis? (Carrier, Present or Absent?)
1
2
3
4
5
6
7
8
9
10

Questions:

1. How many of the trials resulted in offspring with CF?
2. How many were normal?
3. How many were carriers?
4. How does the percentage above compare to the percentage you get when you calculate using a Punnett Square approach?
5. In the real-life stories you read at the beginning of this lesson, David and Shelley were not positive that David was a carrier. Do you think they made the right decision to use a donor sperm? Why or why not?

IV. When a Genetics Evaluation Saves a Life: Marfan Syndrome

Marfan syndrome can be inherited in the same way as Huntington's disease, with one dominant, defective gene. Often Marfan's Syndrome results from a spontaneous mutation, meaning neither parent has a defective gene. However, in this case study, because both children in the family have Marfan's, we can conclude that the chances are very slim that two spontaneous mutations occured within one family--more likely, one parent has the defective gene, but we don't know which one.

Questions:

1. Why are sports especially dangerous for people with Marfan's Syndrome? Should students who play sports be tested for Marfan Syndrome?
2. Would testing each of the parents for the presence of the bad gene have been a good idea in Tad's case?
3. Why or why not?

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