Sometimes it is All in the Genes

Nancy had found it hard to concentrate at work that afternoon and it was almost impossible to think about what she was doing as she prepared dinner that evening with her husband Jake.  Suddenly Jake's voice snapped her back to the kitchen, "Nancy, you're about to put the carrots and cucumbers in the garbage."  Jake surveyed the salad bowl where she had been working and smiled at the rare opportunity of catching his wife in a mistake.  "And is vegetable peel salad your latest craving?"

"Oh, honey, I need to talk to you about something."  The stress in Nancy's voice quickly wiped the smile off of Jake's face.  "Remember when I went for my pre-natal checkup a couple of weeks ago, and I told you I was participating in a genetic study?"

"Yeah, something about MS or something, right?"

"No, CF, cystic fibrosis."  Nancy corrected him.  "Well, it turns out that I tested positive."

"What!?  How did this happen?  You don't seem sick at all."

In response to his panic, Nancy calmed down and explained, "No, no, of course I'm not sick.  Remember, I told you that they were testing to see if I was a genetic carrier.  How it happened was that I inherited one bad version of the gene, a bad 'allele' they call it, for cystic fibrosis from one of my parents."

He felt he should know why, but slowly Jake asked, "If you don't have CF, what makes you so worried that you're throwing sliced cucumbers into the garbage?"

"Because there is a chance that I could pass my bad CF allele to our child.  And if you are a carrier and pass on your CF allele, then she or he will have CF."

"CF is pretty serious, right?"

"Yes, pretty serious."

Trying to be optimistic, Jake asked, "Yeah, but what's the chance that I'm a carrier too?"

"Dr. Kwin says the latest research indicates 1 in 29,"  Nancy answered soberly.

Jake's jaw dropped. "Can I get tested?  What is the likelihood that you'll pass on the CF gene?  What is the chance that I'll pass on the gene?  What is the chance our baby will have CF?"

"Yes you can, and will get tested.  I don't care if you are afraid of needles."  Nancy left no room for further questions there.  "I talked to Dr. Kwin at length today and I have a pretty good handle on this genetics thing.  So, turn down the stove, sit down with me, and I'll answer your questions."

"Let's start with the basics,"  Nancy began.  "We all have tens of thousands of different genes in each of the cells in our body.  These genes provide the instructions on how to make all the components of our body.  For almost all of those genes, we have two copies of each different gene.  I inherited one copy of the gene that can cause CF from my mother and one from my father.  One of them, I don't know which, was a defective copy, what they call a 'mutant' allele.  One of the alleles I got from my parents was a normal, working copy.  Because CF is what they call 'recessive,' I'm fine because I have that one good copy.  It takes two bad copies of the gene to give you cystic fibrosis."

Questions:

Help Nancy finish answering Jake's questions.

1. The normal or good copy of the CF gene can be written shorthand as "F", and the mutant or bad copy of the CF gene can be represented as "f".  Using this shorthand style, write out Nancy's genetic make-up for this gene.
2. What is the chance that Nancy passed on the CF allele to her baby?
3. What is the chance that Jake passed on the allele if he is a carrier?
4. What is the chance that their baby will have CF if they are both carriers?  If their first child has CF, what is the chance that their second child will have CF?  What is the chance that the baby will inherit CF if only Nancy is a carrier?

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