Friday, March 25, 2011

How is CF passed down?

I had a question thrown out to me yesterday on Facebook and I thought I'd share my answer here as well.

What if one parent has CF and the other parent is a carrier or isn't even a carrier, are your chances for having a CF child still the same? I'm so confused about this.

Since CF is a genetic disease it takes "bad genes" from both parents to have a baby with CF. As a CFer I have TWO copies of the bad gene. As a "carrier" you would only have ONE copy of the bad gene. Those who have no "bad genes" at all are neither carriers or CFers.

So, if two people with with TWO bad genes (both with CF) make a baby, the baby would virtually have a 100% chance of having CF since neither parent is able to pass down a "good gene".

If a CFer (TWO bad genes) makes a baby with a carrier (ONE bad gene and ONE good gene) the baby would have a 50% of getting TWO bad genes (the baby would have CF) and a 50% of getting ONE bad gene and ONE good gene (the baby would be a carrier)

If a CFer (TWO bad genes) makes a baby with a person who doesn't have CF and is not a carrier (TWO good genes) the baby would virtually have a 100% of getting ONE bad gene and ONE good gene therefore making the baby a carrier (ONE bad gene and ONE good gene).

If a carrier (ONE bad gene and ONE good gene) makes a baby with another carrier (ONE bad gene and ONE good gene) the baby would have a 25% chance of getting TWO bad genes (the baby would have CF) a 50% chance of getting ONE bad gene and ONE good gene (the baby would be a carrier) and a 25% of getting TWO good genes (the baby would not have CF and it would not be a carrier).

If two people who do not have CF and are not carriers make a baby, meaning they both have TWO good genes, then the baby would have a 0% chance of having CF.

Now this is obviously a very stripped down explanation but hopefully a simple enough one that is very easy to understand!
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Lindsay's avatar

Lindsay · 731 weeks ago

For the visual learners, punnett squares like this one on CFF.org help: http://www.cff.org/UploadedImages/ImageGallery/CF...
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RunSickboyRun 96p · 731 weeks ago

Thanks Lindsay!!
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danielle10 31p · 731 weeks ago

An additional tidbit: for CFer siblings, they have a 2/3 chance of being a carrier (1 bad gene), and 1/3 chance of not being a carrier (0 bad genes).

Since they have a sibling with CF, we know that the parents are carriers. There are three possible gene combinations that could end up giving a child with no CF: only dad gives his bad gene, or only mom gives her bad gene, or they both give their good genes.

Hope I made sense there?!
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RunSickboyRun 96p · 731 weeks ago

Not sure I follow... :)

Each sibling has an equal chance of having CF as the sibling before. Each live birth is independent of the other live births in terms of the "chances".
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Mandi's avatar

Mandi · 731 weeks ago

If two carriers parents have kids. Each kid that couple has together has a 25% chance of having CF. 25% chance of not having CF, and a 50% chance of being a carrier. Each sibling of a kid with CF has a 50% chance of being a carrier (1 bad gene) because all children are independent of each other - so the odds always stay the same.
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Danielle's avatar

Danielle · 731 weeks ago

What I mean is if an existing sibling, knowing that a sibling has CF and he/she does not, has a 2/3 chance of being a carrier.

For subsequent births, the probability is of course the same as for the sibling before.
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RunSickboyRun 96p · 731 weeks ago

You are correct that the sibling would have a higher chance of being a carrier of CF than having CF or not being a carrier, but the chance when that sibling was born was still 50% of being a carrier.
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Jackie's avatar

Jackie · 731 weeks ago

I can see where you'd get the 2/3, as there only three 'options' but it's out of 4 possibilites. Thus 2/4 or 50%. :)

The punnett square is how I always think of it.
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Tessa's avatar

Tessa · 731 weeks ago

Lindsay is absolutely right, if you are a healthy sibling of a person with CF, then you have a 2/3 chance of being a carrier because you are healthy (so don't have CF) meaning one of the possibilities is erased. Since you don't have CF, you know that you didn't get two bad genes, so the options are: bad gene from mom only, bad gene from dad only, or two good genes. So, a 1/3 chance of not being a carrier and a 2/3 chance of being a carrier.

This is important because if you have a sibling with CF and are thinking of having your own children, you need to know that your risk of being a carrier is 2/3, which is higher than 1/2.
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Tessa's avatar

Tessa · 731 weeks ago

Danielle is absolutely right, if you are a healthy sibling of a person with CF, then you have a 2/3 chance of being a carrier because you are healthy (so don't have CF) meaning one of the possibilities is erased. Since you don't have CF, you know that you didn't get two bad genes, so the options are: bad gene from mom only, bad gene from dad only, or two good genes. So, a 1/3 chance of not being a carrier and a 2/3 chance of being a carrier.
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Jenny Livingston · 731 weeks ago

Just an interesting tidbit:

My parents are both carriers (so they had a 25% chance of having a baby borh with CF). My 2 sisters and I were born with CF, and my 2 brothers are not even carriers of the gene. I was once told by another CFer that this wasn't even possible... well, it certainly must be.

Two parents (with 1bad gene each) happened to pass BOTH BAD genes on to three kids and BOTH GOOD genes on to two kids. Which all goes to say that sometimes the "chances" go right out the window. ;)
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RunSickboyRun 96p · 731 weeks ago

That makes perfect sense Jenny. Remember, each time they created the "chance" of passing down the gene started over as each creation is independent of the other. Meaning that if you have four kids, it doesn't mean that one will have CF, two will be carriers and two will be totally clean. Each birth will have a 25% of resulting in CF. The body doesn't remember where it stands with the chances :)
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Rhonda · 731 weeks ago

My husbands parents were both carriers as well. My husband and his brother also both have/had CF. They each had their own mutations though.
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Diana's avatar

Diana · 731 weeks ago

The "independence" factor is accurate. CF is a recessive gene. If both parents are carriers of a CF gene, there is a 25% chance that each child (independently) could have CF. Both parents could be carriers, have multiple chilrden and none could have CF while all could. Both parents could be carriers and have all of their children born with CF. To put it in the simplest terms, the same holds true for a child to be born as a carrier. If both parents have CF, the baby will have CF.

Families can go a couple of generations with no one born with CF but are still carriers. If someone inherited the gene and has a child with that special someone who also has the gene, the odds continue.

Praise God for the advancements in CF research in just in the last forty, thirty, twenty, ten, five and two years. It was only in the last couple of decades that the CF gene was identified (and that is just the most common mutation). Research advances everyday due to the generosity of many. Fight on!
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Michael D's avatar

Michael D · 731 weeks ago

Yay for Punnet squares and Mendel's laws! Check these pages for some visuals that may also help clear things up.
http://en.wikipedia.org/wiki/Genetics#Discrete_in... http://en.wikipedia.org/wiki/Punnett_square
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Cara's avatar

Cara · 731 weeks ago

If one bad CFTR gene (from one parent) plus a deletion also equals CF, how are the deletions accounted for?
Are they passed on as good and bad genes are, or are deletions spontaneous mutations, or both?
Thanks and congrats on the sprat.
2 replies · active 731 weeks ago
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RunSickboyRun 96p · 731 weeks ago

The presence of a single known CF mutation in an asymptomatic individual identifies that person as a carrier. The relevance of a single novel mutation in this setting is not known. However, as described above, large deletions or duplications would be expected to negatively affect CFTR function and lead to disease in the presence of a second mutant allele.
http://www.questdiagnostics.com/hcp/intguide/jsp/...
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Tessa's avatar

Tessa · 731 weeks ago

In the easiest terms, having no gene because it's deleted is bad, so a deletion is a bad gene.
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Cara's avatar

Cara · 731 weeks ago

I was miscategorizing deletions as "neutral" genes, neither good nor bad. So, genes that have any imperfection are considered "bad". --also from the Qwest site: deletions and duplications would be expected to impair CFTR protein function and thereby lead to disease in the presence of a second mutant allele.
Thank you for the link.

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