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PKU test - Page 3

post #41 of 44

I am expecting our 6th child. I do not understand a lot about the difference between genetic disorders and metabolic disorders, however, from what I have read and can understand, that if none of these disorders are caused by the mixing of the parents DNA in the first child that it won't be present in any other children. Is this true? If so, why is each child, of the same parents, tested? I'm also a little suspicious about where all this genetic information is stored and what could be done with it....that's a lot of "information" on almost every single baby (in the US). 

post #42 of 44

No, I have seen babies who have a disorder that their older siblings do not.

post #43 of 44
Quote:
Originally Posted by Katinhat View Post
 

I am expecting our 6th child. I do not understand a lot about the difference between genetic disorders and metabolic disorders, however, from what I have read and can understand, that if none of these disorders are caused by the mixing of the parents DNA in the first child that it won't be present in any other children. Is this true?

 

No. These disorders are typically recessive disorders. Here's how it works:

 

Each of us carries two copies of each gene, one inherited from each parent. A recessive disorder is one in which you need a gene for a particular job, but only need one good copy of the gene to do it. So, if you inherited a good copy of the gene from one of your parents and a faulty copy from the other, you won't have the disorder. However, you will still be able to pass on your faulty copy to your children, which means you're a carrier of the condition. Of course, if you do pass on the faulty copy but your child gets a good copy from the other parent, then they won't get the disease either, because their good copy of the gene will be enough.

 

So, to have a recessive disorder, a person needs to get two copies of the faulty gene - i.e. get a copy from both parents. Typically, this happens when two carriers have a child together and each pass on their faulty gene rather than the good one. However, it's completely chance which of your two copies of each gene ends up in which egg or sperm produced by you - half of your eggs/your partner's sperm will have the good gene and half will have the faulty gene. Then, of course, it's pot luck which egg and sperm join together to make the baby. So, if both parents are carriers of a faulty gene, the four possible scenarios are:

 

1. Both egg and sperm happen to have the good gene. Baby is fine.

2. Egg has the good gene, sperm has the faulty gene. Baby is fine, but is a carrier (won't affect them but may affect their children if they marry another carrier).

3. Egg has the faulty gene, sperm has the good gene. This works out exactly the same way as scenario 2.

4. Both sperm and egg have the faulty gene. Baby has the disorder.

 

This is complete chance with each child. It's like tossing two coins and seeing how often they both come down tails. On average, it's a one in four chance with each toss. But you could get lucky and avoid it lots of times in a row... and it might still come up on the next time. So, they have to test each baby.

post #44 of 44
Quote:
Originally Posted by JenniferLS View Post

Secondly, she said that if BOTH parents are negative for PKU, then the baby won't have it. That definitely sounds simple...but maybe for the parents it's more than just a blood test for PKU, maybe it's a genetic test...does that make sense?

 

That's quite correct. If both parents have been tested for the PKU gene and don't have it, their baby cannot have it. However, genetic testing is a darned sight more complicated and expensive than doing a PKU test on a newborn, so you wouldn't normally test this way.

 

What's worrying me a little is that she would say something so potentially misleading and not clarify. I mean, I wasn't there for the conversation so maybe I'm misinterpreting it, but... was this in the context of whether your baby needed the PKU test or not? If she's using an argument like this to try to make out that babies don't need the PKU test, then that raises some worrying doubts about her knowledge base.

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