Naked Science Forum
General Science => General Science => Topic started by: vj_tu on 18/01/2006 17:07:12
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What is the explanation for a birth of a normal child which is depicted as a white square in the Pedigree from the parent who both are phenotypically affected from a particular disease(it is not identified in the exam),depicted as black square and circle.??
I was told today from my lecturer this case did NOT exist, but I was asked about this question for Oxford interview. I couldn't answer it and I remember roughly what the tutors explained to me that there was something to do with the connection of non-defective genes from maternal and paternal chromosomes to give rise to a normal unaffected child.
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I am not familiar with the visual aids that you describe and hope this helps.
Normal cells contain two copies of their genes matched together in the double helix
A carrier of a genetically linked defect has one good gene and one defective gene and someone who suffers from the problem two defective genes.
Gametes (sperm and egg cells contain only one copy of the DNA from the parent) either parent can supply a good gene or a defective one. Normally 50% of the gametes from each parent will contain a defective gene so on a statistical basis any child from these parents has a 25% chance of getting the problem from both parents 50% chance of getting a defective gene from one parent and being a carrier and a 25% chance of being completely clear of the problem.
Learn, create, test and tell
evolution rules in all things
God says so!
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You're right, you can get unaffected children of affected parents in rare cases involving, as you say, crossing over of genes.
The example they gave us in first year cell biology was deafness since the enclosed nature of the signing community tends to result in deaf people marrying each other.
We're talking about a condition caused by a defect in a recessive gene. Deafness can be caused by "misprints" in any one of several genes. For this to work, both the parents have to be deaf due to the same genetic cause.
Right. I hope this is going to be comprehensible.
I'm going to label the generations 1, 2 and 3. Generation 1 is the "grandparent" generation. Any or all of these individuals may be deaf, but in any case they must all have at least one copy of the defective gene. The result of their breeding is two individuals of generation 2 who are the phenotypically affected mum and dad in your family tree.
Both mum and dad are deaf, due to faults in the same gene (I'll call it deaf1).
So.. the grandparents pass faulty copies of deaf1 to their offspring, who get two each, but just because the same gene is faulty doesn't mean the fault lies in the same place in the DNA code. There might be miscodings at the 5' end, or in the middle, or in the 3' end..
During meiosis the chromosomes in the generation 2 member, which up to this point (in all his or her body cells) have been straight forward copies of the chromosomes from the gametes which met when they were conceived, pair up with their opposite number and *swap bits* so that the same genes are present, but a whole new allele (sp? I've forgotten!) may be created.
---------- ---------\/_______
__________ =>_________/\-------
If one of the parents has two copies of deaf1 which are faulty at opposite ends, when meiosis occurs then if this crossover happens within that gene the chromosomes in the gamete will be entirely new relative to those of the individual in generation 2. One will have errors at both ends and one will have no errors at all... it's been fixed and a generation 3 individual conceived from this gamete will be able to hear.
I think that's a bit rambling, I could do a better explanation but not without diagrams.
If any of it doesn't make sense, shout and I'll try to explain further.
Once you've got your head round it, ask your lecturer again!
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I don't know wether this is what you are asking cause I don't really understand the question butt it could have something to do with the genes being recessive or dominant so when they come in pairs and you could have the two parents with one dominant(disease)and recessive(healthy) gene , the combination of the two recessive genes would give a healthy child.
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quote:
Gametes (sperm and egg cells contain only one copy of the DNA from the parent) either parent can supply a good gene or a defective one. Normally 50% of the gametes from each parent will contain a defective gene so on a statistical basis any child from these parents has a 25% chance of getting the problem from both parents 50% chance of getting a defective gene from one parent and being a carrier and a 25% chance of being completely clear of the problem.
A good rule of thumb but as I said above not actually the complete answer!
(And only applies to recessive conditions due to defects in a single gene)
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Nicely answered Rosy. Gold Star !
I'm going to move this to medicine shortly, by the way.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx