Genetics as it applies to evolution, molecular biology, and medical aspects.
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A father has mitochondrial disease and some of his children inherit the disease. Mitochondrial disease is passed from mother to offspring. How can the father be passing on the mitochondrial disease?
The father may possess a recessive gene and show no symptoms of the disease. He may be able to pass on the mitochondrial disease to his children if the mother also possesses the mitochondrial disease, however the father should not be able to pass on a mitochondrial disease to his children, therefore this theory is irrational.
A mutation may have occurred during the development of the fetus and caused a mitochondrial disease.
I do not know if my two theories are correct or not, can you please help me? If they are incorrect, can you please explain? Thank you in advance!
“The father may possess a recessive gene and show no symptoms of the disease.”
No. You said “A father has mitochondrial disease”, so he is double recessive.
“He may be able to pass on the mitochondrial disease to his children if the mother also possesses the mitochondrial disease”, - correct if mother is heterozygous for it.
“however the father should not be able to pass on a mitochondrial disease to his children, therefore this theory is irrational” – not true. It can be caused by either mitochondrial genes (in which case it’s maternally inherited) OR by nuclear genes (in which case it’s not gender specific).
“A mutation may have occurred during the development of the fetus and caused a mitochondrial disease” – possible as well.
i dont know....
Statement: Mitochondria have their own genome, and this has is analogous to a bacterial genome, regardless of your views on evolution.
with ref to Wikipedia
Anna55 - if this is your question from school, then you should point out that Cat has a very good point, mitochondrial diseases are caused not only by mtDNA errors so your question is misleading.
However, as the question clearly says (NB username:Cat) 'Mitochondrial disease is passed from mother to offspring', that is the clue to the answer they want. Therefore the answer they are looking for relates to the fact that the we (and the father) might have been under the impression that he could not pass the disease to the children, yet sometimes some mitochondria pass into the cell from the sperm, it is rare though. The other alternative explanation that is likely is simply that the mother was mosaic- ie. carried 'good' and 'bad' mitochondria - this is likely in related individuals, and a billion or more times more likely that the same mutation arose spontaneously in the mother, partly because identical mutations are infinitisemally unlikely, but also because each egg carries multiple mitochondria usually, so there.
From what wikipedia says, though, alternative explanations that your questioner probably doesnt want you to include , but for the record , are
- that it is drug induced and the mother took the same drugs during pregnancy
- that is is disease induced and the father infected the children but not the mother
Cat... if you read the full question you will see the error in your reply. But you are right that not all mitochondrial genetic diseases are caused by genes in the mitochondria - many have over evolutionary time been replaced by genes in the genome (we know not how they got there, mind you) as you probably know.
your wrote: “He may be able to pass on the mitochondrial disease to his children if the mother also possesses the mitochondrial disease”, - correct if mother is heterozygous for it. if we consider these diseases like Cat, then the mother would have to have at least one defect in the gene - heterozygous as you say. Incidentally, if the mother was homozygous for the defect, then she would have the disease and (regardless of whether it is chromosomal or mitochondrially-encoded), that would be sufficient for the children to have the disease given that the father has it . We are not given this information but it is implied that the mother does not have the disease. HOwever I have to say that this is a terribly worded question, as it is ambigious. It should read:
A father has a SPECIFIC mitochondrial GENETIC disease and some of his children inherit the disease. This specific Mitochondrial disease, like many such diseases, is passed from the mother to offspring. How can the father be passing on this mitochondrial disease?
Finally, in terms of recessive/doublerecessive - you are getting your terms confused - the word you want is not recessive but defective (I am a geneticist!) . If a gene causes a phenotype due to being defective, then in the case of autosomal diseases (all non sex chromosome or mitochondrial diseases) then the presence of the defective gene is recessive in its effect. But a mutation can also cause disease and be dominant, and we dont know whether this disease in question is a dominant-mutant (in other words is it broken always off or broken always on, = recessive or dominant mutant). We are assuming that this is a recessive in the nature of its effect, but actually we dont know so it may not be safe to do so.
Now if it were autosomal , then a broken dominant gene would only require the father to have one 'bad' copy (heterozygous). If recessive then he would need two broken copies. But then if sex chromosomal, then : if it is on the X part of the Y chromosome, then if dominant, all boys would have the disease and girls would never have the disease.
(or on the X-homologous part of Y chromosome)
It could be that it is on the part of the X chromosome that is missing in the Y chromosome; thus if dominant, the father would pass it on only to 100% of of his girls (who would all be carriers) and non of his boys!
The final possibility then is that it is recessive sex chromosomal, and this would normally describe circumstances where the males only have one copy per cell- and if this is broken (and recessive as I said) then girls would rarely manifest the condition, whereas boys would depend on getting a good copy from their mothers. So, if a childs mother is related to the father, then this is much more likely (they can not be helped by their fathers). In this regard it is very similar to the mitochondrial encoded diseases the question asks about (as I understand it!).
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