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I am having some problems understanding recombination, and I am not sure what element I am missing here. This figure is an example from my text book. The pedigree belongs to a family with an autosomal dominant disease, typed for a marker with alleles A1-A6.

Figure 14.9 Human Molecular Genetics, 4ed. (© Garland Science)

So, all meioses are phase-known, meaning we know which combination of alleles was inherited from each parent. Apparently this leads us to be able to unambiguously score III1-III5 as non-recombinant (N) and III6 as recombinant (R).

This is were I get lost, I don't understand how it is possible to unambiguously score each individual as N or R. Also, if the disease is dominant, how is it possible that III6 is affected, as both alleles originate from the both unaffected I1 and II2?

EDIT: Blue colour in the pedigree means the person is affected by disease, white symbolizes unaffected individuals. I believe the red stars symbolizes that the person carries the disease gene. That would mean blue corresponds to not carrying the disease gene. If this is the case The red colour of III2 is a fault in the figure.

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    $\begingroup$ Could you add some details to your question to annotate the image? Also in my opinion this is quite homework question, which is off topic here at Biology SE, unless you share your efforts / research to solve the question. $\endgroup$ – Nandor Poka Mar 29 '15 at 18:58
  • $\begingroup$ I don't understand how recombination scoring works, and I thought I'd use this image from my text book to provide a basis for illustrating it. It wasn't meant as help with home work, but I understand your remark :-). How can you deduce from given alleles which is recombinant or not? $\endgroup$ – Wolgast Mar 29 '15 at 19:04
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    $\begingroup$ Okay so not homework :) but it would still help to know what all the stars etc mean on the image. For me at least... $\endgroup$ – Nandor Poka Mar 29 '15 at 19:08
  • $\begingroup$ To be honest I have no clue... I believe its purpose might be in emphasizing heterozygosity, but I don't think this is correct: if blue star corresponds to allel A2, then III2 and III5 should also have a blue star... $\endgroup$ – Wolgast Mar 29 '15 at 19:12
  • $\begingroup$ You need to add what the stars and colors are referring to. $\endgroup$ – fileunderwater Mar 30 '15 at 16:23
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Ns are not non-recombinant at the genomic level. What you know is that they do not show recombination events at the risk allele locus as subjects carrying the marker A1 show signs of the disease and similarly healthy subject do not carry the marker A1. Hence they are called non-recombinant. On top of that 3rd generation subjects (excepted III6) show combinations of alleles reflecting inheritance of one allele from each parent.

What is very important is that A1 is not the allele, it is the marker for the risk allele. This means that A1 is not directly the allele but a region of DNA close to the allele provoking the disease, sufficiently close to be co-inherited most of the time with the disease allele.

For III6 you actually know that a recombination must have happened as this subject obviously carry the risk allele (shows the disease) yet not the marker associated with it, therefore a recombination event happened between the marker and the disease allele.

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  • $\begingroup$ -I've re-read the question a few more times, and what you say makes sense. I've focused too much on the picture than the text itself. My bad, guess I'm a bit tired. I've deleted my answer as yours is obviously better. $\endgroup$ – Nandor Poka Mar 29 '15 at 19:42
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    $\begingroup$ One slight remark though - On molecular level basically everybody is recombinant, but multiple recombinations can restore the original gene / allele combination, practically only those offsprings / individuals are considered recombinant who exhibit different combinations than his/her parents. $\endgroup$ – Nandor Poka Mar 29 '15 at 19:45
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    $\begingroup$ @poka.nandor You are exactly right. In the context of genetic linkage, the term recombinant is usually used at a given locus and not at the genome-wide level this is why III1-5 are considered non-recombinant, they actually are non-recombinant at the disease risk locus. Good point. $\endgroup$ – cagliari2005 Mar 29 '15 at 19:51

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