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                      GrandParents                  -- 1
                            |
                            |
                -------------------------
                |S                      |D          -- 2
            ---------                ----------
            |S(A)   |D               |S(A)    |S(A) -- 3
            |S      |D                              -- 4

S = Son D = Daughter (A) = Affected with a degenerative disease.

The question is how to identify the type of inheritance of the disease - whether X-based, Autosomal dominant or recessive. Which sex of fourth generation are likely to have the disease? Does it depend on the disease?

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2  
This pedigree can not lead to one absolute answer. There should be more information. –  biogirl Dec 10 '13 at 16:56
    
And also, Can you present your ideas on this question? –  Satwik Pasani Dec 11 '13 at 11:10

2 Answers 2

We are missing some information, but (by what I recall from my little inheritance genetics knowledge) it looks X-based. I will make up some of the missing information. Lets say, an "affected X" is H (I wanted to use x, but it will be harder to see), HY will be diseased, XH will be carrier, but not diseased. XX/XH means unclear if carrying.

Grandparents:               XY-------------XH
                                    |
                           -------------------
                           |                  |
Parents&Partners      XH---XY                 XH---XY
                         |                        |
                     ---------               ----------
                     |        |              |         |
F1                  HY       XX/XH          HY         HY
                     |        |
F2                  YX       XX/XH

The F2 son is not carrying (because he is healthy). The F2 daughter could be carrying if her mother was, but even then does not have to. Unless her dad was also carrying it.

Please correct me, if I am totally off.

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This would be one possibility. I guess you say that it looks like the disease is X-linked because all sick people are males but we only have three sick people. Among three guys, the probability that all have the same sex is $\frac{1}{4}$! Moreover, for your tree to be likely you would need that the allele causing the disease is present in high frequency in the population (which is rather rare for a X-linked disease)! I think it is better just to say that we don't have enough information to infer an inheritance type. –  Remi.b Dec 11 '13 at 23:56

Short answer

As said @Biogirl, "This pedigree can not lead to one absolute answer". We miss information in order to answer this question.

Long answer

Should we make the assumption that the allele causing the disease (A) necessarily come from the lineage you draw or can it come from the partners of the second generation (offspring of GrandParents)?

Then, depending on your answer to this question...

If yes (= if the allele causing (A) comes from the GrandParents):

  • This genealogy is impossible if the type of inheritance is as simple as you suggested (either recessive or dominant, no epigenetics, no epistasis, no effect of the environment, no protein-mediated disease via the vomb,... even if it can be autosomal, sexual or mitochondrial genes)

If no (= if the allele causing (A) do not come from the GrandParents only)

  • Everything is possible (from the space of possibilities that you suggested)
    • some possibilities would be more probable than others (depending especially on the allele frequencies)
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