2
$\begingroup$

I have the following heredity tree: enter image description here

and I need to decide which heredity model it fits the most, with the least number of assumptions, from the following models:

  • autosome dominant
  • autosome recessive
  • X-linked dominant
  • X-linked recessive
  • Y-linked
  • mitochondrial

I think the answer is autosome recessive because of the following analysis: enter image description here

and the only assumption which is significant (the other assumptions are about the non-family members which are that they are carriers of the hilliness, it isn't said that the illness is rare, so in examining any model different assumptions about the out-of-family members are made) is the assumption on the last generation where there is two ill persons instead of one if we cross their parents alleles: AA, Aa, aA, aa, but we got 2 aa and AA, Aa, but that can be explained by little statistics in the family, so it is hardly even an assumption at all.

Then I've been told that it is a rare illness and that it autosome illness and not a linked one, its also said as a clue that there may be some Mendelic exceptions (known exceptions from the Mendel heredity models). So In my understanding its more probable that the model with this data is autosome dominant with father genomic imprinting the following way": enter image description here As you can notice for the classic autosome dominant model the circled one should be black (ill) but they are not, so my assumption according to the clue that the ill alleles are imprinted in fathers (males) and expressed only when they inherited from mothers (females).

Till this part I would like you to check me did I any wrong in this analysis, but my major question that made me doubt my analysis is the following:

In addition to previous data, is is now given the the ill girl in the 4 generation (which is black circle on the image) got pregnant from one of two males:

male A: healthy male with both parent healthy and and Homozygous both. male B: ill male with one parent Homozygous and the second parent Heterozygous

It is also given that no matter who is the father, the chances of getting an ill child are 50%.

From the previous analysis I decided that the heredity model is dominant autosome, so the first contradiction to that, that one of the parents of male B is Heterozygous, means he carries the ill allel and the healthy allel, but then in dominant model he has to be ill, but it said both parent are healthy.

The second thing that should correlate to the model is the chances computation. If the girl is Aa as was established by previous analysis then if the father is male A, i.e. his alleles are Aa (because he is ill with parents which are aa and Aa), then the chances for a ill child are 3/4 because the options are {(A_father, A_mother),(A_father, a), (a, A_mother), (a,a)} and if the father is male B then his alleles are aa because his parent are both aa, then the chances of an ill child are 1/2, so the total chances are (3/4 + 1/2) / 2 which is 5/8 which isn't 50%.

Or maybe this is wrong for male 1, because if the ill phenotype is delivered only by women (mothers) because of male genome imprinting, then for the option {(A_father, A_mother),(A_father, a), (a, A_mother), (a,a)} only two will make the child ill and not 3 : (A_father, A_mother) and (a, A_mother), then the calculation will be (1/2 + 1/2) / 2 which is 50% indeed, and for the parent of male B being Heterozygous but not ill can be explained by the assumption that he/she got the allel from his father.

Is it correct that the exception of genome imprinting is for the whole illness and not for some ancestry ? I'm not sure if genome imprinting is a phenomena for the ill allele (the whole illness) or per concrete ancestry? (in the second option it will be that actually I do another assumption on the male B ancestry, that the imprinting is also as in the given ancestry tree). But this make some sense' isn't it ?

It made me doubt because if the model was autosome recessive, then the computation would give me exactly 50% without thinking and explaining about the male's B Heterozygous parent (which then would be healthy just a carrier), but then the previous tree seem to be much more complicated because if you only look at the first generation in this model: if the father is AA (A - healthy dominant allele, a - ill recessive allele) and the mother is aa, then they shouldn't have any ill children (because their cross is {Aa, Aa, Aa, Aa}), but they do, that's why I started to doubt is it dominant autosome as I concluded.

$\endgroup$

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.