-4
$\begingroup$

I want serious help here. I'm in first year of medical school and I jut had my second test in Genetics. One question of the test gave us a heredogram and asked us what was the most probable type of genetic disease the family had. There were 2 types of disease that would fit the heredogram. Recessive autosomal and Dominant X-linked. I put both and explained why both them fit. But a lot of my colleagues put only one of them. He said after that he wanted X-linked, because the heredogram had more trends of X-linked diseases. I just don't bite that. This is not the way we calculate probability. For example a autosomal disease can happen in 44 out of the 46 chromosomes while the X-linked only in the X chromossome (1.5 in average). Also, what if recessive diseases are more common than dominant. What if most of the people with X-linked diseases are infertile or if most of them dies. All of these reduces the chance of a given heredogram being Dominant X-linked. We can't say something one based on the trends.

The heredogram is the below.

enter image description here

Do you agree with me? I really need some opinions of other people so we can contest the teacher. Otherwise half of the classroom will get a zero.

$\endgroup$
  • $\begingroup$ This site does not exist to give opinions so people can contest their teachers. It exists to provide objective answers to problems of biology which are of general importance and interest. The title shows a particular petty arrogance which is more likely to antagonise people than gain their sympathy. $\endgroup$ – David Oct 24 at 11:58
4
$\begingroup$

I think you are making this too complicated. You don't have enough data from this diagram to make any sort of rigorous statistical judgement. You are just looking for clues.

Note that in the first generation offspring of the affected male founder, all the female offspring are affected and none of the male offspring are affected. This is exactly the pattern you would expect for a dominant X-linked trait.

For an autosomal recessive trait, you'd expect roughly equal numbers of male and female offspring to be affected in the first generation where once parent is affected and the other is a carrier. This is true regardless of all the red herrings you raise about the number of autosomes, fertility, and lethality. So this is a little odd looking for an autosomal recessive trait. It's a tiny sample size, so not impossible, not definitively ruled out, just a little unlikely.

Diagrams like this don't usually give you a definitive result, but they can indicate where to start looking.

$\endgroup$
  • 1
    $\begingroup$ @JoãoMaldonado No, you are miscalculating the likelihoods. The likelihood of the data given the Dominant X-Linked model is exactly 1. For the first generation of offspring for an affected father and unaffected mother, the female children are necessarily affected, and the male offspring are necessarily unaffected. No other pattern is possible. $\endgroup$ – Charles E. Grant Oct 23 at 17:35
  • 1
    $\begingroup$ "The question is: what is the probability that an affected father and a non-affected mother have 4 children, 3 females affected and one male not affected" No, you're calculating the wrong thing. All we're interested in here is the probability of observing the disease or not given the inheritance model. The genders of the offspring are treated as a given for the purposes of this estimation. I'd suggest that you review this material with your professor, because you're working from some fundamental misunderstandings. $\endgroup$ – Charles E. Grant Oct 23 at 18:52
  • 1
    $\begingroup$ Sorry but you are misunderstanding statistics at least as involved in interpreting the diagram. In formal terms we're using the diagram to do a maximum likelihood test to choose between two models. You assume one model, calculate the probability of the observed pattern of disease given model, then assume the other model and calculate the probability of the observed pattern of disease. The probability of the observed pattern of disease given a model is called the likelihood. You choose the model with the greater likelihood. No need to take ratios. $\endgroup$ – Charles E. Grant Oct 23 at 19:53
  • 1
    $\begingroup$ You are introducing a host of complications that are unneeded for this question. Given two models we're simply asking which model "better" explains the data right in front of us. Check out any text on statistical genetics and really, please talk to your professor. $\endgroup$ – Charles E. Grant Oct 23 at 19:57
  • 1
    $\begingroup$ You are introducing a host of complications that are unneeded for this question. Given two models we're simply asking which model "better" explains the data right in front of us. Check out any text on statistical genetics and really, please talk to your professor. I hate to engage in credentialism, but you started it: Me: MS Applied Math and 15 years working as a bioinformatics programmer, and of course your professor is presumably a trained geneticist, so they "outrank" us both. Of course, I could be wrong, but so far it's two against one. Seriously, consider that you may be wrong. $\endgroup$ – Charles E. Grant Oct 23 at 20:04

Your Answer

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

Not the answer you're looking for? Browse other questions tagged or ask your own question.