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Pedigree analysis is a process of establishing a virtually correct inheritance pattern of a genetic condition in a proband's family. I call it virtually correct because in many a cases we assume the genotype of a parent to be something by weighing the genotypes of the offsprings.

Now in a given problem like in this one where the trait follows autosomal dominant inheritance pattern I have problem deciding the genotype of I-3. enter image description here

My opinion:

The genotype of II-6 is AA'. (Obtained from a calculation which I have decided to not to mention as it will make it lengthy and complicated)

(here A=Normal allele on autosome and A'=Mutant allele on autosome)

II-6 receives allele A from mother I-4 (who is obviously AA) and allele A' from father I-3. Now the genotype of the father could be either AA' or A'A'.Since the probability of having a AA' female child becomes 100% if we assume the genotype of the father to be A'A' so the genotype of the father I-3 is A'A'.

But one of my instructor says that it cannot have a genotype A'A' because it is fatal and I am not convinced with his reasoning.

In Huntington's disease presence of one HTT gene is enough to cause death of the person. It is a dominant lethal allele. While in achondroplasia, an autosomal dominant disorder the mutant allele when present in two copies is fatal.It is a recessive lethal allele. A heterozygous person survives with symptoms of Achondroplasia.

So death is not always caused by two copies of a mutant gene of a dominant autosomal trait. It may be caused by only one copy of gene as in Huntington's disease or may not be caused at all(I've no proof of this. Its my assumption).

So in a pedigree where nothing has been mentioned about the lethality of the genes concerned we shouldn't assume them being recessive lethals. This is all I think. Am I correct?

Finally: Should the genotype of I-3 be A'A'?

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I-3 can be either A'A' or AA'. In Huntington's disease or Achondroplasia, the father could still survive to adulthood and reproduce. If the father had a chromosomal disorder such as Kallman's syndrome, he would be unable to reproduce. However, this is not a simple mutation, and it is not heritable. You should ask your instructor to clearly define the problem.

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