Question

Question 3: (4 points) Consider the following pedigree: O O 1 2 3 4 5 Characterize each of the following modes of inheritance as: Impossible, Possible, or Most likely. Justify your answers.

this is all that im given

Answer 1

As the I and IV generation doesn't have any affected persons the mode of inheritance cannot be dominant but to be recessive type hence option a. Autosomal dominant and option c. X-linked dominant are impossible.

Option b. Autosomal recessive is also not possible, as only the male were affected in each generation. Therefore, the disease is getting transmitted through the sex chromosmes (X or Y) rather than autosomal chromosomes ( first 22 pairs)

Option e. Y-linked is also impossible. As, if it occurs then all the males in the pedigree should have the disease which is not the case here.

Option d. X-linked recessive is the correct mode of inheritance for the above pedigree. Because male has only one set of X-chromosome any defect in this whether it is dominant or recessive will result in the diseased condition in male while female may act as a carrier whithout showing any diseased symptoms as they have another X-chromosomes free of defect which compensates for the defective chromosome. In generation I, I-1 carries one defective X-chromosome and one normal X-chromosome and she passed the defective one to the II-4 individual which results in a diseased condition. Same as thas with II-2 acting as a carrier, passing disease to  III-5, III-6. Here, III-4 doesn't show any diseased symptoms as he got the normal X-chromosome from the II-2. In IV generation the IV-1 individual acts as a carrier having one defective X-chromosome as she got it from mother whose generation has the defective chromosome, thereby passing it to their three male children in the V generation namely V-2, V-3 and V-4.

Now, coming to the last option f. Assuming that individual V-1 marries to the phenotypically normal male, as we know that V-1 acting as a carrier with one defective X-chromosome if they have a male child then their is a 50 % chance for the male child to be affected considering one normal and one affected X-chromosome within her and if they have a female child then the child will be 100% acts a carrier and doesn't develop disease as the defective X-chromosome will be compensated by the normal X-chromosome from her father.