Question

Study the pedigree given below. What is the most likely mode of inheritance for this disease?

• A. Autosomal recessive
• B. Y linked disorder
• C. Autosomal dominant
• D. X-linked recessive

Answer 1

The correct option is D X-linked recessive

A pedigree chart is a graphical representation of a family history showing the transmission of a particular disease or trait. The pattern of inheritance of Mendelian disorder can be traced in a family by pedigree analysis.
We know from Mendelian studies that recessive traits are the ones which can express themselves only in homozygous condition. The dominant traits can express themselves even in heterozygous condition.
Autosomal traits are the ones whose genes are located in the autosomes and sex linked traits are the ones whose genes are located on the sex chromosomes. Sex chromosomes include both X and Y chromosomes.
In the given pedigree chart, we can see that only males are affected. This could easily give us an idea that it is Y-linked, but it is not. If it were a Y linked trait, all the males would have been affected.
The males in generation II are affected but the parents in generation I are not affected. Also, in generation II, the individuals 5 and 6 are parents to individuals 5, 6, 7, 8, 9 in generation III, the male parent is affected, but none of the progeny are affected. Here, we can see that the trait is skipping generations. This is a characteristic feature of recessive traits. So, the given pedigree indicates a recessive trait.
It is not an autosomal trait. In autosomal traits, both males and females have an equal chance of being affected, which is not seen in this case.
Hence, eliminating all possibilities, we can say that the pedigree chart depicts the transmission of an X-linked recessive trait.