Question

Identify the type of inheritance pattern given in the following human pedigree.

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

Answer 1

The correct option is B Autosomal recessive

A pedigree chart is a graphical representation of the inheritance pattern of a particular trait in the members of a family. In the given pedigree, members are represented by different geometrical figures and the relationship that they share among themselves is represented by horizontal and vertical lines.

In the given pedigree chart, we can see that in generation I, the male parent is affected, whereas the female parent is normal. In the progeny, the sons and daughters are affected with an approximately equal frequency. This shows it to be an autosomal trait.

It is not an X-linked trait, because the father contributes the X chromosome only to the daughter, not to the son. Similarly, the Y chromosome is passed down to the son, not the daughter. Since the affected father has affected both the son and daughter, it is not a sex linked trait, rather it is an autosomal trait.
Had it been a dominant trait, all the progeny would have been affected. Here we can see that in generation II, a son is normal. Also, in the next generation we can see the trait skipping a generation. This happens in case of a recessive trait. Recessive traits require two identical alleles for their expression. An affected child can have unaffected parents.
Hence, we can conclude that the given pedigree represents the transmission of an autosomal recessive trait.