This lecture uses the auditory system to illustrate Mendelian inheritance. First the earlobes -- a classic example in teaching laboratories, where attached earlobes and pendulous (or free) earlobes are supposed to be inherited as a Mendelian character. Except we now know more than a half dozen genetic markers on different chromosomes that influence earlobe form. This lays the groundwork for a discussion of what Mendel accomplished with his experiments, and the exceedingly rare conditions that allowed him to study dominant and recessive traits. Deafness is a true Mendelian character in many families, but like other genetic conditions we've seen, it's caused by different genes in different families.
The genetic pathway uncovered by the study of deafness allows us to investigate the evolution of hearing on the human lineage. Some fossil evidence is also relevant to this question, and I briefly introduce the site of Sima de los Huesos, where micro-CT study of the middle ear suggests the appearance of a human-like auditory capacity.
- Other features of human anatomy, such as the curvature of the thumb when flexed backward (hitchhiker's thumb) have been proposed to be inherited in a Mendelian fashion. What genetic factors impact this hypothesis?
- Before micro-CT scanning made it possible to study the middle ear bones of ancient people, what other evidence might have been relevant to the evolution of the auditory system?
- How important is hearing to human survival and reproduction?