Final answer:
Ellis Marsalis was the head of a prominent jazz dynasty in the mid 1900s, known for his contributions to modern jazz and his family of talented musicians.
Step-by-step explanation:
The head of a musical jazz dynasty in the mid 1900s was Ellis Marsalis. As an influential jazz pianist and educator, Marsalis played a significant role in modern jazz. He also raised a family of renowned musicians, with sons Branford and Wynton Marsalis becoming prominent figures in contemporary jazz.
Unlike the other choices listed such as Johann Sebastian Bach, Berry Gordy, and Ludwig von Beethoven, who were associated with classical music and Motown respectively, Ellis Marsalis contributed directly to the jazz genre, which has its roots in African American communities and had a stronghold in areas like Harlem during the early 20th century.