I find one McDowell family living in Murphysboro, Jackson Co. when the census was taken on June 22, 1870. The family reads as follows:

Richard 31 Born-IL Coal Miner US Citizen
Casandra 24 Born-Ohio Keeping house
Lizzie 3 Born-IL
Eisak 1/12 Born-IL

As you know the information on the census is not always correct so could Richard have been born in Scotland ? Could he be an older brother, cousin, etc. of Alexander?

I do not find this RIchard McDowell living in the state of Illinois on the 1880 census.

Good luck in your search.