His parents were probably Samuel and Elizabeth. They were found in Jackson County, GA, in 1850 and Milton County, GA, in 1870. In 1870, Andrew and Mary C. had 5 children, and his parents were living with them. Andrew reported his birthplace as GA in 1870, and both parents reported theirs as SC, but they were more likely correct in 1850 when Andrew reported being born in SC and Samuel reported being born in Ireland.
I have no known relationship to this Alexander family and hope that you will consider Roger's urging to do the DNA test.