Originally a chemist, he was recruited after the Second World War to work on animal genetics on behalf of the British government, and continued in this sphere until his retirement in 1985.

Annie Robertson died within days of his birth, and Alan was brought up by his aunt Bessie Grace on the family farm at Halewood, near Liverpool.

[6] In an obituary study of Robertson, William G. Hill wrote, "At this stage Alan could have had no more than a passing acquaintance with genetics, but he did have a firm understanding of practical farming.

"[6] After spending nine months in the US receiving training in genetics and animal breeding with Sewall Wright and Jay Laurence Lush, Robertson took up his work at NABGRO, based in Edinburgh.

The organisation was split into two parts, and Robertson became a member of the Unit of Animal Genetics, where he spent the rest of his career.

[8] At first he worked on improving dairy cattle using mathematical and statistical methods to devise the most effective breeding programmes.

He did much to introduce widespread use of artificial insemination in dairy cattle,[6] and worked for many years on estimating genetic effects that influence quantitative traits, and he developed what became known as the "secondary theorem of natural selection.