The male sterile maintainer line, N69-2774, segregates 1:1 for fertility and sterility (Brim and Young, 1972). The fertile and sterile siblings are indistinguishable, except for pollen grain morphology, until the onset of maturity. For controlled hybridization the phenotypic similarity is an obstacle.
Brim, C. A. and Kenworthy, Sharon
"Research Notes: Identification of male sterile soybean plants by pollen
Soybean Genetics Newsletter: Vol. 4
, Article 29.
Available at: https://lib.dr.iastate.edu/soybeangenetics/vol4/iss1/29