29 CFR § 1604.3 - Separate lines of progression and seniority systems.
(a) It is an unlawful employment practice to classify a job as “male” or “female” or to maintain separate lines of progression or separate seniority lists based on sex where this would adversely affect any employee unless sex is a bona fide occupational qualification for that job. Accordingly, employment practices are unlawful which arbitrarily classify jobs so that:
(1) A female is prohibited from applying for a job labeled “male,” or for a job in a “male” line of progression; and vice versa.
(2) A male scheduled for layoff is prohibited from displacing a less senior female on a “female” seniority list; and vice versa.
(b) A Seniority system or line of progression which distinguishes between “light” and “heavy” jobs constitutes an unlawful employment practice if it operates as a disguised form of classification by sex, or creates unreasonable obstacles to the advancement by members of either sex into jobs which members of that sex would reasonably be expected to perform.