While this appears to be a form of FRD, it is probably not illegal. In most states, marital status or familial discrimination is not against the law. Even if your state does recognize these forms of discrimination as illegal, being forced to temporarily cover for another employee is not likely to be considered serious enough to succeed in a discrimination complaint. Many companies have adopted “flextime” or other “family-friendly” policies which make it easier for workers with children to balance work and family commitments. If you still believe you have been treated unfairly, you may wish to discuss this situation with coworkers, your supervisor, or your company's personnel department to determine whether the company can adopt leave policies or practices that treat employees with and without children the same, or whether the department's work can be reallocated so that no one person is required to assume the burden of a worker's absence for family reasons.