The combined effects of dominance and inbreeding on covariances between relatives are still poorly understood in maize (Zea mays L.) populations. Our objectives were to address the following questions: (i) What is the importance of dominance in a maize synthetic? (ii) How does inbreeding affect the genetic variance among individuals in a maize synthetic. (iii) How do the covariance parameters compare between populations? (iv) How does breeding design impact estimators? We estimated covariance components for inbred relatives in the maize synthetic BSCB1(R)C13. Previous estimates of covariance parameters have been used to explain the ineffectiveness of inbred progeny selection in the stiff-stalk population BS13. We found that the dominance variance was larger than the additive variance for grain yield, whereas the additive variance was larger than the dominance variance for all other traits. Negative estimates of the covariance between additive and homozygous dominance deviations were found for all traits with the exception of traits associated with reproductive maturity, suggesting a negative relationship between inbred and outbred performance. The correlation between genotypic values and breeding values was lower for grain yield than for any other trait. Our results were similar to previous results found in the stiff-stalk maize population BS13, suggesting similarity in structure among populations.