Altogether 115 animals representing 5 genetic groups: 3 purebred Booroola Merino, Corriedale and Olkuska, and 2 Booroola crossbreds were included in the studies. In total 6 alleles from 97 bp to 119 bp in microsatellite OarAE101, and 5 alleles from 162 bp to 174 bp in BM1329 were identified. The marker of FecB gene presence seems to be an allele of 97 bp in the case of microsatellite OarAE 101 and 162 bp in the case of BM1329. Significant differences FecB carriers (Booroola-Corriedale) and non-carriers (Corriedale) in birth weight and at weaning at 100 days (males and females from twins) as well as weight gain during the first 28 days and 100 days were found. Purebred lambs showed higher values of the investigated traits.