We designed and tested the polymorphic gene MC4R in relation to live weight and weight gain of Slovak native chicken breed Oravka kept at the farm of NAFC-RIAP Nitra, which was selected for future breeding by phenotype. We monitored the weight of chickens at the age of 5, 12 and 20 weeks and calculated the weight gain for each period. The design of detection oligonucleotide primers for MC4R gene detection in DNA samples was based on the sequence available in GenBank, NCBI. Statistical analysis was done using the SAS 9.2 Stat software. We identified the GG genotype in only one hen. The number of GT heterozygotes was significantly higher in both hens (110) and roosters (16) after a long-time directive selection. The TT genotype was found in 9 hens and 5 roosters. The GG genotype was detected with a frequency of 0.71%, the GT genotype with a frequency of 89.36%, and the TT genotype with a frequency of 9.93%. The frequency of allele G was 0.454 ± 0.030; and that of allele T was 0.546 ± 0.030. At the age 12 weeks, homozygous hens were heavier (1111.33 ± 44.96 g) than heterozygous hens (1108.15 ± 13.17 g). Similarly, rooster homozygotes were on average heavier (1581.47 ± 59.11 g) than heterozygotes (1389.69 ± 34.40 g). WG 1 was significantly higher in homozygote animals TT (P < 0.01). The same pattern was also observed at the age of 20 weeks. Homozygous TT animals achieved a higher average weight, i.e., the weight of hens was 1947.25 ± 74.35 g, the weight of roosters was 2719.91 ± 87.75 g; meanwhile, for heterozygous animals, the weight of hens was 1888.78 ± 21.78 g, and the weight of roosters was 2525.99 ± 56.89 g. We concluded that in the selected Oravka “mutant” allele T is preferred.