The effect of genotype and slaughter age on gonadal development, body development and the correlations between these measurements were studied in five pig genotypes (Genotype 1, Genotype 2, Genotype 3, Genotype 4 and Genotype 5) consisting of 112 gilts and 112 boars with initial live weights varying between 25 – 30 kg. The pigs were group-housed in commercial type grower houses and fed a diet consisting of 14 MJ/kg energy, 18 % CP and 1.1 % lysine during the growth period up to a live weight of 65 kg, followed by a diet consisting 13.5 MJ/kg, 16 % CP and 0.9% lysine from 65 kg to 90 kg and then a diet consisting of 13.2 MJ/kg, 15 % CP and 0.7% lysine from 90 kg onwards. Pigs were slaughtered at 116, 130, 144, 158, 172, 186, 200 and 214 days of age. Gonadal growth and development were measured in gilts (ovary length, ovary width, ovary thickness, ovary weight, ovary volume, follicle number, and size of the largest follicle), boars (testis length, testis width, testis weight and testis volume) and body development parameters (slaughter weight, warm carcass weight, carcass length, chest depth, dressing percentage and P2 backfat thickness) were compared. Differences between means were tested using breed, sex and slaughter age as fixed effects, while the relationships between gonadal and body development parameters were evaluated by means of correlation analysis. Genotype 5 had a significantly shorter ovary length than Genotype 4 and Genotype 2. Genotype 2 gilts also had heavier ovaries and larger ovary volumes than Genotype 5 gilts. In boars, Genotype 2 had significantly heavier testes weights than Genotype 5 boars. Genotype 5 boars also tended to have smaller testis volumes than Genotype 2 boars. In body development, Genotype 2 gilts and boars were superior to the Genotype 5 in terms of slaughter and warm carcass weights, while Genotype 3 seconded Genotype 2. The average P2 backfat thicknesses were 11.88 mm and 13.68 mm for boars and gilts respectively. Correlations between gonadal and body development parameters were low to moderate in the gilts (r = -0.305 to 0.555) and moderate to high in boars (r = 0.560 to 0.871). However, dressing percentage, follicle number and size of the largest follicle correlated poorly with all other measurements. It is concluded from the study that although Genotype 5 do not grow to the same size and at the same rate compared to the other genotypes, they appear to be the most suitable for the production of top quality pork in terms of its low backfat thickness. Genotype 5 pigs were also characterized with a slower gonadal growth and body development compared to Genotype 2 pigs. Results from this study suggest that selecting against backfat may delay gonadal development and sexual maturation in pigs. / Dissertation (MSc (Agric) Production Physiology)--University of Pretoria, 2007. / Animal and Wildlife Sciences / unrestricted