INTRODUCTION Brassica juncea (L.) Czern & Coss (AABB) is the second most important edible oilseed crop in India after the soybean (Saroj et al., 2021). Cytological studies have revealed that B. juncea is a natural allotetraploid or amphidiploid (2n=36), of two diploid species viz., Brassica rapa (also known as Brassica campestris) (AA) (2n=20) and Brassica nigra (BB) followed by natural chromosome doubling (2n=16) (Tomar et al., 2017; Tarkeshwar et al., 2022a). The species has probably evolved in the Middle East, where its putative diploid progenitors are sympatric (Prakash and Hinata 1980; Singh et al., 2022). India is the second-largest rapeseed–mustard-cultivating country in the world after China and third in production after China and Canada (Kumari et al., 2019). During 2018–2019, rapeseed–mustard was cultivated over an area of 5.96 million hectares with production and productivity of 8.32 million tons and 1,397 Kg/ha, respectively in India (Directorate of Economics & Statistics, and Dac & Fw., 2019). The existence of genetic variability in any plant population or genetic stock is the pre-requisite for any crop improvement programme. Available genetic stocks with effective variability are the store house of desirable genes for various traits. Hence, before planning a breeding programme, knowledge of important genes and extent of variability present among genotypes is utmost criteria. Thus, the present investigation has been conducted to estimate the extent of variability and heritability for various traits in Indian mustard. MATERIALS AND METHODS Experimental Details. The experiment was carried out at the Research Farm, Institute of Agriculture Science, Bundelkhand University, Jhansi (U.P.). The basic material in the present investigation comprised ten varieties/strains of Indian mustard namely, Urvashi, Azad Mahak, R.H.30, Pusa Mustard-25, Kranti, Pusa Mahak-7, NDR-8501, Bio-902, CS-52 and NRCDR-2 were taken from the germplasm maintained at Oilseed Section, Department of Genetics and Plant Breeding, Chandra Shekhar Azad University of Agriculture and Technology, Kanpur (U.P.). Ten genetically diverse genotypes of Indian mustard were subjected to diallel fashion mating design (excluding reciprocals) was attempted during rabi 2019–2020. The F1 seeds of 45 crosses were advanced during the Rabi season of 2020-2021 to raise the F1's and were selfed in order to obtain F2's seeds. The parents were also maintained through selfing. The experimental material comprising 100 treatments viz., (10 parents + 45 F1's + 45 F2's) were evaluated in RBD with three replications during rabi 2021-2022. Each parent, F1's and F1's planted in one row of 5m long 45cm apart; plant to plant distance was maintained 15cm by thinning. All the recommended packages of practices were adopted for raising a good crop. Observations Recorded. Five competitive plants in parents, F1's as well as F2’s were taken randomly for each treatment in each replication and tagged for recording fourteen quantitative traits viz., days to 50% flowering, days to maturity, leaf area index (cm2), plant height (cm), number of primary branches per plant, number of secondary branches per plant, length of main raceme (cm), number of siliquae on main raceme, number of seeds per siliqua, biological yield per plant (g), harvest index (%), 1000-seed weight (g), oil content (%) and seed yield per plant (g). Statistical Analyses. Theaverage from the five competitive plants was calculated and data were then subjected to estimate the various statistical analyses as Analysis of variance (ANOVA) for R.B.D. (Panse and Shukhatme 1988), Heritability in broad sense (Hanson, 1963) and Genetic advance and genetic advance in percent of mean (Johnson et al., 1955), genotypic coefficient of variation (GCV) and phenotypic coefficient of variation (PCV) (Burton, 1952) respectively. RESULTS AND DISCUSSION Analysis of Variance. The combined analysis of variance (ANOVA) for parents and their forty-five each F1's and F2's for all the above fourteen characters’ mean sum of squares are presented in Table 1. Significant differences were observed among the treatments (parents, F1's and F2's) for all the fourteen attributes under investigation. ANOVA for parents and hybrids showed significant differences among all the traits studied except length of main raceme in F1 and plant height and oil content in F2. Similar observations were also reported by Mahendra et al. (2020). Mean performance of Parents, F1's and F2's. The mean values for all the fourteen characters for parents, F1's and F2's, and variability expressed in terms of mean and ranges are presented in Table 2. The variation amongst the parents, F1's and F2's was found highly significant for all the fourteen attributes. However, the magnitude varied from character to character. In comparison with parents and F1, the variation in parents was ranged from 51.00 to 56.00 days for days to 50% flowering with mean of 53.40 days, 124.67 to 132.67 days with mean value of 128.47 days for days to maturity, 172.97 to 204.41 cm with mean of 187.93 cm for plant height, 72.42 to 91.04 cm with mean of 80.60 cm for length of main raceme, 1.52 to 2.02 with mean of 1.80 for leaf area index, 5.76 to 8.29 with mean 6.89 for number of primary branches/ plant, 16.26 to 21.27 with mean of 18.92 for number of secondary branches/plant, 264.74 to 375.24 with mean of 333.96 for number of siliquae/plant, 13.46 to 16.45 with mean of 15.10 for number of seeds/siliqua, 3.16 to 4.94 with mean of 4.30 g for 1000-seed weight, 52.07 to 60.57 g with mean of 57.03 g for biological yield/plant, 25.46 to 29.19 with mean of 27.39 per cent for harvest index, 37.05 to 41.56 with mean of 39.62 per cent for oil content and 13.40 to 17.45g with mean of 15.62 g for seed yield/plant. Among the F1's, the range lies between 46.67 to 54.33 with mean of 50.82 days for days to 50% flowering, 117.33 to 130.67 with mean of 124.98 days for days to maturity, 162.65 to 200.71 with the mean of 183.12 cm for plant height, 73.83 to 87.34 with mean of 80.32 cm for length of main raceme, 1.64 to 2.13 with mean of 1.90 for leaf area index, 6.10 to 9.08 with mean of 7.35 for number of primary branches/plant, 17.64 to 22.92 with mean of 20.01 for number of secondary branches/plant, 288.38 to 392.98 with the mean of 349.85 for number of siliquae/plant, 14.36 to 17.54 with 15.92 for number of seeds/siliqua, 3.38 to 6.04 g with the mean of 4.68 for 1000-seed weight, 55.67 to 67.64 g with mean of 60.82 g for biological yield/plant, 24.18 to 33.44 with mean of 28.32 per cent for harvest index, 37.45 to 44.12 with mean of 40.47 per cent for oil content and 14.48 to 20.48 g with the mean of 17.21 g for seed yield/plant. In comparison with parents and F2, the variation in parents ranged from 52.67 to 57.67 days for days to 50% flowering with mean of 54.93 days, 126.33 to 134.33 days with mean value of 130.13 days for days to maturity, 168.00 to 199.44 cm with mean of 182.96 cm for plant height, 67.20 to 82.19 cm with mean of 74.56 cm for length of main raceme, 1.35 to 1.84 with mean of 1.865 for leaf area index, 4.60 to 7.12 with mean 5.72 for number of primary branches/ plant, 13.57 to 18.88 with mean of 16.23 for number of secondary branches/plant, 245.33 to 362.24 with mean of 312.12 for number of siliquae/plant, 12.51 to 15.17 with mean of 13.79 for number of seeds/siliqua, 3.05 to 4.86 with mean of 4.33 g for 1000-seed weight, 50.72 to 57.59 g with mean of 54.70 g for biological yield/plant, 23.86 to 28.15 with mean of 26.61 per cent for harvest index, 36.60 to 40.81 with mean of 39.27 per cent for oil content and 12.56 to 16.14g with mean of 14.56 g for seed yield/plant. Among the F1's, the range lies between 45.00 to 53.67 with mean of 49.53 days for days to 50% flowering, 116.00 to 127.33 with mean of 122.39 days for days to maturity, 159.32 to 195.87 with the mean of 178.66 cm for plant height, 73.20 to 90.85 with mean of 80.41 cm for length of main raceme, 1.62 to 2.19 with mean of 1.94 for leaf area index, 6.09 to 9.22 with mean of 7.42 for number of primary branches/plant, 17.96 to 23.38 with mean of 20.27 for number of secondary branches/plant, 292.14 to 401.75 with the mean of 355.53 for number of siliquae/plant, 14.13 to 17.86 with 15.99 for number of seeds/siliqua, 3.47 to 6.13 g with the mean of 4.73 for 1000-seed weight, 55.95 to 68.68 g with mean of 61.62 g for biological yield/plant, 23.56 to 33.09 with mean of 28.26 per cent for harvest index, 37.46 to 44.69 with mean of 40.62 per cent for oil content and 14.66 to 20.73 g with the mean of 17.41 g for seed yield/plant. When all the three populations i.e., parents, F1's and F2's were considered simultaneously, the extent of variability differed from parents’ vis-a-vis F1 hybrids and parents’ vis-a-vis F2 progenies. Similarly, F1 hybrids expressed greater variability in eleven attributes out of fifteen in comparison of the F2 progenies in their investigation had also reported the almost similar results for most of these traits for parents and their F1’s. Heritability and Genetic advance. The estimates of heritability (broad sense) and genetic advance in per cent over mean of the trait for all the 14 characters in F1 and F2 generations were determined. The findings on these parameters are given in Table 2. The estimates of heritability in broad in sense varied from 54.15% (oil content) to 95.27% (1000-seed weight) in F1 and 35.15% (days to maturity) to 95.66% (1000-seed weight) in F2 generation, respectively. In F1 generation estimates of heritability in broad sense were observed high for 1000-seed weight (95.27%) followed by number of primary branches per plant (89.77%), length of main raceme (88.01 %), biological yield/plant (86.51%), plant height (85.1%), number of siliquae/plant (84.05%), seed yield/plant (83.11%), leaf area index (80.66%), days to 50% flowering (79.30%), number of secondary branches/plant (78.93%) and harvest index (76.58%). Moderate values of heritability were observed for days to maturity (57.14%) and oil content (54.15%). In F2 generation estimates of heritability in broad sense were observed high for test weight (95.66%) followed by number of primary branches/plant (93.66%), number of siliquae/plant (88.29%), seed yield/plant (86.83%), number of secondary branches/plant (85.66%), leaf area index (84.36%), length of main raceme (82.35%), days to 50% flowering (79.75%), and biological yield/plant (79.09%). While, moderate values of heritability were observed for oil content (54.84%) and low values for days to maturity (35.15%). The estimates of genetic advance in per cent over mean was calculated for all 14 characters. The arbitrary scale for genetic advance suggested that if estimated value of genetic advance if, less than 10%, it is low, if it is ranged between 10 to 20 per cent, it is medium and above 20%, it is high. The estimates of genetic advance in per cent over mean of the character ranged from 3.92% (days to maturity) to 35.05% (1000-seed weight) in F1's and 2.28% (days to maturity) to 35.33% (1000-seed weight) in F2 generation. High estimates for genetic advance were recorded for 1000-seed weight and number of primary branches per plant (23.02 and 28.15% in F1 and F2) respectively. Moderate genetic gain was observed for number of leaf area index (14.54 and 15.83%), number of secondary branches per plant (14.07 and 17.23%), number of siliquae/plant (16.52 and 18.28%), harvest index (13.01 and 13.02%) and seed yield per plant (16.74 and 17.85%) in both F1 and F2 generations, and plant height (10.39%) in F1 generation only. Rest of the traits in both the generations exhibited low estimates of genetic advance in percent of mean. Similar findings were reported by Rout et al. (2019); Gadi et al. (2020). Genotypic and Phenotypic coefficient of variation. The higher values of genotypic coefficient of variation (GCV) along with phenotypic coefficient (PCV) were recorded for 1000-seeds weight followed by number of primary branches per plant as presented in Table 2.