Abstract: ObjectiveThe effects of inbreeding on growth traits,genetic diversity and biomineraliazation gene expression were evaluated with an aim to clarify genetics principle of trait degradation in first filial generation caused by inbreeding and provide valuable information for breeding shellfish varieties.MethodTwo yellow colored full-sib families (M and N) were taken as parents,four family combinations(F1,M1 ♀ ×M1♂;F2,M1 ♀ ×N1♂;F3,N1 ♀ × M1♂;F4,N1 ♀ ×N1♂)were constructed based on factorial design.Growth traits of the shells at adult stage from the four families were compared.Genetic diversity of the four families were analyzed by nine pairs of SSR primers.The expression differences among nacrein,pearlin and pif1 77 genes were evaluated by real-time PCR.ResultThere were significant differences on mean shell length,mean shell height,mean shell width,mean total weight and mean shell weight among the four families(P<0.05).Mean shell length,mean shell height,mean shell width,mean total weight and mean shell weight of inbreeding families (F1 and F4) were smaller than those of hybrid families (F2 and F3),growth trait degradation rate (ID) was 11.7%-29.4%.The observed heterozygosity values of F1,F2,F3 and F4 were 0.2525,0.3116,0.3178 and 0.2752,while the expected heterozygosity values of F1,F2,F3 and F4 were 0.3451,0.3822,0.4005 and 0.3549.Average polymorphism information content(PIC) of F1,F2,F3 and F4 were 0.4591,0.5389,0.4878 and 0.4810.The expressions of biomineraliazation genes nacrein,pearlin and pifl77 in hybrid families(F2 and F3) were higherer than those in inbreeding families(F1 and F4).There was positive correlation between mean growth traits and relative expressions of the three biomineraliazation genes.