Familial Incidence of Stuttering
At least 25 studies have investigated whether stutterers are more likely to have relatives who stutter. The results varied so widely that they don’t prove anything with certainty. 
A 1940 study of five generations of an Iowa family, with stuttering in all generations, “proved” that the family had a “tone” (parental behaviors) that was conducive to stuttering. A 1986 reexamination of the study concluded that “genetic transmission has been judged to provide an equally viable explanation for the data.” 
More recent research has found that when a preschool stuttering child has several family members who stuttered into adulthood, there is a greater chance that the child’s stuttering will persist. If the child has family members who stuttered as children and then recovered, there is a greater chance that the child will follow this pattern. 
Dopaminergic Genes and Stuttering
A 1996 study  of 225 persons with Tourette syndrome, their families, and controls investigated three genes associated with dopamine.
The three genes were associated with stuttering, Tourette’s, and obsessive-compulsive disorder (OCD).
A 2009 study  of 112 Han Chinese persons who stutter associated stuttering with two dopaminergic genes, one of which was associated with stuttering in the 1996 study.
A 2011 study questioned the association of stuttering with the dopamine D2 receptor DRD2 gene. No significant differences between stutterers and controls were observed in Brazilian and western European populations.
Lysosomal Enzyme Mutations and Stuttering
A 2010 study  investigated a family in Pakistan in which 44 of the 82 family members stuttered. The family is inbred: three siblings married three siblings who were their parents’ first cousins; three sons and three daughters of these three couples then married each other.
This study found that 52 of the 82 PKST72 family members have a mutation called G3598A in the GNPTAB gene. 41 of the 44 stutterers (93%) in the PKST72 family had the mutation. 11 of the 38 non-stutterers (29%) in the family had the mutation.
Outside the family, 8 of the 123 (7%) unrelated Pakistani stutterers had this mutation; 1 of the 96 (1%) Pakistani non-stutterers had this mutation; 2 of the 270 (1%) American and British stutterers had this mutation (one of whom was of South Asian descent); and none of the 276 American and British non-stutterers had this mutation.
Other mutations in GNPTAB and GNPTG have been associated with the rare inherited lysosomal storage disorders mucolipidosis types II and III. Lysosomes are the waste disposal system in animal cells, using acid hydrolase enzymes to break up waste materials and cellular debris.  None of the subjects examined had mucolipidosis.
The researchers concluded that their “results can explain only a small fraction of cases of stuttering.”
 Wingate, M. (1986) “Physiological and genetic factors.” In G.H. Shames & H. Rubin, Eds.), Stuttering Then and Now (pp.49-69). Columbus: Charles E. Merrill Publishing Co.
 Yairi, E. Ambrose, N., & Cox, N. (1996) “Genetics of Stuttering: A Critical Review,” Journal of Speech and Hearing Research, 39, 771-784.
 Yairi, E., & Ambrose, N.G. Early Childhood Stuttering for Clinicians by Clinicians. (2005, ISBN 89079-985-7), page 309.
 Jie Lan, Manshu Song, Chunhui Pan, Guoqing Zhuang, Youxin Wang, Wenzhan Ma, Qiaoyun Chu, Qingxuan Lai, Feng Xu, Yanli Li, Lixin Liu & Wei Wang. (2009). Association between dopaminergic genes (SLC6A3 and DRD2) and stuttering among Han Chinese. Journal of Human Genetics 54, 457-460 (August 2009), doi:10.1038/jhg.2009.60.
 Comings, David E., S. Wu, Connie Chiu, Robert H. Ring, Radhika Gade, Chul Ahn, James P. MacMurray, George Dietz & Donn Muhleman. (1996). Polygenic inheritance of Tourette syndrome, stuttering, attention deficit hyperactivity, conduct, and oppositional defiant disorder: The additive and subtractive effect of the three dopaminergic genes DRD2, D H, and DAT1. American Journal of Medical Genetics 67:3, 264-288. DOI: 10.1002/(SICI)1096-8628(19960531)
 Changsoo Kang, Sheikh Riazuddin, Jennifer Mundorff, Donna Krasnewich, Penelope Friedman, James C. Mullikin, and Dennis Drayna. (2010). Mutations in the Lysosomal Enzyme Targeting Pathway and Persistent Stuttering. New England Journal of Medicine 362:8, February 25, 2010.
 “Lysosome,” Wikipedia, accessed 2011 March 19.
Changsoo Kang, Bianca Santos Domingues, Eduardo Sainz, Carlos Eduardo Frigerio Domingue2, Dennis Drayna1, and Danilo Moretti-Ferreira. (2011) Evaluation of the association between polymorphisms at the DRD2 locus and stuttering. Journal of Human Genetics advance online publication, 10 March 2011; doi:10.1038/jhg.2011.29