A man gunned down his daughter-in-law and infant grandson over petty dispute here said on Saturday. Police arrested the accused. According to details, the incident took place in Shergarh area of Mardan where an accused after exchange of hot words with wife of his son opened fire at her and her infant child. As a result of firing, both mother and her infant child were killed on the spot. The bodies were shifted to hospital for postmortem. The police arrested the accused and after registering a case against him started investigations.