Heredity is of biological significance and hence internal.
Enviornment relates to external factors.
Personality traits and development in a person in psychology, is in relation to a persons reaction to the enviornment in which he grows. Subjecting his heredity traits to a certain external factors or influence, will determine the personality he develops eventually. But still two identical twins may behave differently even if remain in the same enviornment, which will be so because the worldly reactions could be different to both of them and both may have diffrent I Q applied to acertain situation and behave differently yet. Heredity- genes application to I Q is still a subjective issue. Heredity is constant subject to a person which is over with the birth. So how one uses his own I Q to react to his sitaution to either have a good , bad or avaerage personality is matter of individual choice to a far degree on relative levels. And Genetically, science has enough left to qualitatively measure personality traits vs genes, leave alone quantitatively. Hence in Psychology terms it is the reaction to the external factors that helps determining the personality issue, as heredity being a constant feature in an individual is a personality born feature and not developed feature or an issue.