Departing from Lenneberg’s biological conception of language and its de- velopment, this paper first reviews select examples from research on lan- guage development and its interface with genetics before making some specific proposals with regard to how the genetics of grammar could be investigated. The central proposal of this paper is that an important, per- haps necessary, avenue for studying the genetics of grammar is to study the genotypes corresponding to phenotypes of child (and genetically im- paired) versions of the computational system of grammar, as opposed to strictly descriptive measures of a construction or standardized linguistic tests. In some cases, these phenotypes have wide explanatory ability, sug- gesting that they directly involve parts of the computational system of lan- guage. The primary example discussed is the phenotype of the Unique Checking Constraint (UCC). In particular, it is proposed that one could usefully start to investigate the genetic basis for he development of finite- ness, object clitic omission, and related phenomena of the UCC. A second, less developed example here, corresponding to a much later developmen- tal stage, is the Universal Phase Requirement (UPR), regulating verbal pas- sives and many other phenomena in children.