How do I find variation information and associated phenotypes for a human gene at NCBI?

If you are focusing on an individual human gene, consider searching the Gene database for your gene record (example). 

The graphic display (Sequence Viewer) that is located in the Genomic regions, transcripts, and products section of the record offers several configurable variation tracks. For example, you can choose the Clinical track from the NCBI Recommended Tracks  as shown in this video.

The Phenotypes section of the record (accessible from the Table of contents, top right) will list the known phenotypes for the gene and, if available, link you to the pertinent literature information in the following medical genetics literature databases: MedGen, OMIM, and GeneReviews. Within the Phenotypes section, you will also be able to access the Phenotype-Genotype Integrator (PheGenI) tool that will display any genotype- phenotype association data for the gene that were discovered in Genome-Wide Association studies or genotype associations with expression quantitative trait loci (eQTL).  The "Compare labs" link (if available) will take you to the testing laboratories' information in Genetic Testing Registry (GTR®).

The Variation section on the Gene record provides additional links to other variation resources. See this article for more information: NCBI variation services.