TOPICS: Alleles, genotype, phenotype, variability, phenotypic manifestations, tumor suppressor genes, gametes, mutations, locus, loci, mitochondrial genomes, dominant, codominance, incomplete penetrance, variable expressivity, pleiotropy, anticipation, loss of heterozygosity, germline mosaicism, somatic mosaicism, locus heterogeneity, allelic heterogeneity, heteroplasmy, dominant negative mutation, neurofibromas, retinoblastoma, Gomori trichrome stain, breast cancer, BRCA1 gene, neurofibromatosis type 1 (NF1), homocystinuria, Marfanoid habitus, Huntington disease, retinoblastoma, Turner syndrome, McCune-Albright syndrome, albinism, beta-thalassemia, mitochondrial disorders