Principles Codominance both allelic copies are expressed login to view 1 more bullet Variable expression nature and degree of phenotype vary from 1 individual to another with the same mutation login to view 1 more bullet Incomplete penetrance not all individuals with a mutant genotype have diseased phenotype login to view 1 more bullet penetrance can be calculated by ( # with symptoms) / (# with disease genotype) login to view 3 more bullets observed in recessive and dominant diseases Pleiotropy single mutation has diverse effects upon several organ systems login to view 1 more bullet Anticipation changes in disease presentation in succeeding generations login to view 2 more bullets caused by trinucleotide expansion login to view 2 more bullets Loss of heterozygosity "two-hit model" individual inherits or develops a mutation in one copy of gene login to view 1 more bullet e.g. tumor suppressor diseases (Li-Fraumeni, retinoblastoma) Dominant negative mutation mutant gene product antagonizes wild-type gene product login to view 2 more bullets De novo mutation genetic disease in an individual with no familial history recurrence risk for offspring of same parents is low Locus heterogeneity different mutations can produce the same phenotype login to view 2 more bullets Heteroplasmy presence of both normal and mutated mitochondrial (mt)DNA in the same cell login to view 1 more bullet Uniparental disomy offspring receives both copies of a chromosome from 1 parent login to view 3 more bullets Polygenic inheritance multiple genes are responsible for inheritance of a disease e.g.) androgenic alopecia login to view 1 more bullet Heritability can measure the relative effect of genetic vs. environmental factors on a phenotype login to view 7 more bullets