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Both alleles contribute to the phenotype of the heterzygote

Examples- blood groups, alpha antitrypsin def, HLA groups


Patients with the same genotype have varying phenotypes

Variable expressivity
Ex: 2 patients with NF1 have differing severities of disease


Not all individuals with a mutant genotype show the mutant phenotype

Incomplete penetrate- % penetrance x probability of inheriting genotype= risk of expressing penetrance
Ex: BRCA1 genes don’t always result in breast or ovarian cancer


One gene contributes to multiple phenotypic effects

Ex: untreated PKU manifests with light skin, intellectual disability, and must body odor


Increased severity or earlier onset of disease in succeeding generations

Ex: Huntington’s disease with trinucleotide repeats


If a patient inherits or develops a mutation in a tumor suppressor gene, the complementary allele must be deleted/mutated before cancer develops. (Not true of oncogenes)

Loss of heterozygosity
Ex: Rb and two hit hypothesis, HNPCC, Li-Fraumeni syndrome


A heterozygous produces a nonfunctional altered protein that also prevents the normal gene product from functioning

Dominant negative mutation
Ex: mutation of a TF in its allosteric site. Non functioning mutant can still bind DNA, preventing wild type transcription factor from binding.


Tendency of certain alleles at 2 closely spaced loci to occur together more or less often than expected by chance. Measured in population, not in a family, and often varies in different populations

Linkage disequilibrium


Presence of genetically distinct cell lines in the same individual

Somatic- mutations arises from mitotic errors after fertilization and propagates through multiple tissues or organs
Gonadal- mutation only in the egg or sperm cells. If parents and relatives do not have the disease, suspect germline mosaicism
Ex: McCune Albright syndrome- due to mutation affecting G protein signaling. Presents with cafe-au-last spots with ragged edges, polyostotic fibrous dysplasia, and one endocrinopathy. Lethal if mutation occurs before fertilization, but survivable in mosaicism.


Mutations at different loci can produce a similar phenotype

Locus heterogeneity
Ex: Albinism


Different mutations in same locus produces the same phenotype

Allergic heterogeneity
Ex: Beta thalassemia


Presence of both normal and mutated mtDNA, resulting in variable expression in mitochondrially inherited disease

MtDNA passed from mother to all children


Offspring receives 2 copies of a chromosome from 1 parent and no copies from the other parent.

Uniparental disomy- euploid(correct # of chromosomes) so most can have normal phenotype
Heterodisomy (heterozygous) indicates a meiosis I error. Isodisomy (homozygous) indicates a meiosis II error or postzygotic chromosomal duplication of one of a pair of chromosomes, and loss of the other of the original pair.

Ex: UPD in an individual manifesting a recessive disorder when only one parent is a carrier- angel an or prader willi syndromes.


One gene copy is silenced by methylation, and only the other copy is expressed to get the parent of origin effects

PW syndrome- maternally imprinted (maternal genes are silenced) so disease occurs when PATERNAL allele is mutated/deleted. Chromosome 15 mutation on paternal allele or maternal uniparental disomy.
Angelman- paternal imprinted (paternal UBE3A gene is silenced) and disease occurs if MATERNAL allele is deleted or mutated. Chromosome 15 mutations on maternal allele or paternal uniparental disomy.