Genetics Review For Step 1

Front 1

Codominance

Back 1

Neither of 2 alleles is dominant ex. Blood Groups (A,B,AB)

Front 2

Variable expression

Back 2

Nature and severity of phenotype vary from 1 individual to another
ex: 2 patients with NF 1 may have varying disease severity

Front 3

Incomplete penetrance

Back 3

Not all individuals with a mutant genotype show the mutant phenotype

Front 4

Pleiotropy

Back 4

1 gene has > 1 effect on an individual's phenotype
ex: PKU causes many seemingly unrelated symptoms ranging from mental retardation to hair/skin changes

Front 5

Imprinting

Back 5

Differences in phenotype depend on whether the mutation is of maternal or paternal origin
ex: Prader-Willi and Angelman's

Front 6

Anticipation

Back 6

Severity of disease worsens or age of onset of disease earlier in succeeding generations
ex: huntington's diseasee

Front 7

Loss of heterozygosity

Back 7

If patient inherits or develops a mutations in a tumor suppressor gene, the complementary allele must be deleted/mutated before cancer develops. This is not true of oncogenes
Ex: retinoblastoma

Front 8

Dominant negative mutation

Back 8

Exerts and dominant effect. A heterozygote produces a nonfunctional altered protein that also prevents the normal gene product from functioning.
Ex: Mutation of Tx factor in its allosteric site. Nonfunctioning mutant can still bind DNA, preventing wild-type Tx factor from binding

Front 9

Linkage disequilibrium

Back 9

Tendency for certain alleles at 2 linked loci to occur together more often than expected by chance. Measured in a population, not in a family, and often varies in different population.

Front 10

Mosaicism

Back 10

Occurs when cells in the body have different genetic makeup.
Can be a germ-line mosaic, which may produce disease that is not carried by parent's somatic cells.
Ex. Lyonization - random X inactivation in females (ex. hemophilia A)

Front 11

Locus heterogeneity

Back 11

Mutations at different loci can produce the same phenotype
ex: Marfan's, MEN 2B, homocystinuria; all cause marfanoid habitus.
Albinism

Front 12

Heteroplasmy

Back 12

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

Front 13

Uniparental disomy

Back 13

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