Genetic Terms and Examples

Question 1

Codominance

Answer 1

Blood groups A, B, AB

alpha1-antitrypsin deficiency

Question 2

Variable expressivity

Answer 2

NF1 (varying disease severity)
Tuberous sclerosis
Androgenetic alopecia

Question 3

Incomplete penetrance

Answer 3

BRCA1 (do not always result in breast or ovarian cancer)

Tuberous sclerosis

Question 4

Pleiotropy

Answer 4

PKU (one gene contributes to multiple phenotypes)

Question 5

Anticipation

Answer 5

Huntington Disease

Question 6

Loss of heterozygosity

Answer 6

Rb (“2-hit hypothesis”)

Question 7

Dominant negative mutation

Answer 7

nonfunctioning mutant prevents normal gene product from functioning

Question 8

Linkage disequilibrium

Answer 8

tendency for certain alleles at 2 linked loci to occur together more or less often than expected by chance

Question 9

Mosaicism

Answer 9

McCune-Albright Syndrome

Down’s Syndrome

Question 10

Locus heterogeneity

Answer 10

Albinism (mutations at diff loci can produce same phenotype)

Question 11

Allelic heterogeneity

Answer 11

beta-thalassemia (diff mutations at same locus can produce same phenotype)

Question 12

Heteroplasmy

Answer 12

MELAS (presence of both normal and mutated mtDNA resulting in variable expression in mitochondrially inherited disease)

Question 13

Uniparental disomy

Answer 13

individual manifesting a recessive disorder when only one parent is a carrier; Prader-Willi Syndrome and Angelman Syndrome.
Heterodisomy (meiosis I error)
Isodisomy (meiosis II error or postzygotic chromosomal duplication of one of pair of chromosomes and loss of the original pair)

Question 14

Imprinting

Answer 14

Prader-Willi Syndrome (paternal gene deleted or mutated; maternal imprinting); 25% maternal uniparental disomy
Angelman Syndrome (maternal gene deleted or mutated; paternal imprinting); 5% paternal uniparental disomy

Question 15

Polygenic inheritance

Answer 15

Androgenetic alopecia

DM II