Page 82 - Genetic Terms

Question 1

Codominance

Answer 1

Both alleles contribute to phenotype of heterozygote. Example: Blood groups A, B, AB.

Question 2

Variable Expressivity

Answer 2

Phenotype varies among individuals with same genotype. Example: 2 Pts w/ NF1 may have varying disease severity

Question 3

Incomplete Penetrance

Answer 3

Not all individuals with mutant genotype show the mutant phenotype. Ex. BRCA1 mutations don’t always result in breast or ovarian cancer.

Question 4

Pleiotropy

Answer 4

One gene contributes to multiple phenotypic effects. Ex. PKU causes many unrelated symptoms, such as mental retardation to hair/skin changes.

Question 5

Imprinting

Answer 5

Differences in gene expression depend on whether allele is of maternal or paternal origin. Ex. Prader-Willi and Angelman’s syndromes

Question 6

Anticipation

Answer 6

Increased severity or earlier onset of disease in succeeding generations. Ex. Huntingdon’s disease.

Question 7

Loss of heterozygosity

Answer 7

If pt inherits or develops mutation in a tumor suppressor gene, the complementary allele must be deleted/mutated before cancer develops. Not true of oncogenes. Ex. Retinoblastoma and the “two-hit hypothesis.”

Question 8

Dominant negative mutation

Answer 8

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

Question 9

Linkage disequilibrium

Answer 9

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

Question 10

Mosaicism

Answer 10

Occurs when cells in body differ in genetic makeup due to postfertilization loss or change of genetic information during mitosis. Can be germ-line (gonadal mosaicism), which may produce disease not carried by parent’s somatic cells. Mutation in embryonic precursor of bone marrow stem cell leads to hematologic mosaic. Chimeric individual derived from 2 zygotes that fuse.

Question 11

Locus heterogeneity

Answer 11

Mutations at different loci can produce same phenotype. Ex. Marfan’s syndrome, MEN 2B, and homocystinuria all cause marfanoid habitus. Albinism.

Question 12

Heteroplasmy

Answer 12

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

Question 13

Uniparental Disomy

Answer 13

Offspring receives 2 copies of chromosome from one parent and no copies from other parent. Heterodisomy (heterozygois) indicates meiosis I error. Isodisomy (homozygous) indicates meiosis II error or postzygotic chromosomal duplication of one of a pair of chromosomes, and loss of the other of the original pair. Ex. Uniparental is eUploid (correct number of chromosomes). Most occurrences of UPD lead to normal phenotype. Consider in an individual w/ a recessive disorder when only one parent is a carrier.