Genetics Terms

Question 1

Codominance

Answer 1

Both alleles contribute to the phenotype of the heterozygote

Blood groups A, B, AB; α1-antitrypsin deficiency

Question 2

Variable expressivity

Answer 2

Phenotype varies among individuals with same
genotype

2 patients with neurofibromatosis type 1 (NF1)
may have *varying disease severity.

Question 3

Incomplete penetrance

Answer 3

Not all individuals with a mutant genotype show the mutant phenotype

BRCA1 gene mutations do not always result in
breast or ovarian cancer.

Question 4

Pleiotropy

Answer 4

One gene contributes to *multiple phenotypic
effects.

Untreated phenylketonuria (PKU) manifests with light skin, intellectual disability, and musty body odor.

Question 5

Anticipation

Answer 5

Increased severity or earlier onset of disease in
succeeding generations.

Trinucleotide repeat diseases (eg, Huntington
disease)

Question 6

Loss of heterozygosity

Answer 6

If a patient inherits or develops a mutation in
a tumor suppressor gene, the complementary
allele must be deleted/mutated before cancer develops.

This is not true of oncogenes.

Retinoblastoma and the “two-hit hypothesis,”
Lynch syndrome (HNPCC), Li-Fraumeni syndrome.

Question 7

Dominant negative mutation

Answer 7

Exerts a dominant effect. A heterozygote produces a nonfunctional altered protein that also prevents the normal gene product from functioning.

Mutation of a transcription factor in its allosteric site. Nonfunctioning mutant can still bind DNA, preventing wild-type transcription factor
from binding.

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.

Measured in a *population, not in a family, and often varies in different populations.

Question 9

Mosaicism

Answer 9

Presence of genetically distinct cell lines in the same individual.

*Somatic mosaicism mutation arises from mitotic errors after fertilization and propagates through multiple tissues or organs.

*Gonadal mosaicism mutation only in egg or
sperm cells.

*ex. McCune-Albright syndrome

Question 10

Locus heterogeneity

Answer 10

Mutations at different loci can produce a similar
phenotype.

Albinism.

Question 11

Allelic heterogeneity

Answer 11

Different mutations in the same locus produce the same phenotype.

β-thalassemia

Question 12

Heteroplasmy

Answer 12

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

Question 13

Uniparental disomy

Answer 13

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

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.

Uniparental is eUploid (correct number of
chromosomes), not aneuploid.

Most occurrences of UPD–>normal phenotype

Consider UPD in an individual manifesting a *recessive disorder
when only one parent is a carrier.

Question 14

Hardy-Weinberg population genetics

Answer 14

If a population is in Hardy-Weinberg equilibrium and if p and q are the frequencies
of separate alleles, then: p2 + 2pq + q2 = 1 and p + q = 1

The frequency of an X-linked recessive disease in males = q and in females = q2.

Hardy-Weinberg law assumptions include:
ƒ-No mutation occurring at the locus
ƒ-Natural selection is not occurring
ƒ-Completely random mating
ƒ-No net migration

Question 15

Imprinting

Answer 15

At some loci, only one allele is active; the other is inactive (imprinted/inactivated by
methylation).

With one allele inactivated,
deletion of the active allele Ž disease.

Both Prader-Willi and Angelman syndromes are due to mutation or deletion of genes on *chromosome 15.