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13 Cards in this Set

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Codominence

Both alleles contribute to heterozygote phenotype eg. blood group, A1AT deficiency

Variable expressivity

Phenotype varies among individuals with same genotype eg. Neurofibromatosis type I

Incomplete penetrance

Not all individuals with mutant genotype show mutant phenotype eg. BRCA1

Pleiotropy

One gene contributes to multiple phenotypic effects eg. Untreated PKU can manifest with light skin, intellect disability, musty body odor etc.

Anticipation

Increased severity or earlier onset of disease with succeeding generations. Trinucleotide repeat diseases eg. Huntingtons

Loss of heterozygosity

Mutation of tumour suppressor gene requires deletion/mutation of complementary allele before cancer develops. "two-hit hypothesis" eg. Retinoblastoma

"two hit hypothesis"

Dominant negative mutation

Heterozygote produces non-functional protein that also prevents normal gene product from functioning

Linkage disequilibrium

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

Mosaicism

Presence of enetically distinct cell lines in same individual.


Somatic = mutation from mitotic errors after fertilisation.


Gonadal = mutation only in egg or sperm cells


eg. McCune-Albright syndrome

Locus heterogeneity

Mutations at different loci can produce similar phenotype


eg. Albinism

Allelic heterogeneity

Different mutations in same locus produce same phenotype eg. B-thalassaemia

Imprinting

At same loci, only one allele active (other is imprinted/inactivated by methylation).


Deletion of active allele = disease




Prader-Willi: maternal (chr 15)


AngelMan: paternal (chr 15)

Uniparental disomy (heterodisomy vs. isodisomy)

Two copies of chromosome from one parent, nil from other.




Heterodisomy: non-identical chromosomes (meiosis I error)




Isodisomy: identical chromosomes (meiosis II error or postzygotic duplication)




eg. recessive disorder seen when only one parent is carrier.


Prader-Willi/AngelMan syndromes occasionally result from two imprinted genes.