Johns Hopkins Som Genetics

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allelic heterogeneity

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different mutations at the same locus

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allelic series

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when different allels at the same locus produce different phenotypes

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autosomal dominant

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When heterozygote shows dominant trait. Seen in every generation, equally between male and female (50%)

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autosomal recessive

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When heterozygote does not show recessive trait. Only seen in homozygous state. Not in every generation, equally between males and females.

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compound heterozygote

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two allels at same locus, both containing mutations. (a1a2) aka heteroallelic

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digenic inheritance

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phenotype requiring mutant genes at two loci

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dominant gain of function

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mutation which produces a new function or increased function of product

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dominant loss of function

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mutation which produces a 50% loss of function AA-->Aa

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dominant negative allele

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mutation in which the product interferes with normal production thus reducing function by >50%

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genetic heterogeneity

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similar phenotypes caused by different mutant genes

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Heteroallelic

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compound heterozygous

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Homo allelic

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when both allels are the same (homozygous)

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incomplete dominance

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when neither allel is dominant but rather phenotype is an intermediate

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locus heterogeneity

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mutations in two genes at different loci which may produce a similar phenotype

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modifying genes

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when the product of a non allelic gene alters the expression of the phenotype associated with a particular gene (or mutation of that gene)

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mosaicism

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Variable phenotypic expression (chimeric expression) on the cellular level (X inactivation)

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Penetrance

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When a particular gene (mutation) does not always produce a phenotype (the probability that a mutation will show the mutant phenotype)

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pleiotrophy

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when one gene has consequences on many phenotypes

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pseudoautosomal

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regions on x and y c/some that are homologous

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variable expression

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degree to which a phenotype is expressed

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euchromatin

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uncondensed regions of c/somes can be transcriptionally active

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heterochromatin

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condensed regions of c/somes

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conserved noncoding sequences (cNCS)

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areas of genome that are more conserved than would be expected by random mutation (total 5%, about 1.5% protein coding other 3.5% unknown)

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copy number variation (CNV)

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segments of the genome that are duplicated or deleted.

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gene desserts

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regions of genome devoid of genes-usually repetative sequences and heterochromatin

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homolog

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genes which share a common ancestor

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ortholog

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similar gene (w/ similar function) between species

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paralog

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homologous genes within an organism that belong to the same family but serve different functions

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genomic disorders

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problems/diseases which result from mutations in geneome

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haplotype

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the genotype of a set of markers linked together on a segment of the same c/some

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haplotype block

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the haplotype (group of genes) which is usually inherited as a unit

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linkage disequilibrium

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the condition in which the haplotype frequencies in a population deviate from the values they would have if the genes at each locus were combined at random

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Low copy repeats (LCR)

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region specific segmental duplication that occurs at more than one site in the genome and share >90% identity

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LINES

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Long (~6 kb) interspersed nucelear elements (created by insertions of transposable elements)

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SINES

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Short (~300 bp) interspersed nuclear elements (created by insertions of transposable elements)

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Single nucleotide polymorphism (SNP)

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Change in basepair sequence

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Syntenic

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genes on a contiguous piece of DNA-typically in a specific order

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Association Studies

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compare the frequency of an allele in a population of affected individuals with the frequency of that allele in a population of unaffected individuals

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complex traits

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a phenotype that results from the interplay of multiple genes and the environment

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genetic markers

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sequences of the genome which can be inherited with particular genes. These serve to locate genes associated with a phenotype.

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genomic profiling

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the detection of gene variants associated with a greater risk for a particular disease

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Relative risk (lambda)

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the recurrence risk for a relative of an affected person compared to the risk for a person from the general pop.

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linkage analysis

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using markers that co-segragate with a phenotype, it is possible to find the gene associated with that phenotype

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Short Tandem repeat polymorphism (STRP)

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Short (200 bp) of tandem repeats of a simple 1-4 bp DNA sequence (aka microsatellites) can be used as markers

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Susceptibility genes

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makes an individual prone to a disease, but is not sufficient to cause a diseased phenotype

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Transmission disequilibrium test (TDT)

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Comparing parents and affected offspring, the susceptibility allele (or marker) will be transmitted to offspring more often than the expected 1/2

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polygenic inheritence

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variant alleles for multiple genes contribute to a particular phenotype

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phenocopy

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when a phenotype is expressed for reasons unrelated to those with a specific genetic mutation (ie a person has disease X because of mutation in gene A but another person has disease X but is normal in gene A)

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anticipation

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apparent tendency of certain phenotypes to be increasingly severe in successive generations (premutation)

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heterodisomy

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A specific case of uniparental disomy when the pair results from inheritence of both homologs from one parent

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heteroplasmic

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when a cell contains a mixture of mutant and normal mtDNA

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homoplasmic

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when a cell contains only normal mtDNA

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imprinting

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a case of stable transcriptional repression in which the silenced allele is determined by the parent of origin

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isodisomy

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A specific case of uniparental disomy when the pair results from duplication of the same c/some

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loss of imprinting (LOI)

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When silenced gene is lost by malignant transformation making both genes active

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parent of origin

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one parental allele may be preferentially expressed

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epigenetics

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"above" genetics-mitotically/meiotically heritable changes in gene expression not coded in DNA sequence

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uniparental dismoy

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when both members of a c/some pair are inherited from a single parent

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Mosaicism

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The presence of at least two cell ines derived from a single zygte but differeing in genotype and/or karyotype