Genetics Exam 1

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genotype

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DNA

genetic "fingerprint" of an organism

catalog of specific alleles that are inherited

the specific DNA sequence at a given locus

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phenotype

Back 2

behavioral manifestation of the inheritance of specific alleles

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wild-type

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the most common allele in the population for a given locus, "normal"

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polymorphism

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two or more common varients (blood type, hair color, eye color, etc)

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mutant

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rare, discontinuous varient, different

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discontinuous variation

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single gene with alternative alleles (ex - purple vs pink flowers)

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continuous variation

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continuum of phenotype, often polygenic (ex - height)

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meristic, polygenic traits

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quantitative traits, but discrete (ex - number of seeds per pod, can't have half a seed only 1,2,3...)

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threshold traits

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can be quantitative or polygenic, but there are a small number of discrete phenotypes (ex - diabetes, you either have it or you don't)

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complex inheritance

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genetic and environmental influences

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dominance

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phenotype caused by mutant allele is visible even in the presence of another allele

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recessive

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phenotype is only visible in absence of any other functional allele

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incomplete dominance

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phenotype of the heterozygote is intermediate between wild-type and mutant (true of many autosomal dominant diseases, reflecting gene dosage)

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co-dominance

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phenotype of the heterozygote reflects the impact of both the wild-type and mutant allele (ex - two forms of enzyme are present)

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null mutation

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mutation leads to no activity of gene product

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hypomorphic mutation

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mutation leads to reduced activity of gene product

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haploinsufficiency

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when loss of function mutations are dominant, remaining allele does not provide sufficient functional protein

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hypermorphic mutation

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mutation leads to an increased activity of gene product

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neomorphic mutation

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mutation leads to a new or altered activity of gene product

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pleiotropy

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a single mutation in one gene causes multiple phenotypic changes

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allelic heterogeneity

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a single disease or phenotype caused by several different mutations within the same gene (ex - cystic fibrosis)

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locus heterogeneity

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a single disease or phenotype caused by mutations in several different genes (ex - xeroderma pigmentosa)

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incomplete penetrance

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not every person with the mutation genotype shows the associated phenotype or disease (environmental influences)

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variable expressivity

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the severity of the mutant phenotype varies among affected individuals (continuous mutations)

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forward genetics

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"phenotype first" approach

mutagenize population and select or screen for a phenotype, then identify the mutation

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reverse genetics

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"gene first" approach

mutate a specific gene of interest and then select or screen for phenotype

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complementation test

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functional test to determine if two mutations giving the same phenotype lie within the same gene

only works with recessive alleles

if the phenotype is rescued - different genes

if the phenotype is not rescued - same gene

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extragenic supressor

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a second mutation in a different gene that overcomes (suppresses, compensates for) the phenotype caused by the inital mutation

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synthetic lethality

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when two mutant alleles in different genes cause a lethal phenotype when both are present, and a non-lethal phenotype individually

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epistasis

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genetic interaction of mutations at two loci, in which one mutation "masks" the phenotype the other mutation would cause alone

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Mendel's Principle of Segregation

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in the formation of gametes, the paired unit factors (alleles) segregate in such a way that each gamete is equally likely to contain either member of the pair

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Mendel's Principle of Independent Assortment

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during gamete formation, segregating paris of unit factors assort independently of each other

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test-cross

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crossing an organism with an unknown genotype with an organism with a recessive phenotype

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back-cross

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F1 x P1

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monohybrid cross phenotypic ratio

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3:1

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dihybrid cross phenotypic ratio

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9:3:3:1

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monogenic (Mendelian) disorders

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determined primarily by a single gene

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autosomal dominant inheritance

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mutant allele is dominant to wild-type, located on an autosome

on pedigrees:

each affected individual has an affected parent

normal children with affected parent will only have normal children

no gender preference

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autosomal recessive inheritance

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mutant allele is recessive to wild-type, located on an autosome

on pedigrees:

heterozygotes asymptomatic

both parents of an affected individual are heterozygotes

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X-linked dominant inheritance

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mutant allele is dominant to wild-type, located on the X chromosome

on pedigrees:

never passed from father to son

all daughters from affected father will be affected

daughters and sons have a 50/50 chance of being affected if Mom is affected and Dad isn't

females are more likely to be affected than males

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X-linked recessive inheritance

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mutant allele is recessive to wild-type, located on the X chromosome

on pedigrees:

never passed from father to son

generally, half the grandsons of an affected grandpa have the disease

males are more likely to be affected than females

all affected males in a family are related through their mothers

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Y-linked inheritance

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mutant allele is on the Y chromosome

on pedigrees:

only males affected

all sons of affected fathers are affected

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Mitochondrial inheritance

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mutation in maternally inherited mtDNA

on pedigrees:

all children of affected females are affected

no children of affected males are affected

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genetic anticipation

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members of a pedigree exhibit a progressively earlier onset age and an increased severity of the disorder in each sucessive generation

basis - expansion of trinucleotide repeats (unstable mutation)

ex - Marfan Syndrome

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mosaicism

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mutation originated as a somatic mutation during embryogenesis of one of the parents

a mosaic individual may be unaffected, depending on the proportion/type of cell carrying the mutation

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de novo mutation

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mutation originated spontaneously in parental gamete

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chromosomal disorders

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can be associated with aneuploidy (addition or loss of chromosome(s)), deletions/duplications/inversions of chromosomal material, or translocations of chromosomal material

complex phenotypes, hard to pinpoint to one gene

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complex disorders

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caused by mutations at more than one gene locus (polygenic inheritance) or a combination of genetic and envrionmental factors (multifactorial inheritance)

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mitosis

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maintains chromosome number, produces daughter cells identical to original cell

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meiosis

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reduces chromosome number by two, produces daughter cells with one set of chromosomes (haploid)

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screen

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selection process that allows all organisms to survive, but the target organisms are phenotypically different and easily disguinishable

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selection

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selection process that kills all but the target organisms

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missense mutation

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change in coding sequence that results in an amino acid substitiution

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non-sense mutation

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change in coding sequence that results in a premature STOP codon

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frame-shift mutation

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change in coding sequence that results in a reading frame shift, usually due to an insertion or deletion

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dominant negative mutation

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mutation that disrupts function of co-expressed wild-type protein