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genetics- Robin clark- intro


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Autosomal Dominant Inheritance
Heterozygotes express the trait
Vertical pattern of transmission from one generation to the next
Transmitted by both sexes
Offspring are at 50% risk
Structural genes are often involved
New mutations are common
Phenotypic Variability
Incomplete penetrance (def.)
- give example
a trait that is not expressed in the phenotype of a gene carrier
e.g.,Breast cancer occurs in about 70% of BRCA1 gene mutation carriers

-Other genes or environmental factors influence gene expression in these traits.
Complete penetrance (def.)
- give example
a genetic trait that is expressed in the phenotype
-Achondroplasia and neurofibromatosis are highly penetrant mutations
Variable expression
a genetic mutation associated with more than one phenotype
Autosomal Recessive Inheritance
Homozygotes express the trait
Horizontal pattern of transmission within a sibship.
Transmitted by both sexes
Offspring of carrier parents are at 25% risk
Functional genes are often involved
Ex. Sickle cell disease
Consanguinity is more common
X-linked Recessive Inheritance
Hemizygotes (46,XY) are affected; heterozygotes (46,XX) are carriers
When lethal, transmitted by females only - ex. Duchenne Muscular Dystrophy
When not lethal, transmitted by both sexes - ex. Color blindness
Sons of carrier mothers at 50% risk; sons of affected fathers at no risk
The phenotype becomes more severe or evident at a younger age of onset with each successive generation
Ex. The triple repeat mutation can expand with each generation in Myotonic dystrophy.
Ex. The premutation for Fragile X in a normal transmitting male expands during female meiosis in his unaffected daughter to a full mutation in her affected son.
Parent of Origin effect
The phenotype varies depending on which parent transmits the abnormal allele

Methylation patterns differ in male and female meiosis: some genes are active only when inherited from one parent (imprinting)
Ex. Only the maternal copy of the Angelman syndrome gene is active; the paternal gene is inactive.

In utero factors can affect severity
Ex. Congenital myotonic dystorphy
Polygenic traits
many genes each make a small contribution to the phenotype
Ex. Intelligence, height
Multifactorial traits
Few genes make a major contribution to the phenotype in a permissive environment
Ex. Spina Bifida, MTHFR and folate deficiency
A mutation affects only some cells in the body with a variable phenotype
Ex. Gonadal mosaicism in an unaffected parent causes more than one affected child with osteogenesis imperfecta type II, a lethal autosomal dominant trait
Chromosomal translocations
Ex. Down syndrome due to a familial translocation
Maternal inheritance
Ex. Mitochondrial genome is cytoplasmic so mitochondrial defects are passed down in the egg cytoplasm from the mother to all of her children
genetic heterogenity
mutations in different genes can cause same disease
genotype/phenotype correlations
different mutations in same gene can cause different syndromes
carrier frequency
prevolence of an altered disease gene in a given population
founder effect
high frequency of a specific gene mutation in a population founded by a small ancestral group
Comparative Genomic Hybridization (CGH) to Metaphase Chromosomes
Genome scanning technique use for scanning to search for chromosome aberrations
Generally been used to examine chromosome structure in cancer
Microarray CGH
Whole genome hybridization
dna sequence changes that do not alter protein function
change in normal base pair sequence
disease associated mutations
DNA sequence changes that alter protein function
non mutant gene
point mutation
change in single base pair
missense mutation
changes to a codon for another amino acid (can be harmful or neutral)
nonsense mutation
change from an amino acid mutation to a stop codon, producing a shortened protein
frameshift mutations
insertion or deletion of base pairs, producing a stop codon downstream and usually a shortened protein
splice site mutation
a change that results in alterned mRNA sequence
DNA electrophoresis
dna loaded into wells and fragments are separated by size and charge
linkage analysis
looks for pattern of DNA markers near gene of interest that segregate with disease. Requires DNA analysis of multiple family members
alelle specific ogliotide (ASO) hybridization
1)amplify dna
2)add radio labeled normal DNA probes
3)add known mutant dna probes
4) compare (heterozygous, homozygous)
1) isolate & denature DNA
2)anneal and extend primers
3)repeat as necessary
4)amplify segmenta
single strand conformational polymorphism
1)dna is denatured into single strands
2)single strands fold; shape is altered by mutations
3)mobility of mutant and normal strands differ in gel

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