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MM15 - Medelian Genetics 1 (i) (Autosomal Dominant (also in homozygotes…
MM15 - Medelian Genetics 1 (i)
Y-linked conditions
usually don't have pedigrees because sufferers usually can't reproduce
Autosomal Dominant
most cases are heterozygous (Aa leads to disease phenotype, no carriers)
also in homozygotes (aa), but usually don't pass is on as they tend to die before reproductive maturity
selective disadvantage
low frequency
more severe phenotype
both sexes affected equally + both sexes can pass on mutation
familial hypercholesterolaemia
heterozygous
incidence = 1 in 500
double normal plasma LDL levels
MI @ 30-40 y/o
less functional LDL RS
homozygous
incidence = 1 in 1000000
x10 normal LDL levels
fatal childhood MI
polycystic kidney disease
Familial Adenomatous Polyposis (FAP)
precancerous philips in colon + rectum
treatment = removal of colon + rectum
Neurofibromatosis
precancerous skin disease
vertical inheritance pattern
exceptions...
de novo mutations (sporadic cases)
reduced penetrance: person has disease but also has a modifying mutation which cancels it out (genotype, no phenotype)
can still pass disease onto children though ('skips a generation')
variable expressivity: everyone with genotype has phenotype, but differs in nature/severity
Huntington's disease
triple repeat disorder
variable expressivity
more repeats as it gets passed on - earlier onset age
haploinsufficiency
where normal physiology requires > 50% of gene (protein) to be working
1 mutation causes phenotype
dominant -ve effect
abnormal protein interferes with normal protein
1 mutation causes phenotype
gain-of-function mutation
increased mutant protein function
e.g. achondroplasia
growth factor R3 always switched on
inhibits bone growth - dwarfism
loss of heterozygosity
mutant allele inherited + random loss of normal allele
makes cancerous cells - dominantly inherited cancers
genetic counselling
before someone decides to find out if they're a carrier
how would they react if they are one?
is it best to know?
do they want to know the chance of having affected children?