genetics Flashcards
variable expression
nature and severity of phenotype vary from one person to another
incomplete penetrance
not all individuals with a mutant genotype show the mutant phenotype
pleiotropy
one gene has more than one effect on phenotype (i.e. MULTIPLE phenotypes, seemingly unrelated)
imprinting
differences in phenotype depend on whether mutation is of maternal or paternal origin
anticipation
severity of disease worsens OR
age of onset of disease is early
in succeeding generations
loss of heterozygosity
if a pt inherits or develops a mutation in a tumor suppressor gene, the complementary allele must be deleted or mutated before cancer develops (TWO-HIT HYPOTHESIS).
*not true of oncogenes
dominant negative mutation
exerts dominant effect.
heterozygote produces nonfunctional altered protein that also prevents normal gene product from functioning.
linkage disequilibrium
tendency for certain alleles at 2 linked loci to occur together more often than expected by chance.
measure in POPULATION, not family.
often varies in diff pops.
mosaicism
occurs when cells in body differ in genetic makeup due to postfertilization loss of genetic info during mitosis
germline (gonadal) mosaicism
produce disease that is not carried by parents’ somatic cells
locus heterogeneity
mutations at diff loci can produce same phenotype
ex: Marfan, MEN 2B, and homocystinuria (all produce marfanoid habitus)
heteroplasmy
presence of both normal and mutated mtDNA, resulting in variable expression of mitochondrial inherited dz
uniparental disomy
offspring receives 2 copies of chromosomes from 1 parent and no copies from other parent
Hardy-Weinberg law assumes…
- no mutation occurring at locus.
- no selection for any of the genotypes at the locus.
- completely random mating.
- no migration.
cause of imprinting
at some loci, only ONE allele is active - the other is inactive (imprinted/inactivated by methylation).
with one allele inactivated, deletion of the ACTIVE ALLELE = disease.
Prader-Willi syndrome
imprinting.
chromo 15.
maternal allele inactivated.
Paternal allele should be active but is not expressed.
features of Prader-Willi
mental retardation. hyperphagia. obesity. hypogonadism. hypotonia. short stature. thirst. emotional lability.
AngelMan’s syndrome
imprinting.
chromo 15.
paternal allele inactivated.
Maternal allele should be active but is not expressed.
features of Angelman’s syndrome
mental retardation. seizures. ataxia. inappropriate laughter. HAPPY PUPPET.
which mode of inheritance is often due to defects in structural genes?
AD
which mode of inheritance is often pleiotropic?
AD
which mode of inheritance often presents clinically after puberty?
AD
for which mode of inheritance is fam hx crucial for dx?
AD
which mode of inheritance is often due to enzyme deficiencies?
AR
how does AR compare to AD clinically?
AR more severe,
present in childhood.
which mode of inheritance is usually seen in only one generation?
AR
which mode of inheritance has NO MALE-to-MALE transmission?
X-linked R
which mode of inheritance guarantees disease in female offspring of affected father?
X-linked D
hypophosphatemic rickets
X-linked D disorder with increased phosphate wasting at prox tubule. presents like rickets.
which mode of inheritance is ONLY transmitted through MOTHER?
mitochondrial
all offspring may show signs of disease
which mode of inheritance is often due to failures in oxidative phosphorylation?
mitochondrial
why do mitochondrial disease have variable expression?
heteroplasmy
mitochondrial myopathies
ragged red fibers on microscopy due to abn mito accumulation under sarcolemma.
- Leber’s hereditary optic neuropathy.
- myoclonic epilepsy.
- mitochondrial encephalopathy.
mitochondrial myopathy: Leber’s hereditary optic neuropathy
acute loss of central vision
mitochondrial myopathy: mitochondrial encephalopathy
stroke-like episodes.
lactic acidosis.
AR disorders
albinism. ARPKD. CF. glycogen storage. hemochromatosis. mucopolysaccharidoses (except Hunter's). PKU. sickle cell. sphingolipidoses (except Fabry's). thalassemias.
cystic fibrosis
AR defect in CFTR gene.
chromo 7.
deletion of Phe508.
CFTR channel
actively secretes Cl in lungs and GI tract.
actively reabsorbes Cl from sweat.
activated by cAMP-mediated phosphorylation.
gated by ATP.
what does the CFTR mutation do?
cause abnormal protein folding (defective glycosylation), resulting in DEGRADATION of channel before it reaches cell surf.
clinical features of CF
defective Cl channel = secretion of abnormally thick mucus that plugs lungs, pancreas, liver.
recurrent pulmo infx. chronic bronchitis. bronchiectasis. pancreatic insuff. nasal polyps. meconium ileus in newborn.
what microorganisms tend to cause pulmo infx in CF?
Pseudomonas.
S.aureus.
what is the result of pancreatic insuff in CF?
malabsorption.
fat-soluble vit deficiency.
steatorrhea.
failure to thrive in infancy.
how does CF cause infertility in males?
BILATERAL absence of vas deferens
DX of CF
increased Cl ions in sweat test
TX of CF
N-acetylcysteine:
loosens mucous plugs by cleaving disulfide bonds w/in mucous glycoproteins.
X-linked recessive disorders
“Be Wise, Fool’s GOLD Heeds Silly Hope”
Bruton's agammaglobulinemia. Wiskott-aldrich. Fabry's dz. G6PD def. Ocular albinism/OTC def. Lesch-nyhan. Duchenne/becker MD. Hunter's Syndrome. Hemophilia a and b.
why are female carriers rarely affected by X-linked recessive disorder?
random inactivation of X chromo in each cell
Duchenne muscular dystrophy (DMD)
X-linked FRAME SHIFT mutation leading to DELETION of DYSTROPHIN GENE.
accelerated muscle breakdown.
where does muscle weakness begin in DMD?
pelvic girdle mm.
progress superiorly.
features of DMD
muscle weakness. walking difficulties (waddle). calf PSEUDOHYPERTROPHY. cardiac myopathy. kyphoscoliosis.
onset before age 5.
calf pseudohypertrophy in DMD
fibrofatty replacement of muscle
what is the consequence of DMD gene being the longest known human gene?
increased rate of spontaneous mutation
normal function of dystrophin
help anchor muscle fibers, primarily in skeletal and cardiac muscle
Gower’s maneuver in DMD
child uses upper extremities to help in standing up from the ground
DX of DMD
increase CPK (creatine phosphokinase).
muscle biopsy: varied fiber size and shape. increased conn tissue.
Becker’s muscular dystrophy
X-linked mutated dystrophin gene.
less severe than DMD.
onset in adolescence or early adulthood.
fragile X syndrome
X-linked defect affecting methylation and expression of FMR1 gene.
TRINUCLEOTIDE REPEAT disorder (CGG).
findings in fragile X syndrome
macro-orchidism. long face, large jaw. large everted ears. autism. mitral valve prolapse.
X-tra large testes, jaw, ears
what is the 2nd most common cause of genetic MR?
fragile X. #1 is Down.
trinucleotide repeat expansion disorders
Huntington’s.
Myotonic dystrophy.
Friedrich’s ataxia.
Fragile X syndrome.
trinucleotide repeat in Huntington’s
CAG
trinucleotide repeat in Myotonic dystrophy
CTG
trinucleotide repeat in Friedrich’s ataxia
GAA
trinucleotide repeat in Fragile X
CGG
increased #trinucleotide repeats means…?
earlier onset of disease, increased severity (GENETIC ANTICIPATION)
when does EXPANSION of trinucleotide repeats occur?
during parental transmission
Down syndrome
trisomy 21.
most common chromo d/o.
most common cause of genetic MR.
95% Down syndrome cases due to?
meiotic nondisjunction of homologous chromosomes- assoc with advanced maternal age
4% Down syndrome cases due to ?
robertsonian translocation: break near centromeres, transfer of genetic material between chromos.
t(14;21) or t(21;22)
1% Down syndrome cases due to?
Down mosaicism (no maternal assoc)
findings in Down syndrome
MR. flat facies. prominent epicanthal folds. simian crease. gap between 1st 2 toes. cleft palate. hypotonia. *organ involvement*
congenital heart defects in Down syndrome
most common: osmium primum-type ASD.
endocardial cushion defects.
hematologic malignancy in Down syndrome
increased risk of ALL. also AML (M7).
GI defect in Down syndrome
duodenal atresia
neuro defect in Down syndrome
early-onset Alzheimer disease after age 35
preg quad screen in Down syndrome
decrease AFP.
increase b-HCG.
decrease estriol.
increase inhibin A.
ultrasound in Down syndrome
increase nuchal translucency
Edwards syndrome
trisomy 18.
most common trisomy resulting in LIVE birth, following Down.
findings in Edwards syndrome
severe MR. rocker bottom feet. micrognathia (jaw). low-set ears. clenched hands** prominent occiput. cong heart dz.
death usually w/in 1 year.
preg quad screen in Edwards syndrome
decrease AFP.
decrease b-HCG.
decrease estriol.
normal inhibin A.
Patau syndrome
trisomy 13. severe MR. rocker bottom feet. microphthalmia. microcephaly. HOLOPROSENCEPHALY. cleft lip/palate. polydactyly. cong heart dz.
death usually w/in 1 year.
preg quad screen in Patau syndrome
normal AFP.
normal b-HCG.
normal estriol.
normal inhibin A.
robertsonian translocation
nonreciprocal translocation when long arms of 2 acrocentric (centromeres near end) chromos fuse at the centromere and the short arms are lost
chromos usually involved in robertsonian translocation are…?
pairs 13, 14, 15, 21, 22
balanced robertsonian translocation
normally do not cause any abn phenotype
unbalanced robertsonian translocation
miscarriage.
still birth.
chromo imbalance (trisomies).
Cri-du-chat syndrome
congenital microdeletion of short arm of chromo 5 (5p-).
microcephaly. moderate to severe MR. HIGH-PITCHED crying/mewing. epicanthal folds. cardiac abn (VSD).
Williams syndrome
congenital microdeletion of long arm chromo 7 (includes ELASTIN gene).
distinct "elfin" facies. MR. hypercalcemia (sensitive to vit D). well-developed verbal skills. extreme FRIENDLINESS with strangers. cardio problems.
22q11 deletion syndromes
variable presentation: CATCH-22. Cleft palate. Abn facies. Thymic aplasia (T cell def). Cardiac defects. Hypocalcemia second to parathyroid aplasia.
- due to aberrant development of 3rd and 4th branchial/pharyngeal POUCHES.
- includes DiGeorge and velocardiofacial syndromes
DiGeorge syndrome
CATCH-22 with
thymic, parathyroid, cardiac defects
velocardiofacial syndrome
CATCH-22 with
palate, facial, cardiac defects
