Genetics for the USMLE/COMLEX

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  1. Recurrence risk
    probabiltiy that the offspring of a couple will express a genetic disease
  2. Autosomal dominant inheritance
    • There is no carrier state
    • At least 1 parent must be affected 
    • generally observed in multiple generations
    • males and females are affected equally

    • Recurrence risk: for one affected parent is 50%, for 2 affected parents is 75%
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  3. Autosomal Dominant Diseases
    • Acute intermittent porphyria
    • Familial hypercholsterolemia
    • Huntingtons disease
    • Marfan syndrome
    • Neurofibromatosis type 1
  4. Autosomal Recessive inheritance
    • heterozygous individuals are carriers
    • offspring must inherit 2 copies of the disease causing allele - one from each parent
    • the disease skips generations
    • males and females are affected equally
    • more common in incestuous matings
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    • Recurrence risk
    • In a pairing of 2 heterozygotes, 25% chance of having the dieases, 50% chance of being a carrier, and 75% chance of not having the disease
    • If one parent is a carrier , there is a 50% chance of being a carrier
    • If one parent is affected, there is a 100% chance of being a carrier
    • IF one parent is affected and one is heterozygous, there is a 50% chance of being affected and a 50% chance of being a carrier
  5. How to read a pedigreeImage Upload
  6. Diseases that are Autosomal recessive
    • Cystic fibrosis
    • Phenylketonuria
    • Sickle cell anemia
    • Tay-Sachs disease
  7. X-linked recessive inheritance
    • males are considered hemizygous - only 1 copy of the gene
    • more commonly seen in males
    • skipped generations are common
    • male-to-male transmission is not observed

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    • Recurrence risk
    • In a cross between a carrier mother and an unaffected father, there is a 50% chance of a male being affected, and 50% of the daughters will be carriers
    • In a cross between a normal mother and an affected farther, 100% of the daugthers are carriers
    • In a cross between a carrier mother and an affected dad, there is a 50% chance of the boys being affected, and a 50% chance of the daughters being affected as well as a 50% chance of the daughter being a carrier
  8. Diseases with x-linked inhereitence
    • Bruton agammaglobulinemia
    • Duchenne muscular dystrophy
    • G6PD deficency
    • Hemophilia A and B
    • Lesch-Nyhan syndrome
    • Menkes disease
    • Ornithine transcarbamoylase defiecency
  9. X inactivation
    • 1 x gene in a cell is active all others are formed into Barr bodies and inactivated this process is random and occurs during the blastocyst stage of the embryo
    • they are inactivated due to the XIST gene
  10. X-linked dominant inheritance
    • similar looking to a autosomal dominant chart
    • disease si seen 2x mreo often in females as in males
    • no carriers
    • no skipped generations
  11. X-linked dominant diseases
    • Fragile X syndrome
    • Hypophospatemic rickets
    • pyruvate dehydrogenase deficency
  12. mitochondrial inhereitance
    • Always from the mother - so all offspring of an affected female
    • none of the offspring of an affected male

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    heteroplasmy: the mutations can arise in some mitochondria but not others, so there can be variations in expression among cells
  13. Diseases with mitochondrial inhereitence
    • Leber hereditary optic neuropathy
    • mitochondrial encephalomyopathy, lactic acidosis and stroke-like syndrome (MELAS)
    • Myocolonic epilepsy with ragged red muscle fibers
  14. Incomplete penetrance
    a person with a genotype for a disease who does not inherit it,
  15. Variable expression
    the extent to which a diease causing allele affects and individual
  16. locus heterogeneity
    mutations in multipel loci can cause the same disease phenotype
  17. Pleiotropy
    a single genetic defect affects multiple organ systems
  18. Marfan syndrome
    • mutation is in the fibrillin gene
    • individauls are tall, kyphoscoliosis, eye abnormalities (lens dislocation, retinal detachment), cardiac abnromalties (aortic dissection, mitral valve prolapse)
  19. Diseases with delayed age of onset
    • acute intermittent porphyria
    • familial breast cancer
    • hemochromatosis
    • Huntingtons disease
  20. Imprinting
    certain genes are expressed in a parent-of-origin specfic manner

    ex/ prader willi and angelman syndrome
  21. Prader Willi Syndrome
    intellectual diability, hypogonadism, obesity

    deletion in 15q11-13 from paternal chromosome
  22. Angelman syndrome
    Deletion of 15q11-13 fro maternal DNA

    Severe intellectual disability, ataxia and seizures
  23. Cytogenetics
    the study of microscopically observable changes in chromosomes
  24. karyotype
    ordered photographic display of the 23 chromosomes observed during metaphase stage of mitosis
  25. Euploidy
    human cell with 23 chromosomes
  26. Aeuploidy
    A deciation form the euploid number 

    • monosomy - loss of a chromosomy
    • trisomy - a gain of a chromosome
  27. Trisomy 21 (Down Syndrome)
    • Intellectual disability
    • short stature
    • depressed nasal bridge
    • upslanting palpebral fissures
    • epicanthal folds
    • congenital heat defects
    • early onset dementia
    • increased risk of leukemia
  28. Trisomy 18 (Edwards Syndrome)
    • low ridign ears
    • prominent occiput
    • micrognathia
    • flexion of the fingers
    • rocker-bottom feet
    • intellectual disability
    • die during first year of life
  29. Trisomy 13 (patau syndrome)
    • Microphthalmia
    • microcephaly
    • celft lip and palate
    • polydactyly
    • congenital heart diseas
    • die during first year of life
  30. Turner Syndrome (XO)
    • short stature
    • webbed necks
    • cystic hygrp,as
    • ovarian dysgenesis --> primary amenorrhea
  31. Klinefelter Syndrome (XXY)
    tall stature, long limbs, testicualr atrophy, female distribution of body hair
  32. Non-disjunction
    • can occur durign meiosis 1 or meiosis 2
    • leads to trisomoes or monosomies
  33. Translocations
    the physical movement of genetic material on one chromosome to another
  34. reciprocal translation
    genetic material is swapped between 2 chromosomes, no loss of genetic material
  35. roberstonain translocation
    fusion of 2 acrocentric chromosomes, the small p arms are lost in the process
  36. Cri-du-chat syndrome
    • deletion fo the short arm of chromosome 5
    • microcephaly, servere intellectual disability, high pitched cry
    • cardiac abnormalities
  37. Inversions
    • portions of a chromosome are flipped or inverted
    • - pericentric: involve the centromere
    • - paracentric: do not invovle the centromere
  38. isochromosome
    2 copies of 1 arm are created - loss of the other arm
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Genetics for the USMLE/COMLEX
2013-07-26 23:47:59

genetics cards for the USMLE/COMLEX
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