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*USMLE* > Genetics > Flashcards

Genetics Flashcards

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USMLE® Step 1 flashcards
1
Q

Meiosis I nondisjunction results in what?

A

failure of HOMOLOGOUS CHROMOSOMES to separate

after normal meiosis II with sister chromatid separation, 4 gametes form…

2 with 24 chromosomes (doubled chromosome is different due to sister chromatid sep > 2 bands on an RFLP analysis) > cause trisomies

2 with 22 chromosomes > cause monosomies

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2
Q

Meiosis II nondisjunction results in what?

A

failure of SISTER CHROMATIDS to separate

results in 2 gametes

1 with 24 chromosomes (double chromosome is SAME because sister chroms didnt sep > 1 thicker band on RFLP analysis) > causes trisomy

1 with 22 chroms > monosomy

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3
Q

MCC turner syndrome

less common causes + their consequence

A

Meiotic nondisjunction - forms a gamete without an X chromosome > all pts cells are 45, XO

(Early embryonic MITOTIC nondisjunction - mosaicism, some cells are 46, XX

may also be due to having both Xs but one is dysfunctional (ie “x fragments” or “isochromosomes”)

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4
Q

which gene is missing in Turner that results in short stature?

A

SHOX gene

promotes long bone growth

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5
Q

7 common conditions with polygenic inheritance

A
  1. Androgenetic Alopecia
  2. Hypertension
  3. Glaucoma
  4. Epilepsy
  5. Ischemic heart disease
  6. Schizophrenia
  7. Type II Diabetes

AH GEIST

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6
Q

Trisomy 18

name? cause? features?

(musculoskeletal features + organ systems affected; specific changes to organ systems in another card)

A

Edwards syndrome
meiotic nondisjunction of chr. 18 (incr. with maternal age)

Micrognathia
Prominent occiput
Low-set ears
Clenched hands + overlapped fingers
Rocker feet
Limited hip abduction
Overall growth restriction

Heart, renal + GI defects plus INTELLECTUAL disability

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7
Q

Trisomy 18 - Edward’s

Possible defect by organ system (3 systems, 5 defects)

A

Heart - VSD or PDA

GI - Meckel’s diverticulum or malrotation

Kidney - horseshoe

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8
Q

Cri du chat

genetics? s/s?

A

5p deletion

Cat-like cry
Hypotonia
Developmental delay / failure to thrive

Microcephaly
Low-set ears
Hypertelorism
Broad nasal bridge

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9
Q

Trisomy 13

name? externally visible defects + internal organ systems involved?

(Specific effects by organ system next)

A

Patau syndrome

MIDLINE DEFECTS:
Microcephaly / holoprosencephlay
Microphthalmia
Cleft lip/palate

Polydactyly
Rocker bottom feet
Cutis aplasia (scalp lesions)

Cardiac, renal and GI defects

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10
Q

Trisomy 13

effects by organ system?

A

Patau syndrome

GI - omphalocele, umbilical hernia

Heart - VSD, PDA

Urogenital - renal defects + abnormal genitals

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11
Q

Patau syndrome, trisomy 13

what general embryonic defect do most of the phenotypic features suggest?

A

a defect in PRECHORDAL MESODERM FUSION

affects midface, eyes, forebrain and GI (omphalocele)

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12
Q

Williams syndrome

4 features? 2 organs and 2 general features

A

elf-like face
extroverted

supravalvular AORTIC STENOSIS
RENAL ARTERY stenosis

(think open personality, closed arteries…)

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13
Q

DiGeorge

gene? msk / visible changes?

organ systems affected?

A

hypertelorism
micrognathia
cleft palate

CV - interrupted aortic arch, tet of Fallot
immune - thymic hypo-/aplasia
endocrine - hypoparathyroidism

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14
Q

Nucleotide Excision Repair

defective in what? enzyme sequence (3)?

A

defective in XERODERMA PIGMENTOSUM (XPA gene)

  1. ENDONUCLEASE - recognizes helix deformed by UV-induced thymine dimer; cleaves single strand on both sides of segment
  2. DNA POLYMERASE - replaces segment
  3. DNA LIGASE - seals gap
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15
Q

What is the formula for ALLELE FREQUENCY in Hardy-Weinberg analysis?

A

p + q = 1

p = normal allele freq
q = mutant allele freq
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16
Q

What is the formula for PHENOTYPIC FREQUENCY in Hardy-Weinberg analysis?

A

p^2 + 2pq + q^2 = 1

p = normal allele freq
q = mutant allele freq
17
Q

With rare autosomal recessive disorders, what can we assume NORMAL ALLELE FREQUENCY to be?

(remember normal allele is known as “p” in Harvey Weinberg analysis)

A

1

because if the mutant allele frequency is 1/100… or 1/200… or less … then the normal allele frequency is 99/100 or higher and mathematically might as well be 100%

18
Q

What is the formula for calculating MUTANT ALLELE FREQUENCY from disease prevalence in Harvey Weinberg analysis?

A

√q^2 = q

q = mutant allele frequency

diseases pts has 2 mutant alleles, so disease frequency = q^2

19
Q

What is mosaicism?

Somatic vs. Gonadal/germline mosaicism?

A

presence of genetically distinct cell lines in same individual

somatic - mutation from mitotic errors AFTER FERTILIZATION and propagates through multiple tissues/organs

gonadal/germline - mutation ONLY IN EGG OR SPERM CELLS; if parent/relative does not have disease, can be suspected

20
Q

What is an example of a disease with mosaicism?

think endocrine/dermatological

A

McCune Albright

altered G protein signaling > unilateral CALMs (large + ragged), polyostotic fibrous dysplasia, 1+ endocrinopathy (precocious pub)

lethal if mutation occurs before fertilization > all cells; survivable if mosaic

21
Q

What hematological malignancy is most common in Downs’ syndrome?

what other one has increased risk?

A

ALL - acute lymphoblastic leukemia (10-20x risk)

AML also increased

22
Q

what is the formula to find the FREQUENCY OF CARRIERS (heterozygotes) of the allele for a RARE AUTOSOMAL RECESSIVE disease?

A

freq. of carriers = 2q

because dominant allele frequency in rare AR diseases is assumed to be p = 1

so the normal 2pq formula for AR carriers becomes 2q

23
Q

Williams syndrome

genetic abnormality? serum abnormality + why?

A

7q microdeletion - (includes elastin gene)

HYPERcalcemia due to increased vitamin D sensitivity

24
Q

Syndrome with similar genetic defect to DiGeorge but without the hypocalcemia + thymic defect?

A

Velocardiofacial syndrome - 22q11 deletion

palate, facial and CV issues

(3rd/4th branchial pouches)

25
Q

Uniparental Disomy

if pt is HETEROZYGOUS for the 2 chromosome copies inherited from 1 parent…. what is this called? and when did the error occur?

A

Heterodisomy

occurs in Meiosis I

(heterodisomy has one i > meiosis I)

26
Q

Uniparental Disomy

if pt is HOMOZYGOUS for the 2 chromosome copies inherited from 1 parent…. what is this called? and when did the error occur?

A

Isodisomy

occurs in Meiosis II

(isodisomy has 2 “i”s > meiosis II)

27
Q

How can a patient get an autosomal recessive disease WHEN ONLY ONE PATIENT IS A CARRIER?

A

uniparental disomy

specifically “isodisomy” where the pt is HOMOzygous for the 2 chromosome copies inherited from that parent (due to error in Meiosis II)

28
Q

In Hardy Weinberg analysis…

what is the frequency of an XR disease in males? females?

(formulas)

A

in males frequency = q

(q is the frequency of the recessive allele)

in females frequency = q^2

(less frequent! because q is a fraction like… 1/1,000 so multiplying that by itself gives you 1/1,000,000)

29
Q

Prader-Willi

genetics

(what is imprinted, what is deleted?)

A

Maternal genes are IMPRINTED and

Paternal deletion or mutation of chromosome 15

or

Maternal UPD of chr. 15 (25%)

(so remember the DELETION is what goes with the letter, the UPD/imprinting is what is opposite the letter… Prader = Paternal deletion, maternal UPD, imprinting)

30
Q

Angelman syndrome

genetics

A

Paternal UBE3A (chr. 15) is imprinted and

Maternal allele is deleted or mutated

or

Paternal UPD (5%)

(AngelMan = Maternal deletion)

31
Q

Angelman Syndrome

s/s (behavior, neuro issues)

A
  • inappropriate laughter
  • seizures
  • ataxia
  • severe retardation

*USMLE* (28 decks)

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