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NBME 23 Answers

nbme23/Block 1/Question#33 (53.0 difficulty score)
A 25-year-old woman and her 25-year-old ...
α-Thalassemia trait🔍

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submitted by mambabambaboom(10),
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bc het atnsrep rae naai,s yteh are csi eeiltodn srricer.a oS ihrte kdi nca retihe eb a cis aercrri or haev bH rtsB,a its otn pioblsse to gte bHH

submitted by asapdoc(61),
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DjGroan.l ipnexlsa it elylra lewl on hte da.uoi I lilw tsju ivge eth iscba ea.di llA het ilboegsmohn vhae aahpl ni it hoetrreef ouy nnacto netoic it on mehobiolng crisoseeorltpeh

someduck3  Just to add to this; a-thal is due to a deletion. While b-thal is due to a mutation. If they had a b-thal there would be target cells. a-thal just presents as microcytic & hypochromic. +8  
almondbreeze  looks like a-thal can have target cells too. Individuals with alpha thalassemia trait (-α/-α or --/αα) are asymptomatic, with a normal CBC. The peripheral blood smear typically shows hypochromia, microcytosis, and target cells. ( › article › 955496-clinical) +  

submitted by nwinkelmann(285),
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I oufnd a rputeic inwgsho het mnartissnsio by two tooyhesrugez aaplh ealstsmihaa artit sinaa otcd-eslie(ni) trp,asen nda iofdmdei it ot sola hwso eht ithninecrea yb wot yeotuoesrzgh halap assatlimeha ncaiafr sleia(nn-)otdret ean.psrt eeHr ouy og: JTV2rTpIooc:h17cvqnedsUi/.ndddP1/mRJtgW4.oe8lHwOQw9/tpvouEhewg?=9i

makinallkindzofgainz  broken link +1  
drdoom  broken link +  

submitted by masonkingcobra(318),
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yaEs cuiprtes ot ansdnrdtue

submitted by drschmoctor(82),

Both parents have alpha-thalassemia trait. Given they are Asian, they will have the cis-mutation. That is, each parent will have 1 chromosome 17 with 0 alpha chain genes + another chromosome 17 with 2 alpha chain genes).

Thus, each parent can pass on either 0 or 2 copies of the alpha chain gene. So, the offspring will have a

  • 25% chance of having 0 alpha chains (causing hydrops fetalis & death in utero),
  • 50% chance of having 2 alpha chains (alpha-thal trait, answer choice D), and a
  • 25% chance of having 4 alpha chains (normal).

A) Hemoglobin H is when there is 1 alpha chain, so that is not possible.

B, C, D) Both parents had normal hemoglobin electrophoresis, so Hb S, HbSC, and B-thal are not possible.

rockodude  *alpha chain is on chromosome 16, beta chain is on chromosome 11 +1  

submitted by nor16(57),
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in ß meihsasTala rthee is bH2A enreacsi dna bHA earc,sesde neve in ß+, nmoalr heoipolressetcr ulsre hsit tuo. amse rof C-))A

tyrionwill  Yes! The key point is the normal electrophoresis. Hemoglobin will not show any abnormalities until least one single allele of Hb fully develops mutation. alpha chain of Hb is contributed by two points within one single allele, while beta chain of Hb is made of only one point in one single allele, therefore: -- in alpha thalassemia Hb electrophoresis will be normal if only one point gets mutation, i.e., aa/a- which we call it the "minima type". If two points get mutation, i.e., cis aa/--, or trans a-/a-, the "minor type", the Hb electrophoresis will be either abnormal(Hb Barts 3-8%) or still normal. overall, we assume the parent both are the minima type, so their children have 50% chance to be the minor type, 25% to be fully normal, and 25% to be the trans minor. -- However in beta thalassemia, the mutation of one allele will lead to whole allele changed, so we just need the mutation once to generate an abnormal Hb electrophoresis result. +1  

submitted by krewfoo99(90),
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In hwta tiisutsnao wlil bHH be dmerof (3 laaph ncaih s)leiotde?n

ergogenic22  one parent has 2 deletions on the same gene, the other parent has 1 deletion, and the offspring receives all three. In this question, both parents have alpha 1 deletion +  
ergogenic22  actually its possible that they both have 2 gene deletions, but regardless, a-thalassemia trait is more likely +  
ergogenic22  and someone above said Asian people are cis-2 deletion so the offspring will not receive two deletion from one parent +  
ergogenic22  ↑↑ I made a mistake by confusing trans and cis cis has deletions on the same chromosome and can pass two deletions to off spring, therefore a chance of allowing HbH +  

submitted by prolific_pygophilic(8),

Meme: Asian parents with a clear hemoglobinopathy, Me slapping the Alpha Thal Button