NBME 20 1/25 (NBME Answers)

You must be logged in to vote!

Retired NBME 20 Answers

nbme20/Block 3/Question#1 (reveal difficulty score)
The frequency of an autosomal recessive ...
1/25 🔍 / 📺 / 🌳 / 📖
tags: biostats

+16 upvote downvote
submitted by jotajota94(14)

get full access to all content ⋅ pick a username

Use the Hardy-Weinberg equation

Take the square root of 1/1600, and that will give you the frequency of the recessive allele = 1/40.
Calculate the frequency of the dominant allele with p+q=1, which is p= 0.975.
They are telling you to calculate the frequency of the disease carriers, which is with the equation 2pq.
They want only the disease carriers in which deletion is present. To calculate this, use the q value (1/40) and multiply by 80% in this should give you 0.02.
Finally, calculate for 2Pq 2 (0.975)(0.02)= 0.04 = 1/25.

get full access to all content ⋅ pick a username

yex Nice! ...and we are supposed to read the stem and do all this in a minute or so? :-/ +28

charcot_bouchard Allele frequency 1/40. so carrier freq 1/20. 80% of 1/20 is 1/25 (80/100 x 1/20) +15

dickass Ah feck, 2pq got me +1

hello_planet A handy shortcut for Hardy-Weinberg is that you generally can assume p ~= 1 if q if fairly low. It also tends to be easier to work in fractions if the answer choices are in fractions so you don't have to bounce back and forth between fractions and decimals. So with that, you send up with 2pq = 2 * 1 * 1/50 = 2/50 = 1/25. +9

topgunber hate this whole scramble thing: In one line: 2 * q * 80%. This is for diseased individuals (two q alleles).I = 1/1600 = q^2 The frequency of q = 1/40 Now carriers is 2 p q. P is close to one assuming HW eqm (p+q=1). There's an additional step in this question due to the two different mutations. so 2(q) = 1/20. 80% of these carriers are deletions so multiply 1/20 * 0.8 = 1/25 +

@usmlecrasher

1/40 = .025 = q P + q = 1 -> P + .025 = 1 -> P= 0.975

+3/- alexp1101(11)

You must be logged in to vote!

+2 upvote downvote
submitted by ∗topgunber(68)

get full access to all content ⋅ pick a username

hate this whole scramble thing: In one line: 2 * q * 80%.

This is for diseased individuals (two q alleles).I = 1/1600 = q^2 The frequency of q = 1/40

Now carriers is 2 p q. P is close to one assuming HW eqm (p+q=1). There's an additional step in this question due to the two different mutations.

so 2(q) = 1/20. 80% of these carriers are deletions so multiply 1/20 * 0.8 = 1/25

get full access to all content ⋅ pick a username

You must be logged in to vote!

+0 upvote downvote
submitted by thechillhill(1)

get full access to all content ⋅ pick a username

p + q = 1 p^2 + 2pq + q^2 = 1 if q^2 = 1/1600 = 0.00063 then q = sqrt(q^2) = 0.025 solve for p to get p = 1 - r = 1 - 0.025 = 0.975 the heterozygous carriers = 2pq = 1 - p^2 = 1 - 0.95 = 0.5 q^2 can be dropped b/c it's much smaller than p^2. The deletion is responsible for 80% of the mutations. 0.8 x 0.5 = 0.04 = 4/100 = 1/25

There might be an easier way to do this, but it worked for me.

get full access to all content ⋅ pick a username

thechillhill So apparently I don't know how to format very well. Sorry! +1

pakimd So because i couldnt spend more than a minute on this question and honestly didnt recall the Hardy-Weinberg equation this is how i solved it under a minute: so you know in a given population half of them will be carriers since its an autosomal recessive disease Aa Aa= AA aa Aa Aa so of that half 80% are due to deletion mutations and 20% are due point mutations by that logic 80% of half into 20% of half will give you 1/25 +1

draykid 0.8 x 0.5 is 0.4 +

You must be logged in to vote!

Must-See Comments from nbme20

amorah on Cytomegalovirus infection
masonkingcobra on Contact with parakeets
hayayah on Capillary hydrostatic: increased; ...
hayayah on X chromosome-linked isoenzymes
medbitch94 on Mannose 6-phosphate
hello on Capillary hydrostatic: increased; ...
hayayah on Osteoblasts
imgdoc on Hypophosphatemia
hayayah on Missense
celeste on 50%
justgettinby on Omeprazole
andrewk1 on Cold, dry air
yotsubato on Jugular venous pressure of 12 mm Hg
strugglebus on Drug effect