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24. The parents of a son with Duchenne muscular dystrophy
(DMD) are planning to have a second child by in
vitro fertilization. Several viable embryos from the procedure
are produced. The mother does not carry any of
the known mutations in the dystrophin gene, and DNA
testing on four of the best embryos is done using a
restriction fragment length polymorphism (RFLP) that
is informative in this family. On the basis of the results
shown below, what is the best conclusion about the
(A) Embryos 2 and 4 will produce children with DMD
(B) Embryo 3 will produce a child with DMD
(C) Only Embryo 4 will produce a child with DMD
(D) Embryo 1 is a carrier of the DMD allele
(E) Three embryos will produce unaffected children

Attached Files:
question.bmp (1338 KB, 203 downloads)


is it my eyes or those bars are actually different sizes? is that important?
anyways, it seems to me that it'a A


hi babydoc...

good try but can u plz explain the answer ,how A???

the size if the lines does not matter,its jst my mistake..


well, i looked at the pattern of affected child and compared to the embrios
2 and 4 have same patterns with affected child
since you asked i feel it was wrong answergrin


ok babydoc

good u tried

but yes

dats not the right answer

so now we have 4 choices to ans from?


then my second choice is D


hi babydoc

u r the only 1 hu triednod


here is the answer

i got it wrong too

24. The correct answer is E. Duchenne muscular dystrophy

(DMD) is an X-linked recessive disease typically affecting

males. Females who inherit the disease-producing

gene are usually unaffected carriers. In this family, the

mother is a carrier of the DMD gene. Comparing her

RFLP pattern to her affected son shows that the top (larger)

fragment in her pattern is a surrogate marker for the

DMD gene. The lower (smaller) fragment in her pattern

is a surrogate marker for her normal allele. (Note that in

the unaffected father, genetically unrelated to the mother,

the top fragment is a marker for his normal gene.) Three

embryos will produce phenotypically unaffected children:

Embryo 1 (a female homozygous for the normal allele),

Embryo 3 (a male), and Embryo 4 (a female carrier).

Only Embryo 2 will produce a male with DMD.

Embryo 4 will produce a carrier female who is not

affected (thus eliminating choices A and C).

Embryo 3 (choice B) will produce a normal male without

the DMD allele.

Embryo 1 (choice D) will be a female carrier of the DMD

allele. Embryo 1 is homozygous for the normal allele.

and i still cant quite get it thoughmad


well i still don't understand, they said the mother is normal (genetically, not phenotipically)'s too late for mecool


didn't get it.... how could you guys know that 2 is male and 4 is female??


this is another point i didn't understand


babydoc4usmle wrote:
well i still don't understand, they said the mother is normal (genetically, not phenotipically)'s too late for mecool

Exactly my pointnodnod
If the mum ws genetically normal, there should never have been any case of duchene's.

I guess there's something amiss.confused


i ll explain y in a while... 1st lemme put down my thoughts... or i ll forget them..sticking out tongue

Embryo 1 is a girl
Embyo 3 is a boy




AAAARRRRRGGGHHH!!madmadmadgrinshaking head
This is driving me crazy.. n its pretty late at my end.. will try 2moro.sad

Basically, i hv managed to rule out choices B&D so far... rest later.


E.haven't looked at theattachment but it's e. coz it's an X linked disorder. Something to do with the barr chromosome i suppsose.


thickness of the lines matter from what i remember from genetis (kap).can you repost the pic with the thickness almost the same as the qustion...please....


yup thickness does matter,
in fact, i think 4 is a bit thicker than 2 (meaning 2 X chromosomes with top bar frm both mom n dad ) ---- carrier female
n 2 is comparatively thinner.... if its 1 X n that too the top 1, is the diseased one frm mom n its a boy...

This is the only explanation i cn think of... n i hv busted my brains over this...

1=female... normal


here's a figure to explain it a lil better .. sorry fr my crude drawing

Attached Files:
x.JPG (30 KB, 38 downloads)

At least tell me if i ws right or wrong in what i did..

No responses in 2 daysconfusedsad


i am very confused about the whole thing, so i will not say if you are right or not, sorry.....


Ok i guess we all r getting confused so the only person who without looking at the attachment came to the correct conclusion is Mytime. Can Mytime post the line of reasoning adopted while answering this question.

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