Genetics question (reciprocal translocations)

[Ven0m]

A 37yo woman in her 14th week of pregnancy undergoes amniocentesis. Fetal karyotype is revealed to be 46,XYadd(18)(p.11.2), indicating additional chromosomal material on the short arm of one chromosome 18 at band 11.2. All other chromosomes are normal. What is the most likely cause of this fetal karyotype?
(A) A balanced reciprocal translocation in one of the parents
(B) A balanced Robertsonian translocation in one of the parents
(C) An isochromosome 18i(p) in one of the parents
(D) Nondisjunction during meiosis 1 in one of the parents
(E) Nondisjunction during meiosis 2 in one of the parents

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The answer is A. The fetus has unbalanced chromosomal material (additional chromosomal material on one copy of chromosome 18). One of the parents is likely to be a carrier of a reciprocal translocation involving chromosome 18 and one other chromosome (unspecified in stem).

Can anyone explain this further? Here are the 2 chromosomes of the fetus, resulting from a normal mother (egg) and a father who is a reciprocal translocation carrier.

I would imagine chromosome 18 is in red. Only the 2nd one is abnormal - the rest are normal. As you can see, a segment of the 2nd one's p arm has been replaced by a portion of dad's blue chromosome's p arm.
What I'm confused about is when they said this: "Fetal karyotype is revealed to be 46,XYadd(18)(p.11.2), indicating additional chromosomal material on the short arm of one chromosome 18".
- Why did they say "additional" material? Wouldn't that imply this abnormal chromosome has all of its p arm genes present, plus the additional [blue] chromosome material? But what I see is that part of the p arm has been replaced with different material (this is a BALANCED reciprocal translocation) - would this still count as "additional chromosomal material on the short arm"? The word "additional" is what's confusing me, since the answer is balanced.

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[coldcase331]

I am confused too. If what they say is true (balanced translocation), that would imply that for the additional 18p received, that same chromosome must have lost another region in the process. Especially since it goes further on to describe that there's ADDITIONAL material to the chromosome beyond normal.

 

[Naruhodo]

I can check with a friend who is a cytogeneticist, but the way I am thinking about this is that the reciprocal translocation in the dad likely swapped the very tip of one chromosome 18 (basically just telomere) for a big chunk of another chromosome. So you may in fact have almost all of the normal p arm of 18 plus all that additional chromosomal material. I think the picture may be throwing you off, because of course if this were the case you would end up with the structurally abnormal chromosome being larger than the original 18 chromosome (which is not what is shown in the photo). Does this help?