Review Questions for Birth Defects Lecture:

(And I will be very grateful to anyone who suggests more questions!)


A) What fraction of babies are born with birth defects in the USA?

B) What are at least four examples (include names, locations, and symptoms) of birth defects that seem as if some part of the body has been split apart?

C) What are the real explanations of each of these?

D) What are "septal defects"?

E) If a certain protein or other chemical has the effect of inducing formation of a second neural tube when implanted into the blastocoel, why would you predict that this chemical might be a teratogen?

F) For the purpose of testing chemicals to find out whether they might cause human birth defects, please design (and describe) a test using chicken eggs.

G) If mice were discovered that had a consistently high frequency (let's say 50%) of spina bifida or some other birth defect, suggest research projects to use these mice for the following purposes.

    1) To discover more chemicals (equivalent to folic acid) that could reduce the frequency of spina bifida if more of that chemical were in people's food.

    2) To discover chemicals or other treatments what might produce increased frequencies and seriousness of spina bifida.

    3) To discover new kinds of treatments for human babies born with spina bifida.

    4) Please try to invent some other use for such mutant mice... (I can think of two more)

H) Too high a concentration of the chemical retinoic acid can cause birth defects because...?

I) Suggest why birth defects might also result from not enough retinoic acid in a woman's diet.

J) Do developing embryos have a constant sensitivity to exposures that can cause birth defects, or what?

K) Based on your knowledge of normal embryonic development , please invent or imagine a birth defect. Describe what goes wrong and what the symptoms would be?

L) Suggest possible reasons why the polar bodies are so much larger in mammals than in sea urchins. (1) In terms of physical cause? (2) In terms of Darwinian selection pressure, or the lack of it

M) Suppose that a frog was discovered in which part of the body is haploid: What might have gone wrong in meiosis that could have produced this result?

N) Suppose that you had some way to cause development of sperm that have no DNA (or whose DNA has been damaged so badly that it can never replicate or make mRNA). If you fertilized a fish egg with these sperm, what would be the result?

O) What if, in addition, you prevented the first cleavage division from occurring (in one of your oocytes that were fertilized with inactivated sperm), but then let all further cell divisions occur normally.
[Hint: What would be unusual about the genes in the resulting fish?]

P) What are two different ways that you could produce triploid animals?
[Hint: fertilize with two sperm; resorb the second polar body.]

Q) Imagine that an embryo's first polar body was as large enough to become one of the individual embryonic cells, what sort of detectable abnormality could this produce?

R) Can spina bifida be detected based on blood samples from the mother? [Hint: Yes.]

*S) Would you tend to guess that conjoined twinning results from

    (1) One embryo splitting into two, but with one part of the body not separating.
    (2) Two (originally separate) embryos adhering together but merging only parts of their bodies.

*T) What kind of evidence would/could help answer this question?

    (1) Would it help answer this question if a pair of identical twins were born sharing two different parts of the body? (in the sense of being joined at two separate places)

    (2) Or would this disprove both possible mechanisms? [Hint: No]

    (3) Does the Nobel Prize-winning discovery of Proshöldt and Spemann influence your conception of what sorts of process might be the cause of conjoined twins?

*U) Would it influence your opinion if a pair of conjoined twins were born in which one was male and the other female? [maybe there was a hole in their amnions!]

V) In a few percent of identical twins, one of the twins has its aorta on the right side, and all its abdominal organs (stomach, spleen, intestine, chambers of the heart) are mirror images of the normal geometry of these asymetrical organs. This is called situs inversus viscerum, and can also occur in babies that are not twins. Please invent a possible cause, in general terms, it happens because development of these organs is partly controlled by...?

W) Suppose that situs inversus occurred in a high percentage of conjoined twins, but only those that were joined by certain parts (& NOT head to head!), how would you interpret that (if it were true)?

X) Some conjoined twins are joined head to head and others back to back, and others right side to left side. They are never joined head to back, or left side to left side, nor do any of the back to back ones have their heads pointed in opposite directions.

In other words, conjoined twins are (always?) mirror images of each other.

    1) Does this favor incomplete splitting as opposed to incomplete merger?
    2) Or does it favor incomplete merger, rather than incomplete splitting?
    3) Would it influence your opinion if armadillos often produced conjoined twins?
    4) What if an armadillo produced a pair of conjoined twins, plus TWO more embryos?
    5) Instead, what if a litter of armadillos consisted of a pair of conjoined twins, plus THREE more embryos?

Y) Based on pictures you have seen of locations in the embryonic body where different hox genes are transcribed, then in pairs of conjoined twins what would you expect about locations and patterns of hox gene expression?

Z) If a pair of twins were conjoined head to back (which doesn't really happen), in what sort of spatial pattern would you expect hox gene expression to occur? Please make a sketch.

* Does this suggest to you any new or different reasons why conjoined twins are attached by the equivalent parts of their bodies?
(e.g. "Because otherwise it's too difficult for people to draw where hox genes should be expressed?

Or maybe: "So whatever mechanism controls hox gene expression patterns could not create discontinuities in these expression patterns? Or so as not to confuse whatever mechanism controls hox gene expression patterns and thereby create discontinuities?

Or alternatively: "Because if the head were conjoined to any other part of the body, then inductive signals from head tissue would stimulate cells of the other organ (that they are joined to) to change their differentiation toward being brain nerve cells and other cells or tissues that normally only develop in the head.






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