The Hoxd9–13 genes are thought to specify digit identity (see Fig...

Question

The Hoxd9–13 genes are thought to specify digit identity (see Figure 18.18).

You wish to examine the effect of loss-of-function alleles in developing limbs. How would you construct a mouse in which the function of Hoxd9–13 is retained during anterior–posterior embryonic patterning but is absent from developing limbs?

Accepted Answer

Hi, everyone. Welcome back. Here's our next question. Which of the following statements is true about the sonic hedgehog S H H produced by the Z P A zone of polarizing activity region. Choice A overexpression of sonic hedgehog always results in the formation of extra digits. BS H H forms a concentration gradient along the anterior posterior axis of the developing limbed twice. CS H H signaling is not important in adult tissues or choice. DS H H is only produced in the developing limbs. Well, S H H protein is a very important signaling protein involved in many different aspects of development. But we're talking specifically the S H H protein is produced all over different regions of the embryo. But we're talking specifically about the S H H that is produced by the Z P A, the zone of polarizing activity region. And this region is the region at the posterior margin of developing limbs. So we're talking about the very posture region in those developing limbs and it's involved in organizing the polarity of the developing limb. So these two things Sonic Sonic hedgehog in the region of the Z P A, we have the Z P A organizes the anterior posterity, polarity of the limb by the expression of the S H H gene. So that's what's going on in this region we're asking about. So let's just look through our answer choices. We have choice. A overexpression of S H H always results in the formation of extra digits. Well, we always want to be wary of choices with words like always, that's a tricky word and is often going to mean it's an incorrect answer because they're trying to kind of trick you with something that can happen but doesn't always happen. And that's the case here, overexpression of S H H can result in the formation of extra digits. S H H being expressed at the margin of the developing limb. It is important in digit formation and can lead to polydactyly extra digits, but it doesn't always do that. And in fact, because it's involved in the developing limb, it actually can result in the formation of extra limbs depending on when it's over expressed or other issues. So choice A is not a correct answer. Well, looking at choice B, we can see that this is pretty promising because we are talking about the anterior posture axis of the developing limb bud, which we know is what is being organized in this region. And indeed choice B is correct S H H forms a concentration gradient along the anterior posture axis of the developing limb bud. That is how Z P A organizes this polarity. Finally, we have our last two answer choices. Let's just look at them briefly. Uh Choice CS H H signaling is not important in adult tissues and DS H H is only produced in the developing limbs. Well, both of these are incorrect. S H H continues to be expressed even in adult tissues and its signaling is important. Um We mainly know about this when it's disrupted because it is involved in the regulation as a signaling molecule of cell growth and division. And you probably recall from lots of talk about different, regulating genes whenever you talk about a gene that's involved in the regulation of cell growth and division. And when anything goes wrong with that gene, you have involvement in cancers and indeed problems with S H expression in adult tissues leads to various different forms of cancer. So that's why C N D are not our answer. And again, which statement is true about Sonic Hedgehog produced by the Epa Choice BS H H forms a concentration gradient along that anterior posture axis of the developing limbed. See you in the next video.

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