Most cases of Turner syndrome are attributed to nondisjunction of one or more of the sex chromosomes during gametogenesis, from either the male or female parent. However, some females possess a rare form of Turner syndrome in which some of the cells of the body (somatic cells) lack an X chromosome, while other cells have the normal two X chromosomes. Often detected in blood and/or skin cells, such individuals with mosaic Turner syndrome may exhibit relatively mild symptoms. An individual may be specified as 45,X(20)/46,XX(80) if, for example, 20 percent of the cells examined were X monosomic. How might mitotic events cause such mosaicism, and what parameter(s) would likely determine the percentages and distributions of X0 cells?

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Accepted Answer

Hi, everyone. Let's look at our next question. It says, why does mosaic Turner syndrome tend to be less severe than monos Omi Ax Turner syndrome? Well, let's recall from our content video. Um what Turner syndrome is and the difference between mosaic Turner syndrome and monos omi X Turner syndrome. Turner syndrome. Of course is when a fanatic female has just a single X chromosome. Her genotype is X O just missing the second sex chromosome. The monas omi ax version, all cells in the affected individuals body have this genotype. And that's because the non disjunction, the error in the splitting up of the chromosomes that caused the condition occurred in the parents gametes. So it existed in the gametes before fertilization. So once fertilization occurs, the embryo starts developing um right from the beginning, right from the zygote. Um all the cells have X O in mosaic Turner syndrome. The event that caused this error, this non disjunction occurs after fertilization early in embryonic development. So it occurs in one of the main topic divisions as the cells in the early embryo began to divide and differentiate. So as a result, only the cells descended from the cell that has that error will have this Turner syndrome genotype. The other cells in the body descended from cells where there wasn't, this error are not affected. They have the normal um two X chromosomes. So with that in mind, let's look at our answer. Choices. Choice. A says mosaic Turner syndrome is recessive. Um This is not correct again. The syndrome is just because it arose in a cell after fertilization. That condition is present in every cell that's descended because that as a result of mitosis from the cell in which the era arose. Choice B says mosaic syndrome, Turner syndrome is not heritable. Well, this is not our answer. Whether it's heritable depends on which cells contain that error. So if any of the cells that will give rise to the gametes have um have that error in them, then if theoretically uh the woman could conceive, she could pass that on. Um However, in general, most individuals Turner syndrome are infertile, but this doesn't, isn't a difference between mosaic Turner syndrome and monitor me ex Turner syndrome. So not our answer choice. Troisi says mosaic Turner syndrome affects fewer cells. And this is the answer we're looking for again because while the monas omi X version is present in every cell in the body. Mosaic Turner syndrome only affects those cells descended from the cell where the original error occurred. Since it affects fewer cells, the effect of the syndrome tends to be less severe finally, choice D says mosaic Turner syndrome is easier to diagnose. Um well, precisely because the effects are less severe. Mosaic syndrome is harder to diagnose. Since you might not see those symptoms that you expect with Turner syndrome as dramatically as you would in the monos omi X form. So choice D is not an answer. So again, why does mosaic Turner syndrome tend to be less severe than monos Omi X Turner syndrome? Choice C Mosaic Turner syndrome affects fewer cells. See you in the next video.

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Key Concepts

Nondisjunction

Mosaicism

Mitotic Events