Variations in the Sex Chromosomes FAQ

One thing I’ve always enjoyed about teaching is that students ask me questions that I never would have thought of myself. This is a reminder that everyone thinks differently and interacts with knowledge in different ways. One particular subject that frequently gets a lot of questions is unusual arrangements of the human sex chromosomes.

Let’s start at the beginning. All of our DNA is divided into 23 pairs of chromosomes, or 46 in total. Our chromosomes are numbered 1-23 by size, with the smallest being the sex chromosomes. In the sex chromosome “slot” (#23), typical human males have an “X” and a “Y” chromosome (XY) and typical human females have two “X” chromosomes (XX). All chromosomes are actually shaped like an “X” except for the “Y” chromosome — which is really shaped more like a “v” — but “X” and “Y” chromosomes are so named to distinguish the sex chromosomes from one another and from the other 22 pairs.

Typical human male chromosomes. Image from http://www.intropsych.com/ch10_development/10karyotype.jpg

Sometimes people are born with a number of chromosomes other than 46 — either too many or too few. Having too few or too many of chromosomes 1-22 can be pretty bad for you, causing, for example, Edwards, Patau and Down syndromes. Too many or too few sex chromosomes, on the other hand, frequently come with some characteristic physical and psychological traits, but they tend to be within the typical range of human variation.

Here are some questions that I’ve frequently been asked about chromosomal anomalies (the original wording of the questions was preserved to the extent possible):

Question #1: “Are women with three ‘X’ chromosomes super hot?”

Short answer: No.

This isn’t a question that would have occurred to me, but I understand where it comes from. Typical human females have two “X” chromosomes, and are usually more feminine looking than typical human males, who have only one “X” chromosome. Following this trend, females with three “X” chromosomes should be extra feminine and therefore extra attractive (if that’s what you’re into), right?

Not really. When an individual has more than one “X” chromosome, only one is active. The other/s is/are condensed into structures called “barr bodies.” When the DNA is condensed to this extent, it does not function. Females with three “X” chromosomes have two barr bodies instead of one, and therefore don’t have any extra chromosomes from a functional standpoint, and most of these people are totally unaware of the extra one. There are no morphological or psychological traits that are typical of the “XXX” condition, including increased attractiveness.

Question #2: “Are men with extra ‘Y’ chromosomes unusually masculine or violent?”

Short answer: No.

As in the previous question, it seems plausible at its face that the extra “Y” chromosome confers hyper-masculinity. Indeed, people used to believe that men with an extra “Y” chromosome were overrepresented in prison populations, but we now know that this isn’t true.

Biological masculinity broadly has to do with levels of testosterone, and “XYY” individuals do not have more or less testosterone than “XY” individuals. Just as in the “XXX” condition, people with the “XYY” condition are not noticeably different from people with “XY.”

Question #3: “Are people with XXY/XYY/XO sex chromosomes homosexual/transgender/intersex/hermaphrodites?”

Short answer: No.

Genetically speaking, maleness is determined by the presence of a “Y” chromosome, and femaleness is determined by the absence one. Female development is the default position that is altered by the presence of a “Y” chromosome. Unless something else is going on, too, people with no “Y” chromosome develop as females and people with at least one “Y” chromosome develop as males. So having two “X’s” and a “Y” usually leads to male development on account of the “Y” and in spite of the two “X’s.” “XO” refers to a condition called “Turner Syndrome” in which one one “X” chromosome is present and no “Y” chromosome. These individuals develop as females because there is no “Y” chromosome.

As far as we know, none of these combinations of sex chromosomes lead to a higher rate of people who are homosexual, transgender or intersex. “Hermaphrodite” isn’t a word that we use to describe humans anymore. We don’t know a whole lot about what causes people to be homosexual or transgender, and we know some (but not all) of the mechanisms that lead people to be intersex, but there isn’t any evidence that any of these are caused or influenced by unusual arrangements of the sex chromosomes.

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About Christopher Eppig, Ph.D.

I have a Ph.D. in biology and a passion for sharing my knowledge and understanding of the natural world with anyone who will listen. At a time where science is permeating public life more than ever, it is especially important that the public understand what science is, and how its findings intersect with their own lives. In addition to the more practical benefits of scientific literacy, I believe strongly that understanding the natural world enriches peoples lives. The man behind the curtain is not me — it is the real world, which we can discover through science, and it is beautiful. Let me show it to you.  Follow me on twitter @CGEppig. View all posts by Christopher Eppig, Ph.D.

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