mmm@weitek.UUCP (Mark Thorson) (08/01/85)
I have a question about biology that some netter should be able to answer: Do human X and Y chromosomes recombine? For non-experts, recombination is the process via which chromosomes exchange material. Humans have double sets of chromosomes; each cell of your body (other than the sperm and ovum) has 46 chromosomes, half from your mother and half from your father. In the process of creating the sperm and the ovum, a set of 23 is created. The material in these 23 chromosomes is drawn randomly from your 46. Without recombination, each of the 23 chromosomes would be identical with one of the 46. Hence your set of 46 would contain chromosomes identical with chromosomes in your parents, grandparents, etc. An ancestor 10 generations back might make a significant contribution to your genetics (one or two chromosomes) or no contribution at all (zero). But this doesn't happen. Each of the 23 (or at least 22 of them?) has material drawn from both parental chromosomes. Just before the 23 chromosome sets are formed, the chromosomes pair off and swap material (recombination). Thus you are likely to have small bits of DNA from all your ancestors 10 generations back. You may have a greater or lesser amount from each, but you are unlikely to have none and unlikely to have a whole chromosome. But do the sex chromosomes recombine? If they do, this has implications for human sexuality in that sex cannot be contained in the whole chromosome otherwise recombination would render most of the human population andro- genous (maybe it does?). Sex would have to (?) be contained at one site so it would be resistant to splitting via recombination. If they don't, this has implications for sociobiology. It means that all 46 of a woman's chromosomes and 45 of a man's chromosomes are some genetic hodgepodge of all his/her ancestors, but that a man's Y chromosome is identical with his father's, paternal grandfather's, etc. This is relevant to the preponderance of patriarchal societies, the low status of women, etc. Sociobiologically yours, Mark Thorson (...!cae780!weitek!mmm)
hes@ecsvax.UUCP (Henry Schaffer) (08/03/85)
Key to the understanding of this question is that the human Y chromosome is a small chromosome which carries very few genes (if any) other that what it takes to make the individual with it into a male. It does *not* carry copies of all the genes on the X, and so is not really "homologous" to the X (all the other 22 pairs are of homologous chromosomes.) Thus a male does have his father's Y, and an X from his mother (almost certainly resulting from a recombination between her two X's, as was discussed in the original posting. So some of the sociobiological implications may have to be skipped because of the few genes passed on the Y. (Fine point, there may be a small portion of the Y which is homologous to a corresponding small portion of the X. If this is so, then there could be a *tiny* bit of recombination, but this would not fit into the type of effect as in recombination of autosomes.) --henry schaffer n c state univ
werner@aecom.UUCP (Craig Werner) (08/03/85)
> I have a question about biology that some netter should be able to > answer: Do human X and Y chromosomes recombine? > They do, but whether that is in fact normal or innapropriate recombination, I do not know. However, one thing makes much of the implications moot. One, the Y ain't much of a chromosome. It might be the smallest one. (the numbers go from largest to smallest, except that 21 is smaller than 22 - it used to be thought otherwise, and by the time it was discovered, 21 was so intrenched as the chromosome associated with Down's Syndrome, they decided to be inconsistent rather than confusing.) Even if it isn't the smallest one, it is mostly heterochromatin, i.e. inactive. -- Craig Werner !philabs!aecom!werner "The world is just a straight man for you sometimes"
dick@ucsfcca.UUCP (Dick Karpinski) (08/04/85)
In article <241@weitek.UUCP> mmm@weitek.UUCP (Mark Thorson) writes: >I have a question about biology that some netter should be able to >answer: Do human X and Y chromosomes recombine? > I read something about that in the last month or so (Science News? SciAm?) which suggested that the mechanism which keeps the sex related genes from being recombined CAUSED the morphological differences to arise. I think the mechanism is of great interest but very little understood. The recombination is normally a good thing, but with sex you want only male and female, not a little of one and a little of the other. Since the recombination is suppressed, mutations are passed along whole. Over a long time, the differences between x and y became visible at the light microscope level. I'd be interested in a better answer too. Dick -- Dick Karpinski Manager of Unix Services, UCSF Computer Center UUCP: ...!ucbvax!ucsfcgl!cca.ucsf!dick (415) 666-4529 (12-7) BITNET: dick@ucsfcca Compuserve: 70215,1277 Telemail: RKarpinski USPS: U-76 UCSF, San Francisco, CA 94143
sher@rochester.UUCP (David Sher) (08/04/85)
With regard to recombination of the Y chromosome, I remember something from highschool biology. If my memory serves, in the process of meosis (sp?) where sperm and egg are formed there is a stage where there are two sets of genetic material floating about. At this point there are two identical Y chromosomes. These two chromosomes may be able to recombine. Mistakes made in this recombination can result in variance among Y chromosomes. Of course this is still a weaker effect than recombining with a different chromosome. But I really know little about bio anyway. -David Sher sher@rochester seismo!rochester!sher
alexis@reed.UUCP (Alexis Dimitriadis) (08/05/85)
In article <241@weitek.UUCP> mmm@weitek.UUCP (Mark Thorson) writes: > I have a question about biology that some netter should be able to > answer: Do human X and Y chromosomes recombine? I believe they don't. Recombination occurs between _homologous_ chromosomes, that is, chromosomes that are part of a "pair", and carry genes for the same traits. The X and Y chromosomes are completely different chromosomes. They do not pair up during meiosis, and thus no recombination events take place. It is a good thing they don't, because any recombination of parts that are not equivalent would almost certainly make the chromosome useless. (Imagine trading _part_of_ your gene for hemoglobin for part of a gene for eye color!) The Y chromosome is a very unusual chromosome: It is very small, and as far as anybody knows it has (almost) no identifiable genes on it. It is thought that the gene for having a hairy pinnae (sp), (part of the earlobe), may be on the Y chromosome, and one or two even odder things, but apart from those it does not code for any phenotypic characteristics. All the "sex linked" characters that you hear about in Biology class are on the X chromosome. > But do the sex chromosomes recombine? If they do, [...] > Sex would have to (?) be contained at one site > so it would be resistant to splitting via recombination. Note that as I implied above, recombination happens between arbitrary stretches of DNA, not in units of whole genes. A "sex gene" would not be resistant to recombination. At any rate, sex seems to be determined by regulatory mechanisms much more subtle than the possession of a particular gene. Remember, men carry all the genes that women carry. Alexis Dimitriadis -- _______________________________________________ As soon as I get a full time job, the opinions expressed above will attach themselves to my employer, who will never be rid of them again. alexis @ reed ...teneron! \ ...seismo!ihnp4! - tektronix! - reed.UUCP ...decvax! /
molbio@ucbvax.ARPA (Mike Cherry) (08/07/85)
In article <10873@rochester.UUCP> sher@rochester.UUCP (David Sher) writes: >With regard to recombination of the Y chromosome, I remember something >from highschool biology. If my memory serves, in the process of meosis >(sp?) where sperm and egg are formed there is a stage where there are >two sets of genetic material floating about. At this point there are >two identical Y chromosomes. These two chromosomes may be able to >recombine. Mistakes made in this recombination can result in variance >among Y chromosomes. Of course this is still a weaker effect than recombining >with a different chromosome. But I really know little about bio anyway. The above process occuring during meiosis, sister chromatid exchange, can cause mutations at very low levels - as is found with all forms of recombination. The Y chromosome doesn't have all the genes required for "maleness", only those involved in the determiniation of sex. The others "maleness" genes are presumably scattered amoung the other chromosomes. Mike Cherry ucbvax!molbio!mike
charli@cylixd.UUCP (Charli Phillips) (08/09/85)
In article <241@weitek.UUCP> mmm@weitek.UUCP (Mark Thorson) writes: > I have a question about biology that some netter should be able to > answer: Do human X and Y chromosomes recombine? and > But do the sex chromosomes recombine? If they do, [...] > Sex would have to (?) be contained at one site > so it would be resistant to splitting via recombination. If I remember correctly from the Human Medical Genetic course I took in college some years ago, the only known function of the Y chromosome was to "turn on" the production of androgen at an early stage of pre-natal development. All fetuses develop as females before that point, and those with a Y chromosome "become" males as a result of the effects of the androgen. Because some XY fetuses are insensitive to the effects of androgen, it is possible for a genotypic male to be a phenotypic female in every respect, including reproductive ability. (Such XY women are typically infertile but not sterile, since their "Y" eggs are useless.) Recombination of genes across the X and Y chromosomes probably occurs for those few genes that are on the Y chromosome, but it is an extremely small gene and seems to carry very little information.
emigh@ecsvax.UUCP (Ted Emigh) (08/13/85)
Sorry about this reply, but I just returned from the Joint Statistical
Meetings and my office is being painted, so if I can't do anything productive
I might as well do something on the net.
The mechanisms of sex determinism are not very well understood at the
present time. What we do know is:
1) Until 7 weeks of gestation, a fetus has both sets of sex structures
(in primitive form). At that time, individuals who are destined to
become males will develop the male structures. Somewhat later (a week
or two), the rest will develop the female structures.
2) After 12 weeks gestation, the sex of the fetus can be determined
by visual inspection.
3) This time from 7 to 12 weeks is a very critical period for the fetus,
and they are very sensitive to external stimuli. This is the time
when all the thalidomide damage is done. It is possible to interfere
with the sex determinism at this time with various sex hormones (see
work by Money at Johns Hopkins).
4) The factor that "makes a male" is located on the Y chromosome (or,
rather, the factor that starts the process). It is thought that this
gene is at the very tip of the Y chromosome, as there are XX males
who have this small fragment of Y chromosome attached to a normal X
chromosome.
5) During meiosis, the X and Y chromosome will pair up to a certain
extent. No doubt, this is needed to assure that the proper number
of chromosomes are in each daughter cell. While autosomal chromosomes
(not the sex ones) pair as =======, the X and Y will pair as
-------v-- (X chromosome on the left, Y on the right in this diagram,
with a very small area pairing).
I will be glad to expand on any of these.
--
Ted H. Emigh Genetics and Statistics, North Carolina State U, Raleigh NC
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