rene@ascom.uucp (Rene Bach) (04/20/89)
Unfortunately, I haven't studied biology in english, so there might be some semantic misunderstanding in the previous message. Sexual cells, i.e. those that will merge to generate a new individual have undergone MEIOSIS. They have lost their second set of chromosomes. They only have 23 (for humans). Sexual cells are the sperm cells and the ovae (ovum). Checking in the dictionary, it appears that the latter are also called EGGS. To make sure, call it the UNFERTILIZED EGG, then it definitely (if it is normal) has only 23 chromosomes (for humans). Upon fertilization, the two sex cells merge and undergo a process of chromosome grouping into a nucleus. This is the fertilized egg or embryo. It will undergo many, many cell divisions using MITOSIS. somatic cells undergo MITOSIS (duplication of chromosomes and even distribution in the two daughter cells) and if normal, have a double set of chromosomes (i.e. pairs, i.e. 2x 23 = 46). The only pair of chromosomes that look different is the SEX chromosome pair. XY = males and XX for females (valid in many species but I believe not ALL). Rene Bach Switzerland
hnewstrom@x102a.harris-atd.com (Harvey Newstrom) (04/22/89)
In article <169@ascom.UUCP> rene@ascom.UUCP (Rene Bach) writes: >[....] >The only pair of chromosomes that look different is the SEX chromosome pair. >XY = males and XX for females (valid in many species but I believe not ALL). > >Rene Bach >Switzerland Obviously only valid for sexual (not asexual) creatures. In humans the Y chromosome produces males, while its lack produces females. Thus, XXY is male, while XXX is female. XO is female, but I don't think any survive in humans. In some species, such as [some?] birds, it is the two X choromosomes which produce females, while their lack produces males. Thus, XXY is female, as is XXX. XO is male in these species. Sometimes humans can be mottled, with a combination of different cells! This is caused by two cells in the beginning merging into one fetus instead of forming twins!
ogil@tank.uchicago.edu (Brian W. Ogilvie) (04/22/89)
In article <1957@trantor.harris-atd.com> hnewstrom@x102a.harris-atd.com (Harvey Newstrom) writes: >In article <169@ascom.UUCP> rene@ascom.UUCP (Rene Bach) writes: >>[....] >>The only pair of chromosomes that look different is the SEX chromosome pair. >>XY = males and XX for females (valid in many species but I believe not ALL). >> >>Rene Bach >>Switzerland > >Obviously only valid for sexual (not asexual) creatures. The latest issue of NATURAL HISTORY (May 1989) has a fascinating article on a hybrid parthenogenetic lizard found in South America. As it originated from the mating of two different (albeit closely related) species, very few of its chromosomes look the same, although they are similar enough for synapsis to occur reliably. The article focused on the two species from which the hybrid originated. Only one of them is known; the other was predicted from the chromosomal configuration of the hybrid. When the hypothesized species is discovered it will be perhaps the first species whose existence was predicted before its discovery (at least, the first one predicted to such precision; Ernst Haeckel predicted Pithecanthropus years before it was discovered by one of his former students). -- Brian W. Ogilvie / ogil@tank.uchicago.edu "Cartesianism is the most popular 'popular science' ever invented." --Noel Swerdlow
pell@boulder.Colorado.EDU (Anthony Pelletier) (04/22/89)
In article hnewstrom@x102a.harris-atd.com (Harvey Newstrom) writes: > >In some species, such as [some?] birds, it is the two X choromosomes which >produce females, while their lack produces males. Thus, XXY is female, as is >XXX. XO is male in these species. Well...ummmm, I guess this is true if you consider fruit flies a "bird." Drosophila and C. elegans are the most common lab examples of sex being determined by the "X to A ratio." That is, 1X/2A=male (with or without Y)2X (or more)/2A=female. Intermediate ratios can give rise to intersexes. Now, an ornathologist I am not, but isn't it true that the sex chromosomes in birds are call W and Z? The difference, as I recall, is that the *female* is the heterogametic sex (W Z) and the male is the homogametic sex (W W, I think). > >Sometimes humans can be mottled, with a combination of different cells! This Mosaic: see David Bowie (I do, every chance I get) -tony
mkkuhner@codon1.berkeley.edu (Mary K. Kuhner;335 Mulford) (04/22/89)
In article <1957@trantor.harris-atd.com> hnewstrom@x102a.harris-atd.com (Harvey Newstrom) writes: >In humans the Y chromosome produces males, while its lack produces females. >Thus, XXY is male, while XXX is female. XO is female, but I don't think any >survive in humans. XO is Turner's Syndrome in humans, and is viable--individuals are female and nearly normal, but sterile. >Sometimes humans can be mottled, with a combination of different cells! This >is caused by two cells in the beginning merging into one fetus instead of >forming twins! Or by loss or doubling of chromosomes in one cell early in development, which I believe is the more common way for this to happen, perhaps the only one (but "impossible" is a dangerous word). Mary Kuhner mkkuhner@enzyme.berkeley.edu
sue@gtx.com (Sue Miller) (04/24/89)
In article <1957@trantor.harris-atd.com> hnewstrom@x102a.harris-atd.com (Harvey Newstrom) writes: > >In humans the Y chromosome produces males, while its lack produces females. >Thus, XXY is male, while XXX is female. XO is female, but I don't think any >survive in humans. XO is female, but they do survive in humans - the resulting abnormal condition is called Turner's syndrome. The phenotypic characteristics of Turner's syndrome are short stature, wide, web-like neck, poorly developed breasts, and immature internal sexual organs, and sometimes mental deficiency. I used to work with a woman whose daughter-in-law had Turner's syndrome - she was short, but not really "abnormal" in appearance (whatever that means). What surprises me a little is that many organisms are pretty tolerant about wide ranging variations in number/distribution of the sex chromosomes, but such variations are definitely NOT tolerated well in the autosomal pairs. Trisomy-21, familiar to all as Down syndrome, is the only common such aberration, and it involves a fair amt of impairment (depending on the individual, of course). My genetics textbook lists 2 other trisomic abnormalities, trisomy-13 (1/5000 births) and trisomy-18 (1/10000 births). It's my understanding that these 2 conditions have extremely high mortality rates, and surviving victims are quite severely handicapped. Warning - WAG follows: Can it be that there isn't such a big deal about our sex after all? :-) -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Sue Miller {decvax,hplabs,amdahl,nsc}!sun!sunburn!gtx!sue ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~