OK. Yeah, that’s pretty much what it was like in the 20s and 30s. But then, that’s when everyone was searching for “the missing link”, too, thinking that one great find would solve the puzzle. We now know, of course, that the puzzle is much, much more complicated than finding one “missing link”, and the multitudinous proliferation of human ancestors and side branches is something no one could have imagined 100 years ago.
No, it doesn’t. For the offspring to be fertile, the chromosomes need to be able to pair and segregate in meiosis I. In a hypothetical hybrid, with one copy of the human fused chromosome 2, and one copy of the two ape unfused chromosomes, meiosis could still happen just fine. The two ape chromosomes could line up and pair with the two halves of the human chromosome and then segregate to opposite spindle poles.
Not to mention, with laboratory manipulation, at some point, we may not have to rely on nature to create hybrids, and so we may be able to create stable breeds that breed true. Not that anyone wants to do that with apes and humans (well, probably someone does, but no one is giving them grant money), but we might want to do it with mules or hinnies. (BTW, isn’t it mules that are nearly always sterile, and hinnies that are actually frequently fertile, but only female hinnies, so they have to be mated with either horses or donkeys-- you can’t breed a hinny with a hinny, and at any rate, the mule is the more desirable of the two as far as the traits of a work animal go?)
How about milk related because as a Canadian my first flippant response was 2%.
It’s usually the male hybrid that’s sterile - Haldane’s rule.
How do Down’s syndrome people reproduce? IIRC they are not sterile typically, despite mismatched chromosomes. Do they only reproduce with the proper set?
Although it does involve an extra chromosome, Down syndrome is a very different thing than the differing chromosome numbers we’ve been talking about. When looking at the human/chimp chromosome differences, the total information content (for lack of a better phrase) hasn’t changed. It’s just been rearranged slightly. It’s like dividing a book into ten chapters instead of nine, but leaving all of the words the same. If you were to compare the DNA content of a 22-pair individual and a 23-pair individual immediately following the split, they would be essentially the same.
A Down syndrome patient has an extra chromosome. There’s a misprint in their book where several pages have been duplicated. That extra DNA throws lots of things off balance and produces problems.
OK. That said, Down syndrome patients actually often ARE infertile, but that’s an indirect effect of the syndrome rather than a direct consequence of the extra chromosome. It’s a developmental defect thing rather than a “I can’t do meiosis right with this extra bit hanging around” thing. When these patients do make gametes, the extra chromosome segregates randomly, resulting in roughly half euploid gametes and half aneuploid. In other words, if you were to look at sperm cells from a Down syndrome male, roughly half of them should have the normal one copy of chromosome 21, and half should have the abnormal two copies. Fertilization with the first kind of cell would result in a normal, non-Down syndrome child, while fertilization with the second kind would result in Down syndrome.
Before someone jumps in with “I know a DS person with a child,” I just thought I’d note that DS often happens in a mosaic pattern, which means that the person dos not have the extra chromosome in all cells. This is why the syndrome have variable expression. Occasionally you even meet DS people with normal intelligence.
I went to high school with someone who had DS, and was probably actually in the low normal range rather than MMR. She had to take Resource, and was in some remedial classes, but she got a high school diploma, on time (which is to say, in four years, albeit, I don’t recall if she was the same age or maybe a year older than her high school class). She had a daughter, who is not only normal, but actually quite exceptional (and really beautiful). It’s interesting, because this woman, who was sort of in my social group, although not in my kind of inner circle, but we did occasionally go to movies or out to a coffee shop with the same group, always had interesting tastes for a person with DS, I thought at the time, and I’m sure had a big influence on my attitude toward working with mentally retarded and autistic people later in my life. She liked the same art house movies that I liked, and watched programs like Nova on TV. She really was a very interesting person with a lot of personality. She even got an Associates degree in something in her 30s, although it took her several years, because she took a light courseload each semester, but she did it.
Additionally, not all Downs Syndrome* folk have a complete extra chromosome. In some cases, it’s just part of the chromosome, and those generally result in less severe symptoms.
*Trisomy 21, or three copies of chromosome 21
I left barber college
Searchin’ for knowledge,
Went to the university.
I must confess, sir
This lady professor
She turned me on to anthropology.
Now I’m a homo erectus
Got to connect this
Bone that I discovered yesterday.
Tyrannosaurus
Lived in the forest,
Died because it’s heart got in the way.
Yeah. The further down the chromosome line, the less serious the disorders get, which is why trisomy 21, or Down Syndrome, is so survivable, and so is an extra sex chromosome. There is such a thing as trisomy 3, for example, but it isn’t survivable if a person has a whole third chromosome; however, some people have a tiny extra portion of a third chromosome, and it results in deafness, blindness, autism, very short stature, anodontia (lack of teeth), and problems with glands from complete absence of some of them, so that they need medication, like thyroxine (the thyroid gland is the most likely one to be completely missing), to disorders, so they may have a pancreas, but still be insulin-dependent diabetics.
It also depends on which portion of the chromosome you get. The syndrome I described is really rare (probably because of viability problems with fetuses that have it), but there is a less rare problem involving a different portion of the third chromosome-- it also depends on whether it’s a floating strand, or a “Y” bud.
It’s also probably because chromosome 21 is small. They are ranked by size, so it’s one of the smallest.
And chromosome 2 is one of the largest because it’s the fused versions of the two separate “chimp” chromosomes.
That’s what I meant-- I guess I just assumed that people knew that numerically, the chromosomes got smaller as they got numerically larger-- except that the X is as big as 9, and Y is about as big as 20, which is why they have letter designations instead of number designations, I think.
BTW: there are three basic kinds of chromosome disorders: trisomies, deletions, and transpositions. Trisomies are an extra copy, or partial copy of one, loss of one, where a person is missing all or part of one of a pair (males have partial X deletion, in a sense, which is why they have so many problems, like color-blindness and hemophilia, that women almost never have). Transposition is when all or part of a chromosome flips over and reconnects, or connects to a different chromosome. Usually people who have these are not viable, but sometimes they have them, and have disorders that are (or at least as far as the history of medicine goes) sui generis. There’s also a fragmentation disorder, but I think only fragmented X (ie, “fragile X”) is viable.
One of the biggest puzzles in human evolutionary biology is exactly when our lineage acquired the fused chromosome #2. We know that Neanderthals and Denisovans have it, so that puts it back at least 500K years. Plus… what exactly did it do for us, if anything?
I had absolutely no clue. I assumed that the number was related to their location.
Do we know anything about the DNA of any of the species of australopithecines? If they had it, that would suggest they are in human lineage.
The gene that is damaged in people with ALS is on chromosome 2, and the variant gene that produces something called Waardenburg’s syndrome, which is a dominant gene with variable expression (it can cause anything from a small lock of unpigmented hair on the forehead, to a lack of pigment in the center of the forehead, across the top of the head, blue, or partially blue eyes in someone who genetically ought to have brown eyes-- including black people-- and degrees of deafness that don’t necessarily correlate to the degree of pigment loss).
Also, several types of collagen are expressed by the 2nd chromosome, so some diseases that involve problems with skin, like Ehlers–Danlos syndrome, and a congenital form in ichthyosis are caused by defective genes on this chromosome.
There’s a theory that genes passed on the 2nd chromosome are responsible for many of the heritable aspects of longevity, as they are responsible for the ability to repair tissue, and also some seem to effect* hypertension somehow.
If the fusing of the second chromosome resulted in the potential extension of human life, that could have led to a reorganization of social groups, from age cohorts to extended families. Identifying one’s family as an extension of self would lead to lots of things, from the survival of infants whose mothers died, to the willingness to sacrifice for a relative other than one’s child.
Among a lot of other things, it explains how a gene for gayness would be beneficial, if it resulted in a family group having a higher ratio of adults to children than families that did not have members with a sort of “natural birth control.” There’s probably a sweet spot, like 5:1, child:adult, or something, where the most children survive without wasted resources on ones that don’t. You know, families that start out with 5:1 may have an 80% survival rate, while those with a 10:1 may have a 30% survival rate, because they have less food per child, more crowded conditions, so more disease, and a higher maternal mortality rate, which contributes to keeping the ratio as unbalanced as it is.
That last bit is all speculation, but it’s not just me making stuff up. The “grandparent” theory, and the “gay aunt/uncle” theory are all real theories of early human social organizations.
*yes, that’s the right word.
One would expect quite a lot of interesting genes to be on chromosome 2, given its size and the number of chromosomes humans have. Humans have a lot of genes, right? Well, over 1/23 of them will be on 2.
Actually, more than 1/23, because 2 is the second largest chromosome. It represents almost twice that much, 2/25 of our genome. (Give or take, really, since men have only 98% of the DNA women have.)
I don’t think we have any Australopithecine DNA, but I also don’t think there is any doubt that some Australopithicus or another is an ancestor to the human (i.e., Homo) lineage. It would not, however, put them in the Homo genus, if that is what you meant.
The X and Y are sex chromosomes. Sex chromosome aneuploidies are survivable because we already have mechanisms in place to deal with people having different numbers of them.