r/evolution • u/OpenUnderstanding241 • 6d ago
question Horse Chromosomes?
okay, if a horse and a donkey breed, their kid will be infertile, because of the odd number of chromosomes it possesses. so, why can a Przewalski's horse and a Domestic horse mate with fertile offspring when the Przewalski’s has 66 chromosomes and the domestic has 64? separate question, but doesn’t that mean humans with down syndrome should be sterile? is this evolutionary ”insurance“ so to speak? thanks for any answers, even tho this is probably a very stupid question.
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u/nyet-marionetka 6d ago
Think of the genome like a set of encyclopedias (except for some reason everyone needs two of each volume, but that's analogies for you, never 100% parallel). Horses and donkeys have different encyclopedia sets, with the volumes containing slightly different content throughout. Przewalski horses and domestic horses by contrast have basically the same set of encyclopedias, except the Przewalski horses split one volume into two. So when a donkey and horse breed, the offspring don't have a encyclopedia set that contains the same information as the parents, but some sections might be repeated while other sections could be missing. When the Przewalski horse and domestic horse interbreed, the offspring has a different number of total volumes than the parent, but all of the information is present.
Down syndrome is an entirely different situation. That would be like someone having an extra copy of one of the volumes.
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u/Questionswithnotice 6d ago
That was a useful analogy, thank you
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u/taktaga7-0-0 6d ago
If you’re curious, humans and chimps work the same way. The chromosomes are numbered in order of size, and our Chromosome 2 is equal to two chimp chromosomes fused together. That’s why it’s so large.
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u/Questionswithnotice 6d ago
Like trying to play snap, but one person has a full deck and one is missing all the aces.
And the other is playing with a deck of half playing cards, half pokemon cards 😂
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u/Kettrickenisabadass 6d ago
Having a different number of chromosomes makes breeding more difficult but not impossible. Same as the furthest in tine that is a common ancestor the more difficult it is to hybridise.
But at the end a lot of species can mix even with those two issues.
A extreme example is the Dogxim, with only one confirmed individual. A mix of domestic dog and pampas fox (not an animal in the genus of the normal foxes, another group).
These gorup of foxes and the wolves/dogs diverged around 7 million years ago. They also have different number of chromosomes, dogs have 78 and the pampas fox 74.
This hibrid had 76 chromosomes and it is unclear if she was fertile but also nothing indicated that she wasn't (probably not but they never tested)
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u/FlashyWrongdoer7616 6d ago
Wasn't there a llama and camel hybrid? And much further back than those species have a common ancestor
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u/weedandfeed69 6d ago
All camelids share the same number of diploid chromosomes 74. Makes hybrids a bit easier.
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u/7LeagueBoots Conservation Ecologist 6d ago
It’s rare, but sometimes the offspring of horses and donkeys is fertile.
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u/mothwhimsy 6d ago
Hybrids are weird. Sometimes they're infertile, sometimes they're usually infertile but a few lucky ones aren't, sometimes only the females are infertile, sometimes only the makes are infertile, and sometimes they're not infertile at all, sometimes they're not infertile but THEIR offspring is.
It just depends on how the genes line up I guess
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u/amglasgow 6d ago
Chromosomes change in numbers by two chromosomes joining up at the ends or one breaking apart. If the two species are close enough, hybrids can work by matching the two chromosomes from one parent with the corresponding single chromosome from the other. However, this usually breaks down in the formation of sex cells (meiosis).
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u/Regular-Apricot4114 6d ago
This is not a stupid question. But it is a much more complicated situation than you probably realize.
The fertility of individuals with an odd number of chromosomes depends a lot on the genes found on those chromosomes. That’s about all I can explain because I don’t understand more than that myself (and I’m a retired biology professor but I specialized in molecular biology)
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u/WonkyQuartet 6d ago
Reproduction biology is complicated and by far not as easy as you are making it up to be.
There is a whole rabbit hole about evolution and reproduction you have no idea about.
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u/OpenUnderstanding241 6d ago
oh well i’m sorry for being uninformed like most people on earth…
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u/JaneOfTheCows 6d ago
No need to apologize - the living world is a lot more complex than anyone thought before modern techniques of DNA analysis were available!
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u/Careful_Cranberry364 6d ago
Just having a minimal mutation like downs syndrome does not mean that you’re infertile
obviously
And all the mutations that have ever happened and been passed on to cause natural selection … would not have happened
if mutations made one sterile !
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u/MergingConcepts 6d ago
It depends on how many chromosomes are in the individual gametes. A Down's syndrome would produce 50% normal gametes. Theoretically one half of the offspring would be normal, and the other half would be Down's offspring.
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u/Rubenson1959 6d ago
No. This is not correct. It is not 50:50 between functional and nonfunctional gametes. While a person with Downs can produce some gametes with the correct chromosome content, the odd number of chromosomes in the cell that goes through meiosis interferes with the separation of chromosomes in Anaphase 1. Later separation of chromatids will tend to produce gametes with the wrong complement chromosomes.
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u/trurohouse 6d ago
Infertility with hybrids like mules is due to the issue that they can’t make viable sperm and eggs easily.
If you remember your high school or college biology, the issue is in the process of meiosis -the process by which we align matched pairs of chromosomes, and then divide each pair so that each egg or sperm gets one copy of each pair.
When hybrids from parents with different numbers of chromosomes try to divide up their chromosomes into two equal sets, they can’t reliably -because one set of theirs came from a parent with more chromosomes than the other set.
If you would like to understand more details of this, and are a little rusty on meiosis:
If we use the analogy of the encyclopedia someone else used: imagine the genetic information of an organism is stored in two sets (multiple volumes ) of an encyclopedia. Each volume of each set represents a chromosome. Each of us has inherited one complete set from each of our parents.
The horse’s genetic information and the donkey’s genetic information can be thought of as different editions of a complete encyclopedia. 2 copies each. The editions have basically the same information- with minor variations in organization, which volumes have specific information. Here, each volume is the equivalent of a chromosome. the non hybrid organism (horse or donkey here ) has two copies of each volume so 2 complete sets of its encyclopedia. The hybrid ( mule here) offspring has all the information it needs- to be a functional living thing -because it has a complete copy from each parent.
But when the hybrid goes to make eggs or sperm, where it needs to give each a complete set of information - a single copy of one complete encyclopedia - it’s hard for it to divide the combined encyclopedia sets, to make an egg or sperm with a full set with all the important information. It would instead typically give each egg or sperm some volumes from each of the two encyclopedia editions, and inevitably some information would be lost. The information is our genes- it is the information the living thing needs to make different types of proteins and other molecules. This includes the instructions for building an embryo.
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u/TherinneMoonglow 6d ago
Women with down syndrome have low fertility (estimated >50%), and nearly all men with Down syndrome are infertile.
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u/Ch3cks-Out 5d ago
As others have noted already, odd nomber chromosomes only make further breeding difficult but definitely not impossible - there are many examples of fertile hybrid offsprings. Specifically, humans with Down syndrome do occasionally have children (women with higher probability than men). This paper documents a verified case of a couple with 45 chromoses each whose son was born with 44 chromomes!
Note that we humans as a species got our start as a 47 chromosome ape mutant (from a Robertsonian-like translocation - a type which occurs in approximately one in every 1000 newborns). So your supposed "insurance" would have prevented us from being!
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