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    Needing a bit of help from the 2002 paper.

    Cabbage and radish each have a diploid number of 18. these plants can be crossed producing a hybrid.

    chromosome number: cabbage 18 ------------ radish 18

    gametes: -----9--------------------9-----

    chromosome number in hybrid: 18

    which of the following statements is true?

    A: The chromosomes in the hybrid cannot pair during meiosis.


    can someone better my understanding here? i was under the impression a hybrid or polyploid with an even chromosome number was able to pair during meiosis thus is fertile?
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    Surely a hybrid is the stage before polyploidy takes place so the hybrid does not have homologous pairs of chromosomes so cannot pair up for meiosis to occur. But after polyploidy has taken place, the polyploid has homologous pairs of chromosomes and can line up. E.g.

    ABCD is a hybrid
    AABBCCDD is a polyploid.

    I may be wrong as I haven't even revised that since the prelim but it is what I would of guessed.
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    but some hybrids can be sterile so must be able to pair during meiosis, anyone able to clear this up?

    i think i was wrong to mix polyploid and hybrid, i've learned if polyploid are even they are sterile, still no clue about hybrid :P
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    (Original post by JackoGeddes)
    but some hybrids can be sterile so must be able to pair during meiosis, anyone able to clear this up?
    Sterile = not able to produce offspring (which is a consequence of the inability for meiosis to occur as it normally would). I think you're a little bit mixed up about your terminology, and might mean fertile instead!

    Genetics isn't a game of absolutes. Genetic information varies HUGELY across the diversity of life - some of these collections of genetic information are very similar to each other, some is very different.
    With the case of the cabbage and radish; the genome of both species is similar enough that it can produce offspring, but the chromosomes are too different in the progeny for any homologous pairing to go on during meiosis, hence, this hybrid species of vegetable is infertile and incapable of producing offspring.

    However, just by chance, SOME cabbage x radish hybrids will be able to produce off spring - because even variations in the genome amongst the same species, and variation in the gametes of the same ORGANISM, can be pretty big. So, just by dumb luck, some hybrids have chromosomes similar enough that they can synapse (pair together) before division.

    This is the case right across the field of genetic hybrids and aneuploidy: SOME mules, just because of this tricky genetic variation, are fertile. But most aren't because of the aneuploid (abnormal) chromosome number, which is a slightly different reason from our vegetable hybrid, but comes down to the same crux - information encoded in the genome is too different to reproduce.
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    (Original post by Hype en Ecosse)
    Sterile = not able to produce offspring (which is a consequence of the inability for meiosis to occur as it normally would). I think you're a little bit mixed up about your terminology, and might mean fertile instead!

    Genetics isn't a game of absolutes. Genetic information varies HUGELY across the diversity of life - some of these collections of genetic information are very similar to each other, some is very different.
    With the case of the cabbage and radish; the genome of both species is similar enough that it can produce offspring, but the chromosomes are too different in the progeny for any homologous pairing to go on during meiosis, hence, this hybrid species of vegetable is infertile and incapable of producing offspring.

    However, just by chance, SOME cabbage x radish hybrids will be able to produce off spring - because even variations in the genome amongst the same species, and variation in the gametes of the same ORGANISM, can be pretty big. So, just by dumb luck, some hybrids have chromosomes similar enough that they can synapse (pair together) before division.

    This is the case right across the field of genetic hybrids and aneuploidy: SOME mules, just because of this tricky genetic variation, are fertile. But most aren't because of the aneuploid (abnormal) chromosome number, which is a slightly different reason from our vegetable hybrid, but comes down to the same crux - information encoded in the genome is too different to reproduce.
    i did mean fertile, sorry! I understand fully what you are saying, but in relation to the question how do I know when a hybrid is fertile then from chromosome number? or do you just assume hybrid = sterile for that type of question?

    for example Polyploids with even numbers of sets of chromosomes are fertile as the chromosomes can form as homologous pairs in meiosis. so fertile = even, sterile = uneven. is there such a system for hybrids?
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    (Original post by JackoGeddes)
    i did mean fertile, sorry! I understand fully what you are saying, but in relation to the question how do I know when a hybrid is fertile then from chromosome number? or do you just assume hybrid = sterile for that type of question?
    Like I said, there isn't a hard and fast rule, and the questions aren't going to expect you to act like there's a hard and fast rule. I looked up the past paper, and had a look at your question: you can work out that you can't pair up by elimination - the other 3 are untrue for all hybrids, leaving only B: chromosomes can't pair up.

    It'd be dishonest of me to tell you that if A then B. Interbreeding and fertility aren't that simple, but, there's lots of information here.

    If the gods decide that they hate you and decide to give you a question on hybrid fertility, statistics are on your side that the hybrid is infertile.
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    thanks for your help

    while i have you, i came across another puzzler in the 2002 winter diet.. i promise you i'm usually good at this

    q29

    during succession in plant communities a number of changes take place in an ecosystem. which line describes some changes?

    A:
    Species diversity: Falls
    biomass: rises
    food web complexity: rises

    I was under the impression species diversity increased as succession occurred? in my textbook it says : climax communities are usually found to support the greatest diversity of animal species (especially insects) and to support the most complex food webs
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    (Original post by JackoGeddes)
    thanks for your help

    while i have you, i came across another puzzler in the 2002 winter diet.. i promise you i'm usually good at this

    q29

    during succession in plant communities a number of changes take place in an ecosystem. which line describes some changes?

    A:
    Species diversity: Falls
    biomass: rises
    food web complexity: rises

    I was under the impression species diversity increased as succession occurred? in my textbook it says : climax communities are usually found to support the greatest diversity of animal species (especially insects) and to support the most complex food webs
    Sorry. I have zero ecology knowledge now, but I was under the impression that diversity increases, too, because we've got our 'dead' land that we get our specially-adapted colonizers on, who then make the soil more fertile, which gives rise to more complex plant species.
    So, after a while, the ecosystem becomes stable, and we get a stable-ish supply of resources. What's going to happen when all these plant species are competing for the same resources in a big, complex, climax community?

    Also...are you sure that's all the question says? And are you sure that's what the right answer is?
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    yep thats the question straight out the book. i was pretty sure succession was easy, pioneer community, succession occurs causing fertility of the soil to increase and eventually after many bouts of succession the climax community is present. More species as a result of a greater ecosystem basically, ties in with the complex foodweb bit and the biomass increase is obvious.

    i was sure it was rises/rises/rises but no. pretty confused
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    I'd ask your teacher about that q 29 succession one. I think i probably did that paper last year when I did higher and my teacher put corrections in it. What you are saying sounds plausible and is what I think too!
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    right I managed to find a similar question on 2011 which asks about a climax community. and sure enough it's biomass: high, species diversity: high.

    it's the fact that old question only asks about succession rather than climax communities, but i'd have still expected an increase in species diversity with each succession? i hate getting stuck on a question, can't bloody leave it!
 
 
 
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