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    This stuff is confusing. If you can answer any of the following points it would be hugely appreciated!

    1) If you have two plants - both homozygous, one with all dominant alleles, one with all recessive alleles for TWO LINKED characteristics (i.e. on the same chromosome) and you breed them, what will happen (i.e. complete linkage)?

    ...so say u have AABB and aabb (and a/A and b/B are alleles on the same chromosome) - what phenotypic ratio will the second generation have?

    2) What effect do the cross-overs (at chiasmata) during Prophase I of meiosis have on this phenotypic ratio?

    I hope you can understand all of this...I desperately need help!

    Thanks in advance
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    (Original post by KP Nuts)
    This stuff is confusing. If you can answer any of the following points it would be hugely appreciated!

    1) If you have two plants - both homozygous, one with all dominant alleles, one with all recessive alleles for TWO LINKED characteristics (i.e. on the same chromosome) and you breed them, what will happen (i.e. complete linkage)?

    ...so say u have AABB and aabb (and a/A and b/B are alleles on the same chromosome) - what phenotypic ratio will the second generation have?



    Thanks in advance
    Wouldn't they all be AaBb?
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    I dunno - this might be the first generation i suppose.

    Hold on, is a dihybrid cross of two LINKED genes, the equivalent of a monohybrid cross?

    The first step is to work out what gametes would be present from the parents' genotypes?
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    the gametes would be AB and ab, so the f1 generation would all be AaBb

    Then, cross two from the F1 generation =

    AaBb x AaBb

    gametes = AB Ab aB ab

    so
    -------|---AB-----|----Ab----|---aB------|---ab-----
    AB AABB AABb AaBB AaBb
    Ab AABb AAbb AaBb Aabb
    aB AaBB AaBb aaBB aaBb
    ab AaBb Aabb aabB aabb


    just worked out ratio- 9:3:3:1
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    got ya - so the fact that cross overs occur will alter the gametes of the parents so some recombinant phenotypes will occur in F1?

    you will get mostly AB and ab, but a few Ab aB - so u will get the same same phenotypes as you would in the dihybrid cross (described by kohlstream) but not in the same ratio, and in F1, not F2.

    Is this right?
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    (Original post by KP Nuts)
    got ya - so the fact that cross overs occur will alter the gametes of the parents so some recombinant phenotypes will occur in F1?

    you will get mostly AB and ab, but a few Ab aB - so u will get the same same phenotypes as you would in the dihybrid cross (described by kohlstream) but not in the same ratio, and in F1, not F2.

    Is this right?
    I'm not sure, and I don't even know if what I wrote above was right. I based it on an example I have in my notes

    I have to go and do 'sources of new inherited variation' now, but I will be back later to solve once and for all the mysteries of *drumroll* interactions between unlinked genes.
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    Cheers m8, you done the interactions between unlinked genes yet? It turns out its dead easy - just that the two recombinant genotypes formed in a dihybrid cross can interact to form a new phenotype - it takes three damn pages to explain it in the NAS!
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    I think the only gametes the the F1 could produce would also be AB or ab (discounting chiasmata).
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    (Original post by KP Nuts)
    Cheers m8, you done the interactions between unlinked genes yet? It turns out its dead easy - just that the two recombinant genotypes formed in a dihybrid cross can interact to form a new phenotype - it takes three damn pages to explain it in the NAS!
    really? I was looking for some deeper meaning i guess, weeeeeeeeeeee I've sorted that.


    And, (RE: last post) how did you get that?
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    (Original post by KP Nuts)
    This stuff is confusing. If you can answer any of the following points it would be hugely appreciated!

    1) If you have two plants - both homozygous, one with all dominant alleles, one with all recessive alleles for TWO LINKED characteristics (i.e. on the same chromosome) and you breed them, what will happen (i.e. complete linkage)?

    ...so say u have AABB and aabb (and a/A and b/B are alleles on the same chromosome) - what phenotypic ratio will the second generation have?
    It seems to me the question is not asking about actual ratios. It is talking about linked characteristics - those that are more likely to be inherited together. If there is complete linkage, then you can treat your genotype as a single gene in that all of your gametes will be the same. This is like the two genes being right next to each other on the chromosome.



    2) What effect do the cross-overs (at chiasmata) during Prophase I of meiosis have on this phenotypic ratio?

    I hope you can understand all of this...I desperately need help!

    Thanks in advance
    Chiasma can split these linked genes, meaning that they will not be completely linked. This means there will be between a 50% and 100% chance of the two characteristics being inherited together.
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    (Original post by kohlstream)
    the gametes would be AB and ab, so the f1 generation would all be AaBb

    Then, cross two from the F1 generation =

    AaBb x AaBb

    gametes = AB Ab aB ab

    so
    -------|---AB-----|----Ab----|---aB------|---ab-----
    AB AABB AABb AaBB AaBb
    Ab AABb AAbb AaBb Aabb
    aB AaBB AaBb aaBB aaBb
    ab AaBb Aabb aabB aabb


    just worked out ratio- 9:3:3:1
    With complete linkage you will never get Ab or aB gametes from having AABB and aabb to begin with.
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    (Original post by Golden Maverick)
    With complete linkage you will never get Ab or aB gametes from having AABB and aabb to begin with.
    Yeh thats right, unless there are chiasmata.
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    It gets confusing when you have linkage between genes and the parental phenotype is aAbB when the A and B are on different chromasomes
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    Then I think the rule of thumb that there will be 'more parental phenotypes' doesn't apply. Ah what a load of old rubbish.
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    (Original post by richardAndr)
    It gets confusing when you have linkage between genes and the parental phenotype is aAbB when the A and B are on different chromasomes
    Eh? If A and B are on different chromosomes you can't have linkage.
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    This has just confirmed I'll do crap in Biology again :rolleyes:

    rargh
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    No, i mean if one of the bilivalent pair as aB and one has Ab
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    Then they're linked still
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    No, i mean if one of the bilivalent pair as aB and one has Ab
    Oh right. I think the easiest way to look at complete linkage is as if you have 4 possible alleles of 1 gene:
    AB = P
    Ab = Q
    aB = R
    ab = S
    Then just treat it as normal.

    kohlstream: Not at all, it is unlikely a question like this will come up, and you gave a great description of a normal dihybrid cross
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    Ok cheers. It's all pretty simple really if you treat it more like simple maths or logic questions, but easy to get in a pickle over if you aren't concentrating.
 
 
 
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