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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?

    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
    OK, in response to the person who worked out the ratio 9 : 3 : 3 : 1, this occurs with unlinked alleles.

    Linked alleles are on the same chromosome and so two linked alleles will be inherited together.

    Parental genotypes: AABB and aabb

    Parental gametes: AB and ab

    F1 generation: AaBb

    Now crossing 2 of the F1 generation:

    Parental genotypes: AaBb and AaBb

    Now remember that A and B are present on the mothers chromosome and a and b are present on the fathers chromosome. Therefore on separate homologous chromosomes, therefore assuming there is no crossing over in prophase I, these alleles will present in separate gametes.

    Therefore,

    Parental Gametes: AB, ab and AB, ab

    Offspring Genotypes: AABB, AbBb, abab

    Phenotypic Ratio: 1 : 2 : 1

    Hope the colour coding makes things a bit clearer... (Soz, i was wrong before but i think it's right now!)

    Lastly, crossing over in prophase I causes the allels to swap between homogolous chromosome, which will create a variety of different gametes and you'll get all the possibilities shown in the 9 : 3 : 3 : 1 shown above. There will, however, be many more of AaBb that present in dihybrid inheritance as linkage still has a very important role in causing the characteristics to be inherited together.
 
 
 
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