Could someone explain a chromosomal crossing over?


12 Years
Nov 9, 2010
I read what I could find on wikipedia but want to know more. So could it be like if you bred a rooster and hen both with the genotype I/i+ on the I locus and in one of the offspring the alleles on the I locus brake and make a new allele? I may be way off so please correct me. To bad 9th grade biology didnt go deeper.
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you have got the general concept right, except crossovers involve 2(or more) genes at separate loci. One well known poultry example for crossover is P for pea comb and O for the blue eggshell gene. As it happens, those two genes are located on loci very close to each other so they rarely 'separate' because genes tend to break off in sections instead of every single gene being 'separated' and recombined completely. Think of chromosomes being split up in ribbons of different sizes and you get the general concept. The P and O are so close they almost always are on the very same 'cut off ribbon section' during recombination. Result is almost all pea combed hens with this linkage turn out to be colored egg layers, even if crossed/mixed with single combed birds. Just about 99% of the time. It works so well if you are planning to only keep colored egg layers you can cull the single combed chicks at hatch.

Occasionally, two 'linked' genes happen to be 'cut off' in between during recombination. That's called crossing over- one of the previously linked gene gets 'separated' and is 'transferred' to another chromosome. In the case of colored egg(blue or green), pea combed birds mixed with non-colored egg(white or brown), single combed birds, a single combed pullet that lays colored eggs would be evidence of a crossover. In her case, P and O happened to be cut off in between and the O gene 'moved'(crossed over) to the chromosome with p+. making her p+/p+ O/o+.

It can happen the other way too, P and O split up but P crosses over to chromosome with o+ resulting in pea combed laying white or tan egg. This one is harder to be sure of as there are plenty of P o+ birds so it could have been a P o+ chromosome bred into P O linked flock. Single combed colored egg layers are much less common so they stick out like a sore thumb and much easier to be sure of a cross over.

p.s. to answer your I example, yes the same loci can produce completely new mutation, or a mutation can mutate again. IIRC, Smoky is a mutation of I.
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Difficult to explain. If I had you and a chalk board for a few minutes, I believe you would understand.

Remember- meiosis is a nuclear process that produces egg or sperm.

Watch this. This video has an error but it will get the point across. For the biology student- during Metaphase I you should see tetrads lined up at the equator of the cell and during anaphase I the chromosomes moving to the poles are double stranded and not single stranded.

then this

Then this

Before a different combination of genes can be found in an egg or sperm, the organism needs to be hybrid for the two genes that are exchanged during crossing over.

hybrid for blue egg shell O/o alleles and hybrid for the pea comb/single comb P/p.

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The crossing over (C.O.) point can be inside the (coding) sequence of one gene, but the chances are very slim because the chance of a C.O. is proportionate to the distance in basepairs.

Let's say that dom.white I has a single point mutation compared to i+...
In case of I and i+, an intra-gene crossing-over would not make a new allele. The point mutation would just move to the other chromosome.

But if you have two alleles with each a different point mutation, an intra-gene crossing over could make a new allele with both these point mutations while the other chromosome would get a reverted wildtype allele.

e^m||e^n => e^mn||e+

Don't ask how many eggs you'd have to hatch...
Thank you so much everybody for your explanations! It was a very interesting read and I think it's starting to make sense for me... hopefully lol.

I read that a single combed blue egg layer has a chance of about 3%, does anybody know how that was calculated? I have no idea how to link the math part of such a calculation with the biology part...
And did anybody try to calculate how unlikely a crossing over between barring / mulberry skin gene is? Unlikely in the range of 3% or 0,3% does make a difference after all. I like crazy projects, but there's a difference between crazy and next to impossible, so it'd be nice to find out how unlikely this really is.

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