Double Crossover

[justadream]

Got the following from mcat-review.com (it is only of the 3 possible situations for a "double crossover"

"The chromatids exchange alleles during a crossover. Then, one of the crossover chromatid exchanges with a different chromatid. This is called the 3-strand double crossover. Results in 2/4 recombinants."

My question is: Why does this result in only 2 recombinants?

I thought first crossover results in 2 recombinants. Then, when one of these chromatids exchanges with another chromatid, shouldn't it result in THREE recombinants?

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[type12]

Sister chromatids are identical (you've duplicated the DNA from chromosome 1 from dad); homologous chromosomes code for the same genes (chromosome 1 from dad, chromosome 1 from mom). So, during crossing over, you have AA and aa, the big A's are dad, and little a's are from mom. You have a recombinant when A is on mom, or a is on dad. Now, let's do the double crossover as per the directions:

AA -- aa

Step #1:
Aa -- Aa

Now, according to what they say, ONLY ONE OF the bold/underlined chromatids (i.e., the ones that crossed over first) on can crossover, and only to one of the unbolded ones:
aA -- Aa

So, if you take a look, there's no way to make a third recombinant.

Hope that helps!

 

[Czarcasm]

Got the following from mcat-review.com (it is only of the 3 possible situations for a "double crossover"

"The chromatids exchange alleles during a crossover. Then, one of the crossover chromatid exchanges with a different chromatid. This is called the 3-strand double crossover. Results in 2/4 recombinants."

My question is: Why does this result in only 2 recombinants?

I thought first crossover results in 2 recombinants. Then, when one of these chromatids exchanges with another chromatid, shouldn't it result in THREE recombinants?

When discussing a double crossover, we are considering 3 loci (3 pairs of alleles) on the same chromosome (linkage). Say we have gene X, Y, Z (in that order) on Chromosome 1. A diploid individual like humans has two copies (a homologous pair - one inherited from mom, other from dad). These homologous chromosomes are paired up during metaphase I of meiosis (where crossing over takes place). Also recall each chromosome is in replicated form. Therefore:

X1X1 :: X2X2
Y1Y1 :: Y2Y2
Z1Z1 :: Z2Z2

A double crossover would produce:

X1X1 :: X2X2
Y1Y2 :: Y1Y2
Z1Z1 :: Z2Z2

Four gametes result:
2 Parental: X1Y1Z1 and X2Y2Z2
2 Recombinants: X1Y2Z1 and X2Y1Z2

 

[justadream]

@Czarcasm

Why do we need to consider 3 loci when looking at double crossovers?