So recall test crosses because mapping involves test crossing
to reveal the genotype and the phenotype.
But a test cross involves crossing an unknown genotype.
For example, we had our purple pea plants
with a homozygous recessive, the white one to reveal whether it was a
heterozygously purple or homozygously purple plant.
So, we are using test crosses in order to reveal phenotypes
and get recombination information.
So, here we go.
Our language again of fruit flies, wild type versus mutant.
We have wild type and mutant, and the wild type is generally
the dominant form although not always.
The dominant form in this case would be gray body
and the recessive, form black body.
In this case this match up as mutant and wild type.
And the next characteristic we're looking at
is wings, vestigial wings or tiny little wings
that never grow properly versus normal wings which do grow properly.
Normal wings as the dominant case.
So when we take our parental generation and cross them,
we end up with a heterozygote in the F1 generation.
Nothing new there, right?
And we take those crosses and we cross it back in a test crossed
to the homozygous recessive form and we calculate or count the number of offspring
that are recombinant versus the number of offspring that are not.
The recombinant ones have had a crossing over event
on one linkage group or that chromosome.
And so we can use that frequency to calculate
how far away because, obviously, if you are closer together,
you’re gonna have less crossing over events if the genes are further apart,
you would have more crossing over events so we can calculate those crossing
over frequencies or recombination frequencies
by looking for the recombinant phenotype first.
How do we find that?
We find the recombinant phenotype because it is the least frequent thing.
Crossing over is less likely than not crossing over between these two alleles.
So our recombinant phenotype are the ones with the less numbers to start with.
We can then add those two numbers together
to get the total number of recombinant offspring
and divide it by the total number of progeny that we had.
Let’s keep that really simple here and make it one thousand
so all these numbers add up to one thousand,
we take the recombinant ones divide them by the total number
and we have a percent of recombinant progeny.
We used this percent of recombinant progeny, 18% and call it 18 map units.
So relative distance, these guys are pretty close together
as compared to 30 map units.
Now, there’s a little bit of a catch, if these genes are close together.
We are going to get a more accurate number,
recall that this is just an estimate or relative distances from each other.
So the catch here is if genes are really far apart
we could end up with a crossover and then an un-crossover, right?
A double crossover event.
So the further apart the two genes are,
more likely it is that we could miss the crossing over event,
and they essentially exhibit independent assortment.
Now, two of Mendel’s factors actually ended up being on one linkage group
but they were so far apart
that this double crossover event would happen quite regularly.
So, missed double crossovers
will end up in us calculating shorter recombination frequencies
because the double cross ends up uncrossing them
and we don’t see the recombinant progeny.