In this video, I’m going to demonstrate
how to identify the probabilities of producing
gametes from linked loci. For this, I’m
going to need some help from some little friends
called phlobs.
Phlobs are the common name for a diploid eukaryotic
species of microbe that exist slightly beneath
thermal vents in the deep sea, where they
are found in greatest abundance. They have
three autosomal traits that are linked on
the same chromosome, which are flagella length,
colorization, and shape. As shown on the screen,
long flagella is dominant to short flagella,
pink color is dominant to blue, and circular
is dominant to square.
Here, we have two pure breeding phlobs we
want to cross, one with long flagella, blue
colorization, and square shape (with genotype
Hps/Hps), and the other with short flagella,
pink colorization, and circular shape (with
genotype hPS/hPS). Shown on the screen are
their replicated chromosomes. Let’s simplify
this by looking only at the sections of the
chromatids with the loci.
If we cross these two phlobs together, we
will get one chromatid from each parent, resulting
in the following heterozygous progeny. This
individual here, the F1 progeny, is our area
of interest, since there are 8 different gametes
that could be produced from this individual.
Now we want to find the probabilities of producing
certain gametes from this heterozygote.
We create the following map with the parent’s
genotypes across the top and bottom, are given
a coefficient of coincidence of 0.75, and
the following recombinant frequencies to start.
For simplicity, we will look at only the possibility
of creating a gamete that codes for long flagella,
Pink colorization, and square shape, as well
as one for short flagella, blue colorization,
and square shape. For this, we need to determine
the type of crossover occurring. First, we
circle the alleles we are looking for. For
the first gamete, we’re looking for...long
flagella...pink colorization...and square
shape.
From this, we determine this to be a double
crossover event, since there are crossovers
in both regions. Using the table shown on
the screen, we follow the formula using the
recombinant frequencies and coefficient of
coincidence. Here, we convert the frequencies
to decimals by dividing by 100. We then multiply
all these values together to get the double
crossover probability, which is .00675. To
find out the probability of getting ONLY the
long, pink, square gamete, we simply multiply
this value by one half, as we are looking
for one of two possibilities. The result is
.00338.
Next, let’s say we want to find short flagella,
blue colorization, and square shape. After
circling our alleles, we discover that this
is a single crossover event in region one.
Using the formula in the table, we subtract
the probability of a double crossover from
the recombinant frequency in region 1 to get
our overall probability, then multiply by
half to get our end result, which is… .07163.
