Slide
1
To further examine the idea of linked genes, we are going to examine a
dihybrid cross the fruit fly, Drosophila melanogaster. Fruit flies are
common study organisms in genetics laboratories and much of what we know about
genetics we owe to the fruit fly.
Slide 2
Before we look at one
of these crosses, we should review Drosophila nomenclature (you were
introduced to this information in the previous tutorial).
In Drosophila, genes are named after the first mutation that is discovered. For example, normal fruit flies have red eyes (so we say red eyes are the wild-type trait). The first eye color mutation that was found was white eyes. So white eyes are the mutant phenotype.
If the mutation is recessive, as is the case with the allele for white eyes, gene is symbolized with a lowercase letter - in this example, little w.
The wild-type allele is always indicated with a plus sign - so in this example w+ is the wild-type allele and w is the mutant allele.
Slide 3
This
particular dihybrid cross includes the genes for body color (wild-type is gray
and the mutant is black) and wing size (wild-type is normal and the mutant
trait is vestigial wings - vestigial wings are small non-functional wings).
All of the fruit fly crosses in this unit on linked genes will be set up exactly the same way - a female who is heterozygous for both genes (so she is wild-type body color, wild-type wing size) is crossed with a homozygous recessive male. You can see the genotypes of the parents here.
For the moment, assume that these genes are not linked and they assort independently. If this is the case - what gametes can these parents make?
Slide 4
If the genes are not linked, the heterozygous
female can make four different gamete types in a 1:1:1:1 ratio. The male
is homozygous recessive so he can only make single gamete type.
Let's take this assumption to the next step and predict the offspring that will result from this cross.
Slide 5
A Punnett square can help us
visualize the offspring - along the top axis are the female's gametes and
along the side is the male's single gamete type.
This cross can produce four types of offspring - in a 1:1:1:1 ratio. Remember, we are assuming that the genes are not linked and they assort independently during meiosis.
Slide 6
If we look again at the image from this tutorial, we
see these four categories of offspring. If the genes are not linked and
they assort independently - then we expect these four categories of offspring
to occur in approximately equal numbers.
If we observe something significantly different from a 1:1:1:1 ratio - then we know that our assumption was incorrect and the genes do not assort independently and are linked. This cross allowed us to determine that the gene for body color and wing size are linked.
Slide 7
Remember, linked genes are on
the same chromosome and closer than 50 map units apart. Because they are
on the same chromosome, they cannot assort independently.
These crosses can be used to determine more than whether or not two genes are linked. We can also use these crosses to determine the distance between the two genes so that we can ultimately create a genetic map.
Slide 8
Remember,
linked genes can recombine into new combinations as a result of crossing over.
The recombination frequency of linked genes provides information about
the distance between two genes. We can set up crosses that allow us to
determine recombination frequencies and thus distance.
Slide 9
Here
is another view of our previous cross if a heterozygous female and a
homozygous recessive male. This image adds details about how the alleles
are arranged on the chromosomes. In this particular female, the two
wild-type alleles are found on one chromosome and the two mutant alleles are
found on its homologous partner. During meiosis, crossing over
recombines the alleles on two of the non-sister chromatids to create a a
chromatid with the genotype b+ vg and a chromatid with the genotype b vg+.
Meiosis II separates the sister chromatids into individual gametes and
we see the four different gamete types - b+ vg, b+ vg+, b vg+, and b vg.
However, they are not in a 1:1:1:1 ratio. The gametes that reflect
the parental arrangement of alleles (b+vg+ and bvg) occur more frequently than
the recombinant arrangement. We call the orignal arrangement the
parental gametes and those that result from crossing over are the recombinant
gametes. This means that we call the offspring that result from the
parental gametes - parental and those that result from the recombinant gametes
are recombinant offspring.
To determine the distance between the gene for body color and wing size - you use the equation shown here - divide the # of recombinant offspring by the total number of offspring and multiply by 100. This gives the recombination frequency which we can use as a measure of distance.
Slide 10
The gene for body color is 17 map units from
the gene for wing size.
Slide 11
By doing crosses like this one,
we can create a linkage map which provides the order and relative position of
genes along a chromosome.