Slide 1
The topic of this tutorial is linked genes - genes that are linked
to each other as a result of being on the same chromosome.
This is one of the more challenging topics we will discuss this semester so you will want to spend time thinking carefully about this topic.
To understand linked genes we will look at dihybrid crosses in fruit flies. These dihybrid crosses will be different from the dihbrid crosses that Mendel observed. In Mendel's classic dihybrid crosses, the genes were not linked. Each gene was found on a different chromosome - for example, we see in this image that the gene for seed color is found on the pink chromosome pair while the gene for seed texture is found on the blue chromosome pair. Because these genes are on different chromosomes they will assort independently from each other during meiosis (please review independent assortment from Tutorial 2). A pea plant that is heterozygous for the two genes can produce four different gamete types YR, yr, Yr, and yR - and they are produced in a 1:1:1:1 ratio.
Slide 2
Chromosomes do not carry just a single gene - each chromosome has
many genes on it. When these genes are relatively close together (closer
than 50 map units - map units are measurement that we will talk more about
later) they are considered linked because they are physically connected to
each other by the structure of the chromosome.
Because linked genes are physically connected they do not assort independently during meiosis. In this image we see the genes for body color, eye color, and wing size in the fruit fly. These genes are linked because they are on the same chromosome and they are closer than 50 map units.
Slide 3
To understand the behavior of linked genes we need to understand
crossing over between homologous chromosomes. Remember, homologous
chromosomes are the members of a pair with one being inherited from the female
parent and one inherited from the male parent. They are the same size
and shape and have the same genes. However, they can have different
alleles.
In this diagram, we see a homologous pair of chromosomes with three genes - genes A, B, and C. The dark green chromosome has the alleles a, b, and c and the light green chromosome has the alleles A, B, and C (please note - a chromosome does not have to have either all dominant or all recessive alleles - as we see at the end of meiosis in this diagram - chromosomes can have different combinations of dominant and recessive alleles).
Prior to meiosis, the chromosomes replicate themselves. During prophase I of meiosis I, the homologous chromosomes come together and synapse. It is at this time that crossing over takes place. Remember, crossing over is an equal exchange of DNA between non-sister chromatids. In this diagram, crossing over is taking place between the two innermost non-sister chromatids (crossing over only takes place between non-sister chromatids). This particular crossover has taken place between genes B and C. The result is that four different gamete types have been produced - abc, abC, ABc, and ABC. However, unlike unlinked genes - these gamete types are not produced in a 1:1:1:1 ratio.
Slide 4
When we are talking about gamete production we are talking about
what is happening in a large number of cells at once - not just a single cell
going through meiosis.
The reason the gametes are not produced in a 1:1:1:1 ratio is because crossing over is random event. In some cells, a crossover will take place between genes B and C but in other cells a cross over may take place between genes A and B. In other cells a cross over could take place between genes on the other arm of the chromosome.
So some crossover events create gametes with the genotypes abc, abC, ABc, and ABC but other crossover events will not.
The closer two genes are together - the less likely it is that a crossover will occur between them and the less likely it is that the alleles will recombine. So in this example, it is less likely that a crossover will occur between genes A and B than it is that a crossover will occur between genes A and C.
Slide 5
In other words, the recombination frequency will be greater
between genes A and C than it will be between genes A and B.
This means that we can use recombination frequency as a measure of distance between two genes - genes A and C recombine more often so we know that they are farther apart. Genes A and B recombine less often so we know that they are closer together (as we will see - we can observe recombination but we do not have the ability to see genes on chromosomes).