THE gene interaction can change the proportions proposed by mendel in dibridism. If we have cases where two traits are defined by non-allele genes that are located on the same chromosome, we will have a phenomenon called gene link, factorial ligament or linkage.
Index
- How to identify the linkage?
- Permutation or crossing-over
- How to Calculate the Crossing Fee or Permutation Fee
- If so, what is the crossing fee?
- cis and trans position
How to identify the linkage?
If we observe the production of gametes in an individual dihybrid, we'll see that they produce four different gametes in the same proportion. In contrast, if the two pairs of genes are on the same chromosome, the individual hybrid should only produce two different gametes as the linked genes tend to go to the same pole during the meiosis.
Confirmation of whether these genes really migrated to the same pole is done through a cross-test, also known as backcrossing. If a dihybrid individual (AaBb), which has a pair of genes on each chromosome, with a double recessive individual (aabb), we will have the formation of four types of gametes.
Table 1: Backcrossing of an AaBb hybrid with independent genes.
| ab | |
| AB | AaBb |
| ab | aab |
| Ab | Aabb |
| aB | yyyy |
Table 2: Backcrossing of a hybrid with linked genes.
| ab | |
| AB | ABab |
| ab | abab |
In Table 1, we can see that four forms of gametes appear, thus proving that the genes are independent. On the other hand, in table 2, the dominant recombination forms do not appear just for THE and just for B, this proves that AaBb did not form the gametes Ab and aB, therefore, the genes are turned on.
Permutation or crossing over
By the principle of gene linkage, we can imply that genes that are linked on the same chromosome, always go together to the same gamete. That would make the genetic variability was minimal in individuals.
The existence of a phenomenon during prophase I of meiosis called permutation or crossing-over solves this problem. Crossing-over is the breakdown of homologous chromatids, followed by a diagonally interchanged re-soldering, exchanging genetic material between the homologous chromosomes.
The exchange of chromatids during permutation can form the recombination gametes (Ab and aB), But this is not a rule. The formation of these recombination gametes will only happen when the exchange of parts of the chromosome takes place between the two genes that are in linkage.
This is why some meioses from the hybrid AaBb individual form the recombination gametes and others not. This fact determines a different percentage between the parental gametes AB and ab and the recombination gametes (Ab and aB).
This is because the parental gametes will always be formed, regardless of the permutation taking place or not, whereas the recombination gametes only appear when the crossing over happens between the two genes in linkage.
To calculate what percentages of recombination gametes will be formed, we must use a value called crossing fee or permutation fee.
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How to Calculate the Crossing Fee or Permutation Fee
The crossing rate must be calculated from experimental data. Let's use as an example the cross between fruit flies (fruit flies).
If we cross a hybrid drosophila, with gray body and normal wing (NV/nv), with a male with black body and vestigial wing (nv/nv), we will obtain:
- 90 individuals with black body and normal wing (nV/nv)
- 90 individuals with gray body and vestigial wing (Lv/nv)
- 410 individuals with black body and vestigial wing (nv/nv)
- 410 individuals with gray body and normal wing (NV/nv)
If so, what is the crossing fee?
The first thing to be done is to identify the recombination individuals, which are always those resulting from gametes that have undergone permutation. In our example, these gametes formed the two groups of 90 individuals each. So we have a total of 180 recombinants.
In backcrossing, the frequency of recombinant individuals is equal to that of gametes, so just transform this number into a percentage. We have a total of 1000 individuals (90+90+410+410), thus:
1000 – 100%
180 - x
x = 18%
In other words, the crossing rate in these fruit flies is 18%.
It is noteworthy that if we had used as an example a backcross in dihybrid individuals (with independent genes), the four types of offspring would have the same percentage: 25% or ¼ for each. Thus, in these 1000 descendants, we would have 250 with each characteristic.
cis and trans position
The hybrid drosophila gray body and normal wing has the two dominant genes on one chromosome and the two recessive genes on the other chromosome. However, we can have fruit flies like the same phenotype (black body and normal wing) and with a different combination of genes, in which a dominant remains together with a recessive on the same chromosome.
If the two dominant genes are on the same chromosome and the recessive genes on the other, we say that the genes are in the cis position. When the linked genes are a dominant and a recessive on the same chromosome, we call it trans position.
We can discover these positions during cross-testing. In cis position, double-dominant and double-recessive individuals appear in larger quantities. In the trans position, they are the recombinant individuals and appear in smaller quantities. We can represent hybrid individuals, in cis and trans positions, conventionally as:
- AB/ab (cis)
- Ab/ab (trans)
Denisele Neuza Aline Flores Borges
Biologist and Master in Botany
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