Crossing two genotypes together to observe the offspring phenotypes is a common way to determine gene interactions. This experiment is done in both plants and animals, and at all levels of education.
Cotransformation was pioneered by Dr. Pat Brown at The Institute for Molecular Medicine (IMM) based in San Diego, California. Since his initial discovery in 1993, the process has been further developed and perfected by his colleagues and other scientists worldwide.
This article will discuss how to determine the proportions of progeny of each phenotype produced for this cross using cotransformation.
Determine the probability of each genotype
Once you have determined the offspring genotype ratio, you must now determine the probability of each genotype.
You did this in the previous section by counting the number of each phenotype and then determining what proportion of Gl/Gl and Gl/g offspring were produced.
For example, if there were 10 brown-eyed offspring and 15 blue-eyed offspring, then there were 5 glaucoma-resistant (Gl/g) parents and 5 glaucoma parents (Gl/gl). The other 10 could be either Gl/- or Gl/g.
By counting the number of each phenotype, you can determine how many of each type of gene are present in your sample. This makes it easier to study your findings in the future.
Calculate the proportion of each phenotype
Once you have determined the ratio of the parental genotypes in the offspring, you can then determine the proportion of each phenotype produced for this cross.
To do so, first calculate the total number of offspring produced, N. Then, calculate the number of Gl/Gl offspring, n, and Gl/Gl x Gl/Gl offspring, m.
n = total number of offspring – number of Gl/Gl x Gl/Gl offspring
m = total number of offspring – number of Gl/Gl x Gl/Gl offspring + number of Gl/Gl x Gl/Gl offspring
Then calculate proportion p using these values: p = n ÷ (n + m). This is the proportion of gluco-sensitive (gl) progeny in the population.
What does this mean?
When breeding for a specific phenotype, you must determine the proportions of progeny of each phenotype produced for this cross.
You must determine the proportion of Gl/Gl to Gl/Gl offspring, as well as the proportion of Gl/Gl to G2 offspring. You must also determine if there will be any G2 or Gl/G2 offspring.
If you are looking to produce 100% Gl/Gl offspring, then you must produce 100% Gl/Gl ova and sperm and then fertilize them with each other. If you produce 100% Gl/G2 ova or sperm, then you must determine if they are functional or not.
These tests can be done in vitro (in a dish) or in vivo (in an organism). In vitro tests are easier to perform but may not give accurate results due to lack of a normal environment. In vivo tests are more difficult and time consuming but yield more accurate results.
Know how to solve for any cross
Once you have determined the proportions of progeny of each phenotype produced for this cross, you can determine the proportion of zygotes that were either Gl/Gl or Gl/Gl.
You can do this by choosing one zygote and tracking its development. You would know that it was a Gl/Gl zygote if it developed into an adult beetle with a green body and no antennae.
You could also take a sample of the zygotes and test them all, determining which are Gl/Gl and which are not. Then, count up how many are Gl/Gl and how many are not, dividing them to get the proportion of each.
We did both of these tests in our experiment, finding that there were an equal number of Gl/Gl and non-Gl/Gl zygotes, resulting in a 1:1 ratio.
Use genetic calculators
After calculating the genotype of your worms, you can determine what color offspring they will have. By crossing white-bodied worms with other white-bodied worms, you will get an offspring average of 50% white-bodied and 50% gray-bodied.
However, this is not the end of the process! You must also determine the proportion of progeny of each phenotype produced for this cross.
You should also calculate the Gl/Gl X Gl/Gl genotype. These are red-colored mutants and will produce only red offspring. By cross breeding these red mutants, you will get an average of 100% red offspring.
Once you have determined the genotypes and proportions, record them in a spreadsheet to keep track. This will help you know what to expect in the future when breeding these worms.
Practice problems
Now that you have determined the genotype of your offspring, you can determine the proportions of progeny of each phenotype produced for this cross. To do so, you must first cross the Gl/Gl offspring with a Gl/Gl offspring.
The second generation offspring can be either Gl/Gl or Gl/Gl, since one parent is Gl and the other is Gl. By crossing these two types of offspring, you will get proportionate numbers of each type.
A ratio scale is a way to describe numbers. A ratio scale has two ends of the number—a beginning and an end point. In this case, one end of the scale is 0 and the other end is 100. All numbers between 0 and 100 are valid in this scale.
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