Drosophila that are true breeding for the traits straight...

Drosophila that are true breeding for the traits straight wings (S) and red eyes (R) are crossed with flies that are true breeding for curved wings (s) and brown eyes (r). A test cross is then made between the offspring and the truebreeding ssrr flies. A. Use the symbols S, s, R, and r to construct a representation of the parental genotypes in the test cross. B. If these genes are located on different chromosomes, use a Punnett square to construct a representation of the offspring of the test cross. C. Predict the distribution of genotypes and phenotypes resulting from the test cross. D. As it happens, these genes are both on chromosome II as shown below. Use the symbols S, s, R, and r to construct a representation of the parental and recombinant genotypes in the test cross. E. Suppose that 500 flies are produced in the test cross. Apply mathematical methods to calculate the expected number of recombinant offspring using the linear map units (LMU) shown in the diagram below.

Drosophila that are true breeding for the traits straight wings (S) and red eyes (R) are crossed with flies that are true breeding for curved wings (s) and brown eyes (r). A test cross is then made between the offspring and the truebreeding ssrr flies.
A. Use the symbols S, s, R, and r to construct a representation of the parental genotypes in the
test cross.
B. If these genes are located on different chromosomes, use a Punnett square to construct a representation of the offspring of the test cross.
C. Predict the distribution of genotypes and phenotypes resulting from the test cross.
D. As it happens, these genes are both on chromosome II as shown below. Use the symbols S, s, R, and r to construct a representation of the parental and recombinant genotypes in the test cross.
E. Suppose that 500 flies are produced in the test cross. Apply mathematical methods to calculate the expected number of recombinant offspring using the linear map units (LMU) shown in the diagram below.

Key Concepts

Recombination Frequency and Linear Map Units

Recombination frequency refers to the proportion of offspring in which a recombination event has occurred between two linked genes during meiosis. It is expressed in linear map units or centimorgans (cM), where one map unit corresponds to a 1% recombination frequency. This measurement is used to create genetic maps that indicate the relative positions and distances between genes on a chromosome.

Genetic Linkage

Genetic linkage occurs when genes are located close to each other on the same chromosome, causing them to be inherited together more frequently than would be expected under independent assortment. This phenomenon can alter the predicted ratios from crosses and must be considered when mapping gene locations.

Independent Assortment

Independent assortment is a principle stating that alleles for different genes segregate independently of one another during gamete formation, provided the genes are on different chromosomes or sufficiently far apart on the same chromosome. This principle underlies the expected 9:3:3:1 ratio in dihybrid crosses when genes are unlinked.

Punnett Square

The Punnett square is a diagrammatic tool used in genetics to predict the outcome of a cross between individuals by displaying the possible gametes from each parent along with the resulting genotypes of the offspring. It is essential for visualizing and calculating the probabilities of inheriting various allele combinations in simple and dihybrid crosses.

True Breeding

True breeding refers to organisms that, when self-fertilized or crossed with another organism of the same genotype, produce offspring that display only the parental phenotype. This concept is crucial for establishing homozygous genotypes, which are used as starting points in classical genetics experiments.

Mendelian Genetics

This concept involves the principles of inheritance established by Gregor Mendel, which include the use of alleles to represent different forms of a gene and the ideas of dominance, recessiveness, and segregation. It is the basis for understanding how traits are passed from parents to offspring and how genotypes correspond to phenotypes.

Test Cross

A test cross is a genetic crossing experiment used to determine the genotype of an individual with a dominant phenotype by crossing it with an individual known to be homozygous recessive for the trait. This method helps reveal whether the individual in question is homozygous dominant or heterozygous.

Transcript

00:01 Inheritance and in particular flies or drosophilia and here we have a capital s for a straight wing a little s for a curved wing big r for red eyes and little r for brown eyes okay so part a just wants us to very simple use these letters to represent the parental genotypes so in the first cross we're doing to true breeding tells us that in the first sentence so that would be true breeding means homozygous for either the dominant trait or homozygous for the recessive trait so this would be the parent cross and then of course this gives us the standard f1 generation that is heterozygous and so then we make a test cross with this generation and another set of true breeding recessive okay so part a was just to construct these and lb is we're going to do our standard punnant square so if we have this heterozygous that can break down into four possible gametes sort of one of every flavor i'm hoping this a little tick on the s helps uh the capital, the dominant versus the recessive, lowercase.

02:17 I hate using s's because they look relatively the same, big or small.

02:22 Okay, and then, but the gametes for the recessive, we only really have one option.

02:27 I mean, technically it would make four gametes with the exact same recessive in age, but we don't need to make four columns, and so we'll do our square here, and we get a heterozygous for both.

02:51 And then the heterozygous in one, but recessive in the other, and then homozygous recessive.

03:16 Okay, so this would be b, and then c is just to sort of say, well, what are these genotypes and phenotypes, right? so the distribution here is 25 % of each for both the genotypes and phenotypes.

03:43 Phenotypically though, this first row would be the straight and straight winged and red -eyed.

03:57 Then we have the straight -winged because it has the dominant s, but this time the brown eyes.

04:09 Then we get into the curved wings.

04:11 We're recessive, double recessive for s.

04:19 The next two will both be curved wings.

04:21 And then we see the dominant eye color.

04:24 So for this third row would be red eyes.

04:27 And then the last row is tested in both.

04:28 We will have curved wings and brown eyes.

04:35 Okay.

04:36 And so let's hopefully have some practice doing things in the standard way there.

04:41 But then we get to d and we want to mix it up a little bit.

04:45 So d is telling us as it happens, these are on the same chromosome, which means they are linked.

04:50 So now we do a different cross.

04:53 And i tend to write my linked cross.

04:55 Is a little bit differently.

04:57 I like to just group them together.

04:59 So that would mean that our first parent, the very first cross with the true breeding, would, i'm sorry, i forgot the ticks here, would look like this.

05:17 And so then i group these linked traits like this.

05:22 So then the only kind of offspring ends up being this linked dominant with a linked recessive set.

05:37 Course this problem then wanted us to cross this with another true breeding recessive...

Please give Ace some feedback