Four homozygous recessive mutant lines of Drosophila...

Four homozygous recessive mutant lines of Drosophila melanogaster (labeled 1 through 4) showed abnormal leg coordination, which made their walking highly erratic. These lines were intercrossed; the phenotypes of the $\mathrm{F}_{1}$ flies are shown in the following grid, in which "+" represents wild-type walking and "-" represents abnormal walking: $$\begin{array}{rrrrr} & 1 & 2 & 3 & 4 \\ \hline 1 & - & + & + & + \\ 2 & + & - & - & + \\ 3 & + & - & - & + \\ 4 & + & + & + & - \\ \hline \end{array}$$ a. What type of test does this analysis represent?? b. How many different genes were mutated in creating these four lines? c. Invent wild-type and mutant symbols, and write out full genotypes for all four lines and for the $\mathrm{F}_{1}$ flies. d. Do these data tell us which genes are linked? If not, how could linkage be tested? e. Do these data tell us the total number of genes taking part in leg coordination in this animal?

Four homozygous recessive mutant lines of Drosophila melanogaster (labeled 1 through 4) showed abnormal leg coordination, which made their walking highly erratic. These lines were intercrossed; the phenotypes of the $\mathrm{F}_{1}$ flies are shown in the following grid, in which "+" represents wild-type walking and "-" represents abnormal walking:
$$\begin{array}{rrrrr}
& 1 & 2 & 3 & 4 \\
\hline 1 & - & + & + & + \\
2 & + & - & - & + \\
3 & + & - & - & + \\
4 & + & + & + & - \\
\hline
\end{array}$$
a. What type of test does this analysis represent??
b. How many different genes were mutated in creating these four lines?
c. Invent wild-type and mutant symbols, and write out full genotypes for all four lines and for the $\mathrm{F}_{1}$ flies.
d. Do these data tell us which genes are linked? If not, how could linkage be tested?
e. Do these data tell us the total number of genes taking part in leg coordination in this animal?

Key Concepts

Genetic Linkage

Genetic linkage refers to the phenomenon where genes that are located close to each other on the same chromosome tend to be inherited together, rather than assorting independently. Although complementation tests can reveal if mutations are in different genes, they do not provide information on whether those genes are linked. Testing for linkage typically involves recombination frequency analysis in meiotic progeny to determine genetic map distances between loci.

Genotype Nomenclature

Genotype nomenclature in genetics involves assigning symbols to represent the wild?type and mutant alleles for each gene. It is a systematic way to record and communicate the genetic composition of organisms. In complementation tests and genetic analyses, clear notation (such as capital letters for wild?type alleles and lowercase letters for mutant alleles) is essential for tracking which mutations are present in each line and for formulating hypotheses about gene function and interactions.

Gene Function Analysis in Mutant Phenotypes

Analyzing mutant phenotypes through crosses helps researchers deduce how many different genes are involved in a biological process and how these genes interact. When several mutant lines are crossed and the resulting phenotypes are observed, the pattern of complementation or noncomplementation can be used to infer the number of underlying genetic components and their functional relationships in the pathway governing the trait.

Complementation Test

A complementation test is a genetic method used to determine whether two mutations that produce a similar phenotype are in the same gene (allelic) or in different genes (nonallelic). When two homozygous recessive mutants are crossed, a wild?type phenotype in the offspring indicates that the mutations complement each other (and therefore reside in different genes), whereas a mutant phenotype indicates noncomplementation and suggests that the mutations affect the same gene.

Allelism

Allelism refers to the relationship between mutations in the same gene. When two mutant alleles fail to complement each other in a heterozygous state (resulting in the mutant phenotype), it indicates that the mutations are allelic, meaning they affect the same gene. This concept is central to determining the number of genes involved in a phenotype based on the outcomes of crossing different mutant lines.

Transcript

00:01 In drosophila, scientists were interested in understanding how these four homozygous mutation lines were behaving.

00:09 And so we were provided some data in the form of a table where each of the four strains were bred with each other to see what happens to the mutant phenotype.

00:23 And so what we saw was that some strains when bred to other strains produced a wild type phenotype.

00:31 Which is indicated by the plus.

00:36 So that means normal walking.

00:39 And other strains are indicated by a minus, and that is erratic walking.

00:46 The first thing that jumps out to me in this table is that this set of minuses here.

00:52 So the erratic walking is found still when strain two is spread to strain three, you still get a mutant phenotype.

01:02 And the same thing is true when strain three, obviously is bred to strain two, you get a mutant phenotype.

01:07 This tells me that these are alleles of the same gene, and that what we're looking at is a complementation test.

01:23 And remember, a complementation test is used to identify the number of genes that can mutate to a specific phenotype.

01:33 And so if offspring of a given cross, and we can use these two here as our examples, two by three and three by two, express the mutant phenotype, then what we're saying is the mutations found in one strain do not complement the mutations found in the second strain.

01:51 And so you still yield a mutant phenotype.

01:55 However, if you have progeny such as that's found here, say a one by four cross, you yield a wild type phenotype from a mutated one strain crossed with a mutated four strain, that tells me that the mutations in one complement the mutations found in four, and that those mutations together yield a wild type phenotype.

02:21 So they're carrying mutant alleles of separate genes.

02:25 And so we can determine, based on this table, that there are three genes involved in this cross.

02:33 And so i'm going to scoot this up at any bit, and then we can identify the different, genes and alleles.

02:41 So if we have say a and little a, that'll represent gene one, b and little b, and that will represent gene two.

02:50 And i'm going to use d because cs are terrible to tell capital from lowercase, and that's going to represent gene three.

03:00 And i'm going to use, there must be two forms of mutated d.

03:06 So i'm going to use one and two or two and three.

03:10 Okay, so let's look at our cross.

03:13 If we cross strain one with strain two, that has to be a phenotype, our genotype, big a, little a, big b, big b, big d, little d.

03:25 And that tells us that we're going to get a wild type because we have at least one dominant allele from this cross of one to two.

03:35 And then these recessive alleles are found in different strains.

03:40 If we do cross 1 by 3, it's going to be big a, little a, big b, big b, big b, big b, little d.

03:48 And it must be a 3 because you get a wild phenotype that's there.

03:53 And then 1 by 4, it's going to be big a, little a, heterozygous b, and homozygous dominant for d.

04:01 And that yields a wild type...

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