Question

In the fruit fly, Drosophila melanogaster, a spineless (no wing bristles) female fly is mated to a male that is claret (dark eyes) and hairless (no thoracic bristles). Phenotypically wild-type \(\mathrm{F}_{1}\) female progeny were mated to fully homozygous (mutant) males, and the following progeny ( 1000 total) were observed: $$\begin{array}{lc} \text { Phenotypes } & \text { Number Observed } \\ \hline \text { spineless } & 321 \\ \text { wild } & 38 \\ \text { claret, spineless } & 130 \\ \text { claret } & 18 \\ \text { claret, hairless } & 309 \\ \text { hairless, claret, spineless } & 32 \\ \text { hairless } & 140 \\ \text { hairless, spineless } & 12 \end{array}$$ (a) Which gene is in the middle? (b) With respect to the three genes mentioned in the problem, what are the genotypes of the homozygous parents used in making the phenotypically wild \(F_{1}\) heterozygote? (c) What are the map distances between the three genes? A correct formula with the values "plugged in" for each distance will be sufficient. (d) What is the coefficient of coincidence? A correct formula with the values "plugged in" will be sufficient.

Short answer

Calculate the map distances between the three genes and the coefficient of coincidence. Answer: The middle gene is "claret". The genotypes of the homozygous parents are spineless (no wing bristles): aa, claret (dark eyes): bb, and hairless (no thoracic bristles): cc. The map distances are: - Distance between genes "a" and "b": %distance_{a-b} - Distance between genes "b" and "c": %distance_{b-c} - Distance between genes "a" and "c": %distance_{a-c} The coefficient of coincidence (COC) is calculated as COC = (12) / ((321+130+140+12)/1000 times (130+18+309+12)/1000 times 1000).

Step-by-step solution

Identifying the middle gene

To identify the middle gene, first, we need to look for the least occurring double-crossover event. This can be identified based on the least number of observed progeny with two of the mutant phenotypes. In this case, it's "hairless, spineless" with 12 observed cases. Now, let's represent spineless as "a" (allele 'a'), claret as "b" (allele 'b'), and hairless as "c" (allele 'c'). The double-crossover event is between genes "a" and "c", and this indicates that gene "b" must be the middle gene. (a) The middle gene is "claret".

Finding the genotypes of the homozygous parents

To find the genotypes of the homozygous parents used in making the phenotypically wild F1 heterozygote, we need to look at the progenies with recessive homozygous genotype. When a phenotypically wild progeny mates with a fully homozygous recessive, we need to know the genotypes for middle gene in single crossovers: - wild \(F_{1}\): spineless= A/a (homozygous dominant 'AA' mates with spineless recessive 'aa') - claret= B/b (homozygous dominant 'BB' mates with claret recessive 'bb') - hairless= C/c (homozygous dominant 'CC' mates with hairless recessive 'cc') (b) The genotypes of the homozygous parents making the wild F1 heterozygote are as follows: - Spineless (no wing bristles): aa - Claret (dark eyes): bb - Hairless (no thoracic bristles): cc

Calculating the map distances between the three genes

Map distance is calculated as the number of single crossovers divided by the total number of progeny, expressed as a percentage. The map distance between genes "a" and "b" can be calculated as: (321+130+140+12)/1000 times 100 = \%distance_{a-b} The map distance between genes "b" and "c" can be calculated as: (130+18+309+12)/1000 times 100 = \%distance_{b-c} The map distance between genes "a" and "c" can be calculated as: (321+18+140+12)/1000 times 100 = \%distance_{a-c} (c) Map distances: - Distance between genes "a" and "b": \%distance_{a-b} - Distance between genes "b" and "c": \%distance_{b-c} - Distance between genes "a" and "c": \%distance_{a-c}

Calculating the coefficient of coincidence

Coefficient of coincidence (COC) is the ratio between the observed double-crossover event and the expected double-crossover event. The expected crossover event is the product of recombination frequency for two pairs of genes: COC = (Observed double-crossover events) / (Expected double-crossover events) COC = (12) / ((321+130+140+12)/1000 times (130+18+309+12)/1000 times 1000) (d) Coefficient of coincidence: COC

Key concepts

Drosophila melanogaster Genetics

Drosophila melanogaster, commonly known as the fruit fly, is a key organism in genetic research. Thanks to its short life cycle and the ease with which it can be bred in large numbers, this fly has become a mainstay model in studies of genetics and developmental biology. Fruit flies have only four pairs of chromosomes: three autosomes and one pair of sex chromosomes, simplifying genetic analysis.

Genetic traits in Drosophila can be dominant or recessive, like the spineless, claret, and hairless traits in our exercise. When studying the genetics of this species, scientists often investigate patterns of inheritance, mutations, and genetic recombination, which reveals how traits are passed on from one generation to the next. The fundamental principles learned from studying Drosophila genetics are widely applicable across many species, including humans, due to the conservation of many genetic pathways.

In genetic linkage analysis involving Drosophila, researchers cross different strains to observe inheritance patterns of various traits. These crosses can reveal which traits are linked, which are determined by genes located close together on the same chromosome, and thus tend to be inherited together.

Crossover Frequency

Crossover frequency is a measure of how often recombination occurs between two genes on the same chromosome. It is foundational for understanding genetic linkage and gene mapping. During the formation of gametes, homologous chromosomes can exchange genetic material in a process called crossing over. The likelihood of a crossover event occurring between two genes is roughly proportional to the distance between them—the farther apart the genes, the more likely a crossover will occur.

For example, if two genes are close together on the chromosome, they have a low crossover frequency indicating a high degree of linkage. Conversely, genes that are far apart will have a higher crossover frequency and are less likely to be inherited together. In our exercise with the fruit flies, the crossover frequency allows us to understand the relationship and the distance between the spineless, claret, and hairless genes.

Crossover frequency is crucial for genetic mapping because it reflects the physical distance between genes. It's calculated based on the ratio of recombinant offspring to total offspring and is usually expressed as a percentage. In genetic studies like the one in our exercise, accurately determining crossover frequencies is crucial for constructing genetic maps and understanding the genetic basis of traits.

Gene Mapping

Gene mapping is the process of determining the relative positions of genes on a chromosome. This tool is essential in genetics for discovering where genes are located and understanding their correlation with inherited traits. In the fruit fly exercise, gene mapping involves calculating the distances between genes based on crossover frequencies observed in offspring.

To perform gene mapping, we look at crossover events between genes. A single crossover event affects the inheritance of genes flanking the crossover point, while double crossovers can provide information on gene order. The frequency of these crossovers, as seen in the exercise, is used to estimate distances between genes, which are represented in map units or centimorgans (cM). The number of map units between genes corresponds to the average number of crossovers that would occur in 100 meioses.

The key steps in gene mapping include identifying the gene order, which we do by finding the gene that has the least number of observed double crossovers, and then calculating individual gene distances. The coefficient of coincidence (COC) can also be factored in, quantifying the discrepancy between expected and observed double crossovers, thus refining our map calculations. Through these methods, we start to build a comprehensive genetic map that can be used to predict inheritance patterns and understand how genetic diseases and traits may be passed on.