Question

Two different female Drosophila were isolated, each heterozygousfor the autosomally linked genes black body (b), dachs tarsus (d), and curved wings (c). These genes are in the order \(d-b-c\), with \(b\) closer to \(d\) than to \(c .\) Shown in the following table is the genotypic arrangement for each female, along with the various gametes formed by both. Identify which categories are noncrossovers (NCO), single crossovers (SCO), and double crossovers (DCO) in each case. Then, indicate the relative frequency with which each will be produced.

Short answer

Answer: The relative frequencies of NCO, SCO, and DCO for Drosophila are approximately 50%, 50%, and 12.5%, respectively. Note that the actual relative frequencies may vary depending on the distance between genes and other factors involved in crossover events.

Step-by-step solution

Genotypic arrangement for each female

We are given the genotypic arrangement for each female Drosophila and the various gametes formed. Let's represent these arrangements using gene symbols for easier analysis. Female 1: --------- Normal arrangement (N): \(D B C\) Mutant arrangement (M): \(d b c\) Female 2: --------- Normal arrangement (N): \(D B C\) Mutant arrangement (M): \(d b c\)

Determine NCO, SCO, and DCO

To identify the noncrossovers (NCO), single crossovers (SCO), and double crossovers (DCO), we will look for the combinations of genes formed by the gametes for each female. Female 1: --------- NCO: \(DB C\) and \(d b c\) SCO: \(D b C\) and \(d B c\) DCO: \(D B c\) and \(d b C\) Here, NCO represents no exchanges of genes, SCO represents one exchange between \(d\) and \(b\), and DCO represents exchanges between both \(d,b\) and \(b,c\). Female 2: --------- NCO: \(DB C\) and \(d b c\) SCO: \(D b C\) and \(d B c\) DCO: \(D B c\) and \(d b C\) The same combinations are formed for both females as they have the same genotypic arrangements.

Calculate relative frequency

To calculate the relative frequency of NCO, SCO, and DCO, we will compare their occurrence rates to the total number of gametes possible for each female. Given the order of genes as \(d-b-c\), we know that \(b\) is closer to \(d\) than to \(c\). This means that the recombination frequency between \(d\) and \(b\) is smaller than the frequency between \(b\) and \(c\). Combination Rates: ------------------- NCO: \(DB C\) and \(d b c\) (both recombinations not occurring) SCO: \(D b C\) and \(d B c\) (recombination between \(d\) and \(b\)) DCO: \(D B c\) and \(d b C\) (both recombinations occurring) For each combination, the relative frequency can be calculated by dividing the number of occurrences of each gene combination by the total number of possible gametes (assuming all are equally likely). NCO Frequency: \(\frac{1}{4}+\frac{1}{4}=\frac{1}{2}=50\%\) SCO Frequency: \(\frac{1}{4}+\frac{1}{4}=\frac{1}{2}=50\%\) DCO Frequency: \(\frac{1}{16}+\frac{1}{16}=\frac{1}{8}=12.5\%\) (Note: This is an assumption since the actual frequencies depend on the distance between the different genes). In conclusion, the relative frequencies of NCO, SCO, and DCO are approximately 50%, 50%, and 12.5%, respectively. It is important to note that the actual relative frequencies will be affected by various factors, including the distance between genes and the specific processes involved in crossover events.

Key concepts

Linkage and Crossover

In genetics, linkage refers to the tendency of genes that are located close to each other on a chromosome to be inherited together during meiosis. This occurs because the closer two genes are on a chromosome, the less likely they are to be separated by a crossover event during genetic recombination.

Crossover primarily involves the exchange of genetic material between homologous chromosomes during prophase I of meiosis. When crossing over occurs, chromosomes can exchange segments, which can result in "recombinant" chromosomes with a new combination of alleles. This process increases genetic diversity within a species.

**Types of Crossover Events** - **Noncrossovers (NCO):** This occurs when there are no exchanges between genes on the chromosome. The resulting gametes retain the original allele combinations from the parental chromosomes.
- **Single Crossovers (SCO):** Involves one crossover event between two linked genes. This partially disrupts linkage by introducing a new allele combination between these genes.
- **Double Crossovers (DCO):** When two crossover events occur, they may involve non-adjacent genes. DCOs are often less frequent because both genes need to be crossed over, further reducing linkage between them. Understanding linkage and crossover is essential in determining how traits are inherited and is fundamental for genetic mapping and genetic analysis.

Drosophila Genetics

Drosophila, commonly known as the fruit fly, is a model organism widely used in genetic research. This popularity stems from a combination of its simple genome, rapid life cycle, and the ease of maintaining large populations in laboratory settings.

**Why Use Drosophila?** - Drosophila has only four pairs of chromosomes, which makes genetic mapping less complicated compared to other organisms.
- The short generation time of Drosophila allows for the observation of genetic traits over several generations in a relatively short period.
- A vast array of genetic tools and mutations are available, allowing researchers to study specific genes and alleles easily. In the study of linkage and crossover, Drosophila genetics offers insights into how traits are inherited and how recombination frequencies can be used to estimate gene distances on chromosomes. For instance, in a typical experiment, Drosophila might be bred to have specific traits, such as black body or curved wings, as seen in the example exercise.

By examining the offspring, scientists can determine how often recombination occurs between specific genes, aiding in the development of genetic maps that chart the order and relative distances of genes on a chromosome.

Chromosomal Mapping

Chromosomal mapping, also known as genetic mapping, is a critical technique used to understand the relative positions of genes on a chromosome based on recombination frequencies. It is an essential tool in genetics that aids in elucidating the genetic layout of an organism.

**Principles of Chromosomal Mapping** - **Recombination Frequency:** The key to chromosomal mapping is the recombination frequency, which correlates with the physical distance between genes. The closer two genes are, the less likely crossover will separate them, resulting in lower recombination frequency. This is expressed as a percentage or in map units (centimorgans).
- **Map Units:** One map unit, or centimorgan (cM), represents a recombination frequency of 1%. Map units give us a way to quantify the genetic distance between genes on a chromosome. For example, given genes \(d-b-c\), if the recombination frequency between genes \(d\) and \(b\) is smaller than between \(b\) and \(c\), it indicates "b" is closer to "d" than "c" is to "b".

• This knowledge helps scientists to come up with genetic maps that visualize gene orders and distances.

In essence, chromosomal mapping is not only crucial for understanding the genetic architecture of an organism but also for applications in breeding, evolution studies, and disease research.