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Ch. 5 - Chromosome Mapping in Eukaryotes
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 14b
Chapter 5, Problem 14b

In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F1, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F2 generation, and 1000 offspring were counted, with the results shown in the following table.
Table showing offspring counts for eight phenotypes from a Drosophila cross involving three X-linked recessive traits.
No determination of sex was made in the data.
Determine the sequence of the three genes and the map distances between them.

Verified step by step guidance
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Step 1: Identify the parental and recombinant phenotypes. The two most frequent phenotypes represent the parental (non-recombinant) classes, while the less frequent phenotypes are recombinant classes. From the table, the highest counts are 314 (sc s v) and 280 (+++), so these are the parental phenotypes.
Step 2: Determine the double crossover classes. The two least frequent phenotypes correspond to double crossovers. From the table, these are 10 (sc s +) and 14 (++ v). Identifying these will help us find the gene order.
Step 3: Establish gene order by comparing the double crossover phenotypes to the parental phenotypes. The gene whose allele differs in the double crossover classes compared to the parental types is the gene in the middle. For example, compare the double crossover phenotypes to the parental ones to see which gene changes.
Step 4: Calculate recombination frequencies between each pair of genes. Use the formula for recombination frequency (RF): \(\text{RF} = \frac{\text{Number of recombinant offspring between two genes}}{\text{Total number of offspring}} \times 100\) Sum the single and double crossover classes relevant to each gene pair to find the recombinant offspring counts.
Step 5: Convert recombination frequencies into map distances (in centiMorgans, cM). The distances between genes are approximately equal to the recombination frequencies (1% RF = 1 cM). Use these distances to draw the gene map with the correct gene order and spacing.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

X-linked Inheritance

X-linked inheritance refers to genes located on the X chromosome, which show distinct patterns of inheritance, especially in species like Drosophila. Males, having only one X chromosome, express recessive traits if they inherit the mutant allele, while females require two copies. Understanding this helps interpret the phenotypic ratios in crosses involving X-linked genes.
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X-Inactivation

Genetic Linkage and Recombination

Genetic linkage occurs when genes are close together on the same chromosome and tend to be inherited together. Recombination during meiosis can separate linked genes, producing recombinant offspring. The frequency of recombination between genes is proportional to their physical distance, which is used to create genetic maps.
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Chi Square and Linkage

Three-Point Test Cross and Map Distance Calculation

A three-point test cross involves crossing an individual heterozygous for three genes with a homozygous recessive individual to analyze recombination events. By counting offspring phenotypes, one can determine gene order and calculate map distances using recombinant frequencies, which reflect the percentage of crossover events between gene pairs.
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Related Practice
Textbook Question

In a series of two-point mapping crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed:

In another set of experiments, a sixth gene, d, was tested against b and pr:

Predict the results of two-point mapping between d and c, d and vg, and d and adp.

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Textbook Question

Two different female Drosophila were isolated, each heterozygous for the autosomally linked genes b (black body), d (dachs tarsus), and c (curved wings). These genes are in the order d–b–c, with b being closer to d than to c. Shown here is the genotypic arrangement for each female along with the various gametes formed by both:

Identify which categories are noncrossovers (NCOs), single crossovers (SCOs), and double crossovers (DCOs) in each case. Then, indicate the relative frequency in which each will be produced.

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Textbook Question

In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F1, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F2 generation, and 1000 offspring were counted, with the results shown in the following table.

No determination of sex was made in the data.

Using proper nomenclature, determine the genotypes of the P1 and F1 parents.

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Textbook Question

In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F1, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F2 generation, and 1000 offspring were counted, with the results shown in the following table.

No determination of sex was made in the data.

Are there more or fewer double crossovers than expected?

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Textbook Question

In Drosophila, a cross was made between females, all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table.

No determination of sex was made in the data. Calculate the coefficient of coincidence. Does it represent positive or negative interference?

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Textbook Question

Another cross in Drosophila involved the recessive, X-linked genes yellow (y), white (w), and cut (ct). A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. The F₁ females were wild type for all three traits, while the F1 males expressed the yellow-body and white-eye traits. The cross was carried to an F2 progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was constructed.

Diagram the genotypes of the F1 parents.

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