Ch. 5 - Chromosome Mapping in Eukaryotes

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 5 - Chromosome Mapping in Eukaryotes

Problem 11

Chapter 5, Problem 11

In the cross shown here, involving two linked genes, ebony (e) and claret (ca), in Drosophila, where crossing over does not occur in males, offspring were produced in a 2 + : 1 ca : 1 e phenotypic ratio:

These genes are 30 units apart on chromosome III. What did crossing over in the female contribute to these phenotypes?

Verified step by step guidance

Step 1: Identify the parental and recombinant genotypes from the cross. The female genotype is heterozygous for both genes (e ca+ / e+ ca), and the male genotype is the same. Since crossing over does not occur in males, all recombination must come from the female.

Step 2: Understand that the genes are linked and 30 map units apart, meaning the recombination frequency between the two genes is 30%. This implies that 30% of the gametes produced by the female will be recombinant types, and 70% will be parental types.

Step 3: Determine the parental gametes produced by the female: e ca+ and e+ ca. These will be the most frequent gametes, representing 70% of the total gametes.

Step 4: Determine the recombinant gametes produced by the female: e ca and e+ ca+. These will be less frequent, representing 30% of the total gametes, split equally between the two recombinant types (15% each).

Step 5: Use the gamete frequencies to predict the offspring phenotypic ratios. Since males do not undergo crossing over, their gametes are parental types only. Combine female and male gametes to find the expected offspring phenotypes, noting that recombinant phenotypes arise solely from female crossing over.

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Key Concepts

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

Genetic Linkage and Recombination

Genetic linkage occurs when two genes are located close together on the same chromosome and tend to be inherited together. Recombination, or crossing over, can separate linked genes during meiosis, producing new allele combinations. The frequency of recombination between two genes reflects their physical distance on the chromosome, measured in map units or centimorgans.

Sex Differences in Crossing Over

In some species like Drosophila, crossing over occurs only in one sex (females) and not in the other (males). This means recombinant gametes arise only from females, affecting the phenotypic ratios observed in offspring. Understanding this sex-specific recombination is crucial for interpreting genetic crosses involving linked genes.

Interpreting Phenotypic Ratios from Linked Genes

Phenotypic ratios from crosses involving linked genes differ from Mendelian ratios due to reduced recombination. Parental (non-recombinant) phenotypes are more frequent, while recombinant phenotypes appear less often, proportional to the recombination frequency. Analyzing these ratios helps determine gene order, map distances, and the contribution of crossing over to genetic variation.

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Textbook Question

What two essential criteria must be met in order to execute a successful mapping cross?

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Textbook Question

The genes dumpy (dp), clot (cl), and apterous (ap) are linked on chromosome II of Drosophila. In a series of two-point mapping crosses, the following genetic distances were determined. What is the sequence of the three genes?

dp–ap: 42

dp–cl: 3

ap–cl: 39

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Textbook Question

Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green color, whereas the homozygous presence of the recessive y allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained:

colored, green: 88

colored, yellow: 12

colorless, green: 8

colorless, yellow: 92

Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.

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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:

Given that the adp gene is near the end of chromosome II (locus 83), construct a map of these genes.

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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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