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Ch. 5 - Chromosome Mapping in Eukaryotes
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 16
Chapter 5, Problem 16

In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F₁ progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table, Phenotype Number Dichaete 401 ebony, pink 389 Dichaete, ebony 84 pink 96 Dichaete, pink 2 ebony 3 Dichaete, ebony, pink 12 wild type 13 Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

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Identify the genotypes of the parental generation: The Dichaete stock is heterozygous for Dichaete (D) and wild type for ebony (E) and pink (P), so its genotype is DdEePp. The homozygous ebony, pink flies have the genotype ddeepp.
Perform the initial cross: Cross the Dichaete stock (DdEePp) with the homozygous ebony, pink flies (ddeepp). The F₁ progeny will have the genotype DdEePp, showing the Dichaete phenotype due to the dominant D allele.
Backcross the F₁ progeny: Cross the F₁ Dichaete flies (DdEePp) with the homozygous ebony, pink flies (ddeepp).
Determine the expected phenotypes and genotypes: The backcross will produce a variety of phenotypes based on the recombination of alleles. Consider the possible combinations of alleles for each gene (D, e, p) and their effects on phenotype.
Analyze the phenotypic ratios: Compare the observed phenotypic numbers from the table with the expected ratios to infer linkage and recombination events.

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Key Concepts

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

Dominant and Recessive Alleles

In genetics, alleles can be classified as dominant or recessive based on their phenotypic expression. A dominant allele, such as the Dichaete mutation (D), will manifest in the phenotype even if only one copy is present. In contrast, recessive alleles, like ebony (e) and pink (p), require two copies to express their traits. Understanding this distinction is crucial for predicting the outcomes of genetic crosses.
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Variations on Dominance

Homozygous and Heterozygous Genotypes

An organism is homozygous for a gene when it has two identical alleles, while it is heterozygous when it has two different alleles. In this scenario, the Dichaete mutation is lethal when homozygous, meaning that individuals with two copies of the D allele do not survive. Recognizing these genotypic classifications helps in analyzing the inheritance patterns observed in the offspring of the crosses.
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Backcrossing and Phenotypic Ratios

Backcrossing involves mating an F₁ hybrid organism with one of its parental genotypes to analyze inheritance patterns. The resulting offspring's phenotypic ratios provide insights into the genetic contributions of the parents. In this case, the backcross of Dichaete F₁ flies to homozygous ebony, pink flies reveals the segregation of traits and helps determine the linkage and interaction between the genes involved.
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Related Practice
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. Phenotype Male Offspring y + ct 9 + w + 6 y w ct 90 + + + 95 + + ct 424 y w + 376 y + + 0 + w ct 0 Could the F₂ female offspring be used to construct the map? Why or why not?
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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. Phenotype Male Offspring y + ct 9 + w + 6 y w ct 90 + + + 95 + + ct 424 y w + 376 y + + 0 + w ct 0 Were any double-crossover offspring expected?
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Textbook Question
In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F₁ progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table, Phenotype Number Dichaete 401 ebony, pink 389 Dichaete, ebony 84 pink 96 Dichaete, pink 2 ebony 3 Dichaete, ebony, pink 12 wild type 13 What is the sequence and interlocus distance between these three genes?
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Textbook Question
Drosophila females homozygous for the third chromosomal genes pink and ebony (the same genes from Problem 16) were crossed with males homozygous for the second chromosomal gene dumpy. Because these genes are recessive, all offspring were wild type (normal). F₁ females were testcrossed to triply recessive males. If we assume that the two linked genes, pink and ebony, are 20 mu apart, predict the results of this cross. If the reciprocal cross were made (F₁ males—where no crossing over occurs—with triply recessive females), how would the results vary, if at all?
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Textbook Question
In Drosophila, two mutations, Stubble (Sb) and curled (cu), are linked on chromosome III. Stubble is a dominant gene that is lethal in a homozygous state, and curled is a recessive gene. If a female of the genotype is to be mated to detect recombinants among her offspring, what male genotype would you choose as a mate?
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Textbook Question
If the cross described in Problem 18 were made, and if Sb and cu are 8.2 map units apart on chromosome III, and if 1000 offspring were recovered, what would be the outcome of the cross, assuming that equal numbers of males and females were observed?
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