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Ch. 5 - Chromosome Mapping in Eukaryotes
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 17
Chapter 5, Problem 17

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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Identify the genotypes of the parental generation: Drosophila females are homozygous for pink (p) and ebony (e) on the third chromosome, and males are homozygous for dumpy (d) on the second chromosome.
Determine the genotype of the F₁ generation: Since all genes are recessive and the offspring are wild type, the F₁ generation will be heterozygous for all three genes (PpEeDd).
Perform a testcross: Cross F₁ females (PpEeDd) with triply recessive males (ppeddd).
Consider the genetic map distance: Pink and ebony are 20 map units apart, indicating a 20% recombination frequency between these two genes.
Predict the phenotypic ratios: Calculate the expected phenotypic ratios based on recombination frequencies and the testcross setup. Consider the impact of linkage and recombination on the offspring phenotypes.

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Key Concepts

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

Linkage and Genetic Mapping

Linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together. Genetic mapping uses recombination frequency, measured in map units (mu), to determine the distance between genes. A distance of 20 mu indicates a 20% chance of recombination occurring between the pink and ebony genes, which affects the phenotypic ratios of offspring in genetic crosses.
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Chi Square and Linkage

Testcross

A testcross involves breeding an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual. This method helps determine the genotype of the dominant individual based on the phenotypes of the offspring. In this scenario, the F₁ females are testcrossed to triply recessive males to reveal the inheritance patterns of the linked genes.

Reciprocal Cross

A reciprocal cross is a breeding experiment where the sexes of the parents are switched in two separate crosses. This allows researchers to determine if the inheritance pattern is influenced by the sex of the parent. In this case, crossing F₁ males with triply recessive females would yield different results due to the absence of crossing over in males, affecting the distribution of phenotypes in the offspring.
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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 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
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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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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Textbook Question
Are mitotic recombinations and sister chromatid exchanges effective in producing genetic variability in an individual? in the offspring of individuals?
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