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Ch. 5 - Chromosome Mapping in Eukaryotes
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 19
Chapter 5, Problem 19

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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1
Identify the genes involved in the cross: Sb and cu, which are 8.2 map units apart on chromosome III.
Understand that 1 map unit corresponds to a 1% recombination frequency, so 8.2 map units imply an 8.2% chance of recombination between Sb and cu.
Calculate the expected number of recombinant offspring by multiplying the recombination frequency (8.2%) by the total number of offspring (1000).
Determine the number of parental type offspring by subtracting the number of recombinant offspring from the total number of offspring.
Assume equal numbers of males and females, so divide the number of recombinant and parental type offspring by 2 to find the expected number of each type for both sexes.

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Key Concepts

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

Genetic Mapping

Genetic mapping is a method used to determine the location of genes on a chromosome and the distance between them. The distance is often measured in map units, where 1 map unit corresponds to a 1% chance of recombination occurring between two genes during meiosis. In this case, the distance of 8.2 map units between the Sb and cu genes indicates a relatively low frequency of recombination, which affects the expected offspring ratios.
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Mapping Overview

Recombination Frequency

Recombination frequency refers to the likelihood that two genes will be separated during meiosis due to crossing over. It is calculated as the number of recombinant offspring divided by the total number of offspring. In the context of the question, a recombination frequency of 8.2% suggests that 8.2% of the offspring will exhibit recombinant phenotypes, while the remaining 91.8% will show parental phenotypes.
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Recombination after Single Strand Breaks

Offspring Ratios in Genetic Crosses

In genetic crosses, the expected ratios of offspring phenotypes can be predicted based on the principles of inheritance and recombination. For a cross involving linked genes, the expected outcome can be calculated by considering both parental and recombinant types. Given the recombination frequency and the total number of offspring (1000), one can determine the expected number of each phenotype, which helps in analyzing the results of the cross.
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Related Practice
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
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
Are mitotic recombinations and sister chromatid exchanges effective in producing genetic variability in an individual? in the offspring of individuals?
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Textbook Question
What possible conclusions can be drawn from the observations that in male Drosophila, no crossing over occurs, and that during meiosis, synaptonemal complexes are not seen in males but are observed in females where crossing over occurs?
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Textbook Question
An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table. 20 AaBbCc 20 AaBbcc 20 aabbCc 20 aabbcc 5 AabbCc 5 Aabbcc 5 aaBbCc 5 aaBbcc If these three genes were all assorting independently, how many genotypic and phenotypic classes would result in the offspring, and in what proportion, assuming simple dominance and recessiveness in each gene pair?
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