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Ch. 5 - Chromosome Mapping in Eukaryotes
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of GeneticsISBN: 9780135564776Non è quello che usi tu?Cambia libro di testo
Capitolo 5, Problema 15c

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.
Table showing male offspring counts for eight Drosophila phenotypes involving yellow, white, and cut genes, with some phenotypes absent.
Were any double-crossover offspring expected?

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Step 1: Understand the genetic cross. The problem involves three X-linked genes in Drosophila: yellow (y), white (w), and cut (ct). These genes are recessive and located on the X chromosome. The parental cross involves a yellow-bodied, white-eyed female with normal wings (y w +) and a male with normal body and eyes but cut wings (+ + ct).
Step 2: Analyze the F₁ generation. The F₁ females are wild type for all traits (+ + +), indicating that the female inherited the dominant alleles from the male parent. The F₁ males express yellow body and white eyes (y w +), which indicates that they inherited the recessive alleles from the female parent.
Step 3: Examine the F₂ progeny. The F₂ generation includes male offspring with various phenotypes. The phenotypes and their frequencies are provided in the table. These frequencies will help determine the genetic map and identify crossover events.
Step 4: Identify parental, single-crossover, and double-crossover classes. The most frequent phenotypes represent the parental classes (+ + ct and y w +). The less frequent phenotypes represent single-crossover events, and the least frequent phenotypes (or zero-frequency phenotypes) represent double-crossover events. Double crossovers occur when two recombination events happen between the three genes.
Step 5: Determine if double-crossover offspring were expected. Double crossovers are expected based on the genetic distance between the genes. If the genes are far enough apart, double crossovers should occur. However, the absence of double-crossover phenotypes (y + + and + w ct) in the data suggests that the genetic distance between the genes may be small or that the sample size was insufficient to detect them.

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X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. In Drosophila, males have one X chromosome and one Y chromosome, while females have two X chromosomes. This means that recessive traits linked to the X chromosome are more likely to be expressed in males, as they do not have a second X chromosome to mask the recessive allele.
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Genetic Mapping

Genetic mapping is the process of determining the relative positions of genes on a chromosome based on the frequency of recombination between them. By analyzing offspring phenotypes from genetic crosses, researchers can estimate distances between genes, which helps in constructing a genetic map. The closer two genes are on a chromosome, the less likely they are to be separated by recombination during meiosis.
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Mapping Overview

Double Crossover

A double crossover occurs when two separate crossover events happen between two linked genes during meiosis. This can lead to the exchange of genetic material between homologous chromosomes, potentially resulting in offspring with recombinant phenotypes. The frequency of double crossovers can be calculated using the recombination frequencies of the genes involved, and their occurrence can affect the expected ratios of phenotypes in the offspring.
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Double Strand Breaks
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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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Domanda del libro di testo

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.

Could the F2 female offspring be used to construct the map? Why or why not?

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Domanda del libro di testo

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.

Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

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Domanda del libro di testo

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.

Construct a map, assuming that white is at locus 1.5 on the X chromosome.

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Domanda del libro di testo

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.

634
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Domanda del libro di testo

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,

What is the sequence and interlocus distance between these three genes?

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