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Ch. 7 - Sex Determination and Sex Chromosomes
Klug - Concepts of Genetics  12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
Chapter 7, Problem 12

An attached-X female fly, XXY, expresses the recessive X-linked white-eye mutation. It is crossed to a male fly that expresses the X-linked recessive miniature-wing mutation. Determine the outcome of this cross in terms of sex, eye color, and wing size of the offspring.

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Identify the genotypes of the parents: The attached-X female has two X chromosomes attached together and one Y chromosome (genotype: XXY), and she expresses the recessive white-eye mutation on her X chromosomes. The male has one X chromosome with the recessive miniature-wing mutation and one Y chromosome (genotype: X^m Y).
Determine the possible gametes produced by each parent: The attached-X female produces gametes with the attached X chromosomes together (X^w X^w) or a Y chromosome, while the male produces gametes with either the X chromosome carrying the miniature-wing mutation (X^m) or the Y chromosome.
Set up a Punnett square to combine the female and male gametes: The female gametes are either X^w X^w or Y, and the male gametes are X^m or Y. Cross these to find all possible offspring genotypes.
Analyze the genotype of each offspring to determine their sex and phenotype: Offspring with two X chromosomes (one from each parent) will be female, and those with one X and one Y will be male. Use the presence of recessive alleles on the X chromosomes to determine eye color (white if recessive) and wing size (miniature if recessive).
Summarize the phenotypes of the offspring by sex, eye color, and wing size based on the genotypes obtained from the Punnett square.

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Key Concepts

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

X-linked Inheritance

X-linked inheritance refers to genes located on the X chromosome. Since males have one X and one Y chromosome, recessive mutations on the X chromosome are expressed in males if present. Females have two X chromosomes, so recessive traits are expressed only if both X chromosomes carry the mutation.
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X-Inactivation

Attached-X Chromosome Configuration

An attached-X female has both X chromosomes physically connected and inherited together as a unit. This affects the segregation of sex chromosomes during meiosis, influencing the sex and genotype of offspring differently than typical XX females.
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Regions of X Chromosomes

Phenotypic Expression of Recessive Mutations

Recessive mutations, such as white-eye and miniature-wing, are only phenotypically visible when no dominant allele is present. Understanding which offspring inherit these recessive alleles on the X chromosome helps predict their eye color and wing size.
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Mutations and Phenotypes
Related Practice
Textbook Question

An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing (rw) is discovered. Contrast the F1 and F2 generations from a cross between a female with reduced wings and a male with normal-sized wings when the female is the heterogametic sex.

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Textbook Question

An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing (rw) is discovered. Contrast the F1 and F2 generations from a cross between a female with reduced wings and a male with normal-sized wings when the male is the heterogametic sex.

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Textbook Question

When cows have twin calves of unlike sex (fraternal twins), the female twin is usually sterile and has masculinized reproductive organs. This calf is referred to as a freemartin. In cows, twins may share a common placenta and thus fetal circulation. Predict why a freemartin develops.

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Textbook Question

Assume that on rare occasions the attached X chromosomes in female gametes become unattached. Based on the parental phenotypes in Problem 12, what outcomes in the F₁ generation would indicate that this has occurred during female meiosis?

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Textbook Question

It has been suggested that any male-determining genes contained on the Y chromosome in humans cannot be located in the limited region that synapses with the X chromosome during meiosis. What might be the outcome if such genes were located in this region?

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Textbook Question

What is a Barr body, and where is it found in a cell?

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