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Ch. 5 - Chromosome Mapping in Eukaryotes
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 33b
Chapter 5, Problem 33b

The gene controlling the Xg blood group alleles (Xg⁺ and Xg⁻) and the gene controlling a newly described form of inherited recessive muscle weakness called episodic muscle weakness (EMWX) are closely linked on the X chromosome in humans at position Xp22.3 (the tip of the short arm). A male with EMWX who is Xg⁻ marries a woman who is Xg⁺ and they have eight daughters and one son, all of whom are normal for muscle function, the male being Xg⁺ and all the daughters being heterozygous at both the EMWX and Xg loci. Following is a table that lists three of the daughters with the phenotypes of their husbands and children.
Table showing husband’s phenotype, offspring sex, and offspring phenotype for three daughters and their children with Xg and EMWX traits.
For each of the offspring, indicate whether or not a crossover was required to produce the phenotypes that are given.

Verified step by step guidance
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Step 1: Understand the genetic linkage and inheritance pattern. The Xg blood group and EMWX genes are located on the X chromosome and are closely linked. This means that recombination (crossover) between these loci is less likely but still possible. The male parent contributes his X chromosome to all daughters and his Y chromosome to the son. The female parent contributes one of her two X chromosomes to each child.
Step 2: Analyze the parental genotypes. The male parent is Xg⁻ and has EMWX (recessive condition), so his X chromosome carries both the Xg⁻ allele and the EMWX allele. The female parent is Xg⁺ and does not have EMWX, so her X chromosomes carry the Xg⁺ allele and the normal allele for EMWX.
Step 3: Determine the offspring genotypes. All daughters inherit the X chromosome from their father (Xg⁻, EMWX) and one X chromosome from their mother (Xg⁺, normal). This makes them heterozygous at both loci (Xg⁺/Xg⁻ and EMWX/normal). The son inherits the Y chromosome from his father and one X chromosome from his mother, making him Xg⁺ and normal for muscle function.
Step 4: Evaluate the phenotypes of the daughters' husbands and children. For each child, compare the observed phenotype to the expected parental combinations. If the phenotype matches a parental combination, no crossover occurred. If the phenotype does not match a parental combination, a crossover event is required to explain the observed genotype.
Step 5: For each child, determine whether a crossover occurred by analyzing the inheritance of the Xg and EMWX alleles. If the alleles are inherited together as they were in the parent (e.g., Xg⁺ with normal or Xg⁻ with EMWX), no crossover occurred. If the alleles are inherited in a new combination (e.g., Xg⁺ with EMWX or Xg⁻ with normal), a crossover event occurred between the loci on the X chromosome.

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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 the pattern of inheritance for genes located on the X chromosome. Males have one X and one Y chromosome, while females have two X chromosomes. This means that X-linked traits can manifest differently in males and females, with males expressing recessive traits more readily due to having only one X chromosome. Understanding this concept is crucial for analyzing inheritance patterns in the context of the Xg blood group and EMWX.
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Crossover and Genetic Linkage

Crossover is a genetic process that occurs during meiosis, where homologous chromosomes exchange segments of DNA. This process can lead to genetic variation in offspring. Genetic linkage refers to the tendency of genes located close together on the same chromosome to be inherited together. In the context of the question, determining whether a crossover occurred is essential for understanding how the phenotypes of the offspring relate to the parental genotypes.
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Heterozygosity

Heterozygosity refers to the presence of two different alleles at a specific gene locus in an individual. In this case, the daughters are heterozygous at both the EMWX and Xg loci, meaning they carry one allele for each trait. This genetic diversity can influence the expression of traits and the potential for offspring to inherit various combinations of alleles, which is important for predicting the phenotypes of the next generation.
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Related Practice
Textbook Question

Drosophila melanogaster has one pair of sex chromosomes (XX or XY) and three pairs of autosomes, referred to as chromosomes II, III, and IV. A genetics student discovered a male fly with very short (sh) legs. Using this male, the student was able to establish a pure breeding stock of this mutant and found that it was recessive. She then incorporated the mutant into a stock containing the recessive gene black (b, body color located on chromosome II) and the recessive gene pink (p, eye color located on chromosome III). A female from the homozygous black, pink, short stock was then mated to a wild-type male. The F₁ males of this cross were all wild type and were then backcrossed to the homozygous b, p, sh females. The F₂ results appeared as shown in the following table.

The student repeated the experiment, making the reciprocal cross, with F₁ females backcrossed to homozygous b, p, sh males. She observed that 85 percent of the offspring fell into the given classes, but that 15 percent of the offspring were equally divided among b + p, b + +, + sh p, and + sh + phenotypic males and females. How can these results be explained, and what information can be derived from the data?

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Textbook Question

In Drosophila, a female fly is heterozygous for three mutations, Bar eyes (B), miniature wings (m), and ebony body (e). Note that Bar is a dominant mutation. The fly is crossed to a male with normal eyes, miniature wings, and ebony body. The results of the cross are as follows.

111 miniature; 101 Bar, ebony

29 wild type; 31 Bar, miniature, ebony

117 Bar; 35 ebony

26 Bar, miniature; 115 miniature, ebony

Interpret the results of this cross. If you conclude that linkage is involved between any of the genes, determine the map distance(s) between them.

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Textbook Question

The gene controlling the Xg blood group alleles (Xg⁺ and Xg⁻) and the gene controlling a newly described form of inherited recessive muscle weakness called episodic muscle weakness (EMWX) (Ryan et al., 1999) are closely linked on the X chromosome in humans at position Xp22.3 (the tip of the short arm). A male with EMWX who is Xg⁻ marries a woman who is Xg⁺ and they have eight daughters and one son, all of whom are normal for muscle function, the male being Xg⁺ and all the daughters being heterozygous at both the EMWX and Xg loci. Following is a table that lists three of the daughters with the phenotypes of their husbands and children. Create a pedigree that represents all data stated above and in the following table.


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Textbook Question

Because of the relatively high frequency of meiotic errors that lead to developmental abnormalities in humans, many research efforts have focused on identifying correlations between error frequency and chromosome morphology and behavior. Tease et al. (2002) studied human fetal oocytes of chromosomes 21, 18, and 13 using an immunocytological approach that allowed a direct estimate of the frequency and position of meiotic recombination. Below is a summary of information that compares recombination frequency with the frequency of trisomy for chromosomes 21, 18, and 13.

What conclusions can be drawn from these data in terms of recombination and nondisjunction frequencies? How might recombination frequencies influence trisomic frequencies?

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Textbook Question

Because of the relatively high frequency of meiotic errors that lead to developmental abnormalities in humans, many research efforts have focused on identifying correlations between error frequency and chromosome morphology and behavior. Tease et al. (2002) studied human fetal oocytes of chromosomes 21, 18, and 13 using an immunocytological approach that allowed a direct estimate of the frequency and position of meiotic recombination. Below is a summary of information [modified from Tease et al. (2002)] that compares recombination frequency with the frequency of trisomy for chromosomes 21, 18, and 13. (Note: You may want to read appropriate portions of Chapter 8 for descriptions of these trisomic conditions.)

Other studies indicate that the number of crossovers per oocyte is somewhat constant, and it has been suggested that positive chromosomal interference acts to spread out a limited number of crossovers among as many chromosomes as possible. Considering information in part (a), speculate on the selective advantage positive chromosomal interference might confer.

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