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Ch. 5 - Chromosome Mapping in Eukaryotes
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 29
Chapter 5, Problem 29

A female of genotype a b c + + + produces 100 meiotic tetrads. Of these, 68 show no crossover events. Of the remaining 32, 20 show a crossover between a and b, 10 show a crossover between b and c, and 2 show a double crossover between a and b and between b and c. Of the 400 gametes produced, how many of each of the 8 different genotypes will be produced? Assuming the order a–b–c and the allele arrangement previously shown, what is the map distance between these loci?Genetic mapping diagram showing loci a, b, and c with crossover events.

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Identify the parental and recombinant genotypes based on crossover events: Parental (no crossover) = 68 tetrads, Single crossover between a and b = 20 tetrads, Single crossover between b and c = 10 tetrads, Double crossover = 2 tetrads.
Calculate the number of gametes for each genotype: Each tetrad produces 4 gametes, so multiply the number of tetrads by 4 to get the number of gametes for each crossover type.
Determine the genotypes for each crossover type: Parental = a+b+c+ and abc, Single crossover a-b = a+bc and ab+c+, Single crossover b-c = a+bc+ and ab+c, Double crossover = a+bc and ab+c+.
Calculate the map distances: Map distance between a and b = (Number of single crossovers between a and b + Number of double crossovers) / Total number of tetrads * 100, Map distance between b and c = (Number of single crossovers between b and c + Number of double crossovers) / Total number of tetrads * 100.
Sum the map distances to find the total map distance between a and c, considering the double crossovers are counted twice.

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Key Concepts

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

Meiotic Tetrads

Meiotic tetrads are structures formed during meiosis, consisting of four chromatids from two homologous chromosomes. They are crucial for understanding genetic recombination, as they allow for the exchange of genetic material between homologous chromosomes during crossover events. The number of tetrads and the types of crossover events that occur can significantly influence the genetic variation in the resulting gametes.
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Crossover Events

Crossover events occur during meiosis when homologous chromosomes exchange segments of genetic material. This process increases genetic diversity by producing new allele combinations in gametes. The question specifies different types of crossover events, including single and double crossovers, which affect the frequency of various genotypes in the offspring and are essential for calculating genetic map distances.
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Genetic Map Distance

Genetic map distance is a measure of the relative distance between genes on a chromosome, typically expressed in centimorgans (cM). It is calculated based on the frequency of recombination events between genes; a higher frequency indicates a greater distance. Understanding how to calculate map distances is vital for interpreting genetic linkage and predicting the inheritance patterns of traits.
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Related Practice
Textbook Question
In a certain plant, fruit is either red or yellow, and fruit shape is either oval or long. Red and oval are the dominant traits. Two plants, both heterozygous for these traits, were testcrossed, with the following results. Determine the location of the genes relative to one another and the genotypes of the two parental plants.
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Textbook Question
Two plants in a cross were each heterozygous for two gene pairs (Ab/aB) whose loci are linked and 25 mu apart. Assuming that crossing over occurs during the formation of both male and female gametes and that the A and B alleles are dominant, determine the phenotypic ratio of their offspring.
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Textbook Question
A number of human–mouse somatic cell hybrid clones were examined for the expression of specific human genes and the presence of human chromosomes. The results are summarized in the following table. Assign each gene to the chromosome on which it is located.
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Textbook Question
In laboratory class, a genetics student was assigned to study an unknown mutation in Drosophila that had a whitish eye. He crossed females from his true-breeding mutant stock to wild-type (brick-red-eyed) males, recovering all wild-type F₁ flies. In the F₂ generation, the following offspring were recovered in the following proportions: wild type 5/8 bright red 1/8 brown eye 1/8 white eye 1/8 The student was stumped until the instructor suggested that perhaps the whitish eye in the original stock was the result of homozygosity for a mutation causing brown eyes and a mutation causing bright red eyes, illustrating gene interaction (see Chapter 4). After much thought, the student was able to analyze the data, explain the results, and learn several things about the location of the two genes relative to one another. One key to his understanding was that crossing over occurs in Drosophila females but not in males. Based on his analysis, what did the student learn about the two genes?
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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. No other phenotypes were observed. Wild Pink* Black, Black, Pink, Short* Short Females 63 58 55 69 Males 59 65 51 60 *Other trait or traits are wild type. Based on these results, the student was able to assign short to a linkage group (a chromosome). Which one was it? Include your step-by-step reasoning.
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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. No other phenotypes were observed. Wild Pink* Black, Black, Pink, Short* Short Females 63 58 55 69 Males 59 65 51 60 *Other trait or traits are wild type. The student repeated the experiment, making the reciprocal cross, 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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