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Ch. 5 - Chromosome Mapping in Eukaryotes

Klug 12th Edition

Ch. 5 - Chromosome Mapping in Eukaryotes

Problem 33

Chapter 5, Problem 33

How would the results vary in cross (a) of Problem 32 if genes A and B were linked with no crossing over between them? How would the results of cross (a) vary if genes A and B were linked and 20 map units (mu) apart?

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1

Identify the genotypes of the parents in cross (a) from Problem 32.

Understand that if genes A and B are linked with no crossing over, they will be inherited together as a single unit.

Predict the gametes produced by the parents when genes A and B are linked with no crossing over.

Consider the scenario where genes A and B are linked and 20 map units apart, which implies a 20% chance of recombination between them.

Calculate the expected frequency of parental and recombinant gametes when genes A and B are 20 map units apart.

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Key Concepts

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

Gene Linkage

Gene linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together during meiosis. When genes are linked, they do not assort independently, which can affect the ratios of offspring phenotypes in genetic crosses. Understanding linkage is crucial for predicting the outcomes of crosses involving multiple genes.

Recommended video:

Chi Square and Linkage

Crossing Over

Crossing over is a process that occurs during meiosis where homologous chromosomes exchange segments of genetic material. This recombination can create new allele combinations, affecting the genetic variation in offspring. If genes A and B are linked with no crossing over, the parental combinations will be preserved, leading to fewer recombinant types in the offspring.

Recommended video:

Discovery of Crossing Over

Map Units and Genetic Distance

Map units, or centimorgans (cM), measure the distance between genes on a chromosome based on the frequency of recombination events. One map unit corresponds to a 1% chance of recombination occurring between two genes. If genes A and B are 20 map units apart, there is a 20% chance of recombination, which will increase the proportion of recombinant offspring compared to genes that are tightly linked.

Recommended video:

Mapping Genes

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. 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?

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.

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. For each of the offspring, indicate whether or not a crossover was required to produce the phenotypes that are given.

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.) Trisomic Mean Recombination Live-born Frequency Frequency Chromosome 21 1.23 1/700 Chromosome 18 2.36 1/3000–1/8000 Chromosome 13 2.50 1/5000–1/19,000 What conclusions can be drawn from these data in terms of recombination and nondisjunction frequencies? How might recombination frequencies influence trisomic frequencies?

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.) Trisomic Mean Recombination Live-born Frequency Frequency Chromosome 21 1.23 1/700 Chromosome 18 2.36 1/3000–1/8000 Chromosome 13 2.50 1/5000–1/19,000 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.