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Chapter 5: Problem 2

Review the Chapter Concepts list on page \(138 .\) Most of these center around the process of crossing over between linked genes. Write a short essay that discusses how crossing over can be detected and how the resultant data provide the basis of chromosome mapping.

Short Answer

Expert verified
In a short essay format: Crossing over is a critical process that occurs during meiosis, where homologous chromosomes exchange genetic material, resulting in genetic recombination. Linked genes are those located relatively close together on the same chromosome and are often inherited together due to their proximity. To detect crossing over, the offspring resulting from specific genetic crosses are analyzed, and statistical analysis is used to determine the inheritance patterns of different genes. The recombination frequency, representing the ratio of recombinant offspring to the total number of offspring, measures the degree of linkage between genes. By comparing these frequencies for multiple pairs of linked genes, chromosome maps can be created, illustrating the linear order and approximate distances between genes on chromosomes. Chromosome maps, measured in centimorgans (cM), aid in understanding gene functions, inheritance patterns, and the evolutionary relationships among species. In summary, detecting crossing over involves studying inheritance patterns in offspring, calculating recombination frequencies, and using this data for chromosome mapping. This comprehensive approach contributes to a deeper understanding of an organism's genetic makeup and the relationships between genes.

Step by step solution

01

Introduction to Crossing Over and Linked Genes

Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, which leads to the generation of new combinations of alleles called genetic recombination. Linked genes are genes that are relatively close on the same chromosome and tend to be inherited together because they are less likely to be separated by crossing over.
02

Detecting Crossing Over

Crossing over can be detected by studying the offspring produced in genetic crosses between individuals with specific combinations of alleles for different genes. Through statistical analysis of these offspring, it is possible to determine if genes are linked or assort independently. If genes are found to assort independently, it means they are on different chromosomes or far apart on the same chromosome. However, if the genes appear to be inherited together more often than not, it suggests that they are linked and are relatively close on the same chromosome.
03

Recombination Frequency

The degree of linkage between genes can be quantified by calculating the recombination frequency, which is the proportion of recombinant offspring in a genetic cross. This can be calculated by dividing the number of recombinant offspring by the total number of offspring and is usually represented as a percentage. The recombination frequency provides an estimate of the likelihood of crossing over occurring between the linked genes.
04

Chromosome Mapping

The data obtained from recombination frequencies can be used to create chromosome maps, which are diagrams that show the linear order and approximate distances between linked genes on a chromosome. The distances are measured in units called centimorgans (cM), where 1 cM corresponds to a recombination frequency of 1%. By comparing recombination frequencies between pairs of linked genes, researchers can determine their relative distances and positions on the chromosome. This allows for the construction of genetic maps, which can be used to study gene functions, inheritance patterns, and evolutionary relationships among species.
05

Conclusion

In conclusion, crossing over can be detected by analyzing the inheritance patterns of offspring from genetic crosses, and the data obtained can be used to calculate recombination frequencies which provide a basis for chromosome mapping. This allows for the study of gene linkage, gene functions, inheritance patterns, and ultimately, a better understanding of the genetic makeup of organisms.

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Most popular questions from this chapter

Colored aleurone in the kernels of corn is due to the dominant allele \(R\). The recessive allele \(r,\) when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, \(Y\) and \(y\). The dominant \(Y\) allele results in green color, whereas the homozygous presence of the recessive \(y\) allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained: $$\begin{array}{lc} \text { colored, green } & 88 \\ \text { colored, yellow } & 12 \\ \text { colorless, green } & 8 \\ \text { colorless, yellow } & 92 \end{array}$$ Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.

What possible conclusions can be drawn from the observations that in male Drosophila, no crossing over occurs, and that during meiosis, synaptonemal complexes are not seen in males but are observed in females where crossing over occurs?

The gene controlling the Xg blood group alleles \(\left(X g^{+} \text {and } X g^{-}\right)\) and the gene controlling a newly described form of inherited recessive muscle weakness called episodic muscle weakness \((E M W X)\) (Ryan et al., 1999 ) are closely linked on the X chromosome in humans at position \(\mathrm{Xp} 22.3\) (the tip of the short arm \() .\) A male with EMWX who is \(\mathrm{Xg}^{-}\) marries a woman who is \(\mathrm{Xg}^{+}\), and they have eight daughters and one son, all of whom are normal for muscle function, the male being \(\mathrm{Xg}^{+}\) and all the daughters being heterozygous at both the \(E M W X\) and \(X g\) loci. Following is a table that lists three of the daughters with the phenotypes of their husbands and children. (a) Create a pedigree that represents all data stated above and in the following table. (b) For each of the offspring, indicate whether or not a crossover was required to produce the phenotypes that are given.

In this chapter, we focused on linkage, chromosomal mapping, and many associated phenomena. In the process, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How was it established experimentally that the frequency of recombination (crossing over) between two genes is related to the distance between them along the chromosome? (b) How do we know that specific genes are linked on a single chromosome, in contrast to being located on separate chromosomes? (c) How do we know that crossing over results from a physi- cal exchange between chromatids? (d) How do we know that sister chromatids undergo recombination during mitosis? (e) When designed matings cannot be conducted in an organism (for example, in humans), how do we learn that genes are linked, and how do we map them?

DNA markers have greatly enhanced the mapping of genes in humans. What are DNA markers, and what advantage do they confer?
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