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Chapter 7: Problem 4

Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?

Short Answer

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Answer: The distance between genes on a chromosome affects the frequency of crossing over during meiosis because the likelihood of chiasmata formation increases with the distance between the genes. The farther apart two genes are, the more likely a chiasma will form between them, leading to a higher frequency of crossing over and greater genetic diversity in offspring. Conversely, when genes are positioned close together, they are less likely to undergo crossing over and may be inherited together as a group, known as genetic linkage.

Step by step solution

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1. Understanding Crossing Over and Meiosis

Crossing over is an important process that occurs during meiosis, a type of cell division responsible for the production of gametes (sperm and egg cells). During meiosis, homologous chromosomes exchange genetic material through a process called recombination, which can result in new combinations of genes in the offspring. This is a significant source of genetic diversity.
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2. The role of Chiasmata in Crossing Over

During the first stage of meiosis, known as prophase I, homologous chromosomes pair up and form structures called chiasmata, where genetic material is exchanged between the chromosomes. The frequency of chiasmata formation can vary depending on the total length of the chromosome and the distance between genes of interest.
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3. Recombination Frequency and Gene Distance

The recombination frequency between two genes on a chromosome is proportional to the distance between those genes. The farther apart two genes are, the more likely a chiasma will form between them, leading to a higher frequency of crossing over. This relationship is often expressed using a unit called a centimorgan (cM), which represents the relative distance between genes.
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4. The Impact of Gene Distance on Crossing Over

Since crossing over happens more frequently between distantly linked genes, we observe more recombination events for genes located further apart on the chromosome. This increased recombination can result in a greater variety of genetic combinations in offspring, giving them a greater chance of survival and adaptation to changing environments. Conversely, when genes are positioned close together on a chromosome, they are less likely to undergo crossing over and may be inherited together as a group, known as genetic linkage.
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5. Conclusion

In summary, more crossing over occurs between distantly linked genes because the likelihood of chiasmata formation increases with the distance between the genes. This increased recombination frequency ensures genetic diversity and adaptation in offspring.

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Most popular questions from this chapter

Review the Chapter Concepts list on page \(136 .\) Most of these center on 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.

Are sister chromatid exchanges effective in producing genetic variability in an individual? in the offspring of individuals?

Explain the meaning of the term interference.

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 physical exchange between chromatids? (d) How do we know that sister chromatids undergo recombination during mitosis?

In a plant, fruit color 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 results shown in the following table. Determine the location of the genes relative to one another and the genotypes of the two parental plants. $$\begin{array}{lcc} & & \text { Progeny } \\ \text { Phenotype } & \text { Plant A } & \text { Plant B } \\ \text { red, long } & 46 & 4 \\ \text { yellow, oval } & 44 & 6 \\ \text { red, oval } & 5 & 43 \\ \text { yellow, long } & 5 & 47 \\ \text { Total } & 100 & 100 \end{array}$$
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