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Chapter 5: Problem 5

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: More crossing over occurs between distantly linked genes because there is a greater probability for the homologous chromosomes to align at various points and exchange genetic material. As the distance between two genes increases, there are more opportunities for chromosomal exchange without affecting one or both genes, leading to more frequent crossing over events. This process helps to ensure genetic diversity in the offspring.

Step by step solution

01

Introduction to Chromosomes and Genes

Chromosomes are long strands of DNA that contain many genes. Genes are segments of DNA that code for a specific protein or trait. During the formation of gametes (sperm and egg), chromosomes are arranged in homologous pairs, meaning each chromosome in a pair carries the same genes but may have different variations of those genes (alleles).
02

Meiosis and Crossing Over

Meiosis is the process of cell division that produces gametes with half the number of chromosomes of the parent cell. During the prophase I of meiosis, homologous chromosomes pair up, forming a structure called a bivalent or a tetrad. It is during this stage that crossing over occurs. Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes, leading to the recombination of genetic information.
03

Recombination Frequency and Genetic Distance

The frequency of recombination, or crossing over, is related to the distance between two genes on a chromosome. The greater the distance between two genes, the more likely it is for a crossover event to occur between them. This is because the homologous chromosomes have a higher probability of aligning and exchanging DNA at different points when the genes are far apart.
04

Why More Crossing Over Occurs Between Distantly Linked Genes

More crossing over occurs between two distantly linked genes because there is a greater probability for the homologous chromosomes to align at various points and exchange genetic material. When two genes are close together, there aren't many points where chromosomal exchange can occur without affecting one or both genes. As a result, crossing over events are less frequent between closely linked genes as compared to distantly linked genes. This is an essential process to ensure genetic diversity in the offspring.

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Most popular questions from this chapter

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.

In a series of two-point mapping crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed: $$\begin{array}{lc} p r-a d p & 29 \% \\ p r-v g & 13 \\ p r-c & 21 \\ p r-b & 6 \\ a d p-b & 35 \\ a d p-c & 8 \\ a d p-r g & 16 \\ v g-b & 19 \\ v g-c & 8 \\ c-b & 27 \end{array}$$ (a) Given that the adp gene is near the end of chromosome II (locus 83 ), construct a map of these genes. (b) In another set of experiments, a sixth gene, \(d\), was tested against \(b\) and \(p r\) $$\begin{array}{ll} d-b & 17 \% \\ d-p r & 23 \% \end{array}$$ Predict the results of two-point mapping between \(d\) and \(c, d\) and \(v g,\) and \(d\) and \(a d p\)

List some of the differences between a linkage map obtained by analyzing crossovers and a physical map obtained by sequencing the DNA.

Another cross in Drosophila involved the recessive, X-linked genes yellow \((y),\) white \((w),\) and \(c u t(c t) .\) A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. The \(\mathrm{F}_{1}\) females were wild type for all three traits, while the \(\mathrm{F}_{1}\) males expressed the yellow-body and white- eye traits. The cross was carried to an \(\mathrm{F}_{2}\) progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was constructed. (a) Diagram the genotypes of the \(\mathrm{F}_{1}\) parents. (b) Construct a map, assuming that white is at locus 1.5 on the X chromosome. (c) Were any double-crossover offspring expected? (d) Could the \(\mathrm{F}_{2}\) female offspring be used to construct the map? Why or why not?

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.
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