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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

Expert verified
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

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.

The genes dumpy\((d p),\) clot \((c l),\) and apterous \((a p)\) are linked on chromosome II of Drosophila. In a series of two-point mapping crosses, the following genetic distances were determined. What is the sequence of the three genes?

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

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. 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.

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?
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