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Chapter 5: 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: More crossing over occurs between two distantly linked genes than between two closely linked genes on the same chromosome because the formation of chiasmata is more likely between distant genes. The increased likelihood of chiasmata formation between distant genes results in a higher frequency of crossing-over events and genetic recombination, ultimately contributing to greater genetic diversity in the offspring.

Step by step solution

01

Understanding Crossing Over

Crossing over is a process that occurs during meiosis, where homologous chromosomes exchange genetic material. This process increases genetic diversity within the offspring, as it creates new combinations of alleles in the resulting gametes. Crossing over occurs during the prophase I stage of meiosis, where homologous chromosomes pair up and form structures known as chiasmata.
02

Proximity and Crossing Over

The probability of crossing over between two genes is influenced by their physical distance on a chromosome. If two genes are very close or nearby, it is less likely that crossing over will occur between them. This is because the chiasmata, which are the points of exchange, are less likely to form between two closely linked genes. As a result, their alleles will tend to be inherited together, also referred to as genetic linkage.
03

Greater Distance and Increased Frequency of Crossing Over

Conversely, when two genes are further apart on the chromosome, there is a greater likelihood that one or more chiasmata will form between them during prophase I of meiosis. This increases the chance of genetic recombination and crossing over between the two genes, resulting in a greater diversity of alleles in the offspring. The frequency of crossing-over events between two genes is proportional to the distance separating them.
04

Conclusion

In summary, more crossing over occurs between two distantly linked genes than between two closely linked genes on the same chromosome because of the increased likelihood of chiasmata formation between distant genes. This increased probability of chiasmata formation results in a higher frequency of crossing-over events and genetic recombination, ultimately contributing to greater genetic diversity in the offspring.

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Most popular questions from this chapter

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 \(a d p\)

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?

Phenotypically wild \(\mathrm{F}_{1}\) female Drosophila, whose mothers had light eyes \((l t)\) and fathers had straw \((s t w)\) bristles, produced the following offspring when crossed with homozygous lightstraw males: $$\begin{array}{lc} \text { Phenotype } & \text { Number } \\ \text { light-straw } & 22 \\ \text { wild } & 18 \\ \text { light } & 990 \\ \text { straw } & 970 \\ \text { Total } &{2000} \end{array}$$ Compute the map distance between the light and straw loci.

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?

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.
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