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Chapter 7: Problem 6

Why are double-crossover events expected less frequently than single-crossover events?

Short Answer

Expert verified
Short Answer: Double-crossover events are less frequent than single-crossover events mainly due to the increased complexity and precision required for two crossover events to occur in close proximity on a single chromosome during meiosis. Factors such as the stabilization of chromosomal alignment after an initial crossover event, difficulty in detecting events that result in recombinant chromosomes with the same parental allele combinations, and interference between crossover events in close proximity all contribute to the relative infrequency of double-crossover events.

Step by step solution

01

Understanding single-crossover events

Single-crossover events occur during meiosis when homologous chromosomes exchange genetic information in a process called recombination. During this process, parental chromosomes align and cross over at specific points called chiasmata. This exchange of genetic material generates new combinations of alleles on the resulting recombinant chromosomes.
02

Understanding double-crossover events

Double-crossover events, on the other hand, involve two separate crossover events in close proximity on the same chromosome pair. This generates recombinant chromosomes with even more unique combinations of alleles. However, double-crossover events are more complex and require a higher degree of precision in the alignment and interaction between the homologous chromosomes.
03

Reasons for the relative infrequency of double-crossover events

The main reason double-crossover events are less frequent than single-crossover events is the increased complexity and precision required for two crossover events to occur in close proximity. Some important factors affecting the frequency of double-crossover events include: 1. The closer the two crossover points are, the less likely it is for both events to occur during the same meiosis. This is because the initial crossover event can stabilize the chromosomal alignment, making it harder for a second crossover to take place nearby. 2. Double-crossover events can sometimes result in recombinant chromosomes with the same allele combinations as the parental chromosomes, essentially undoing the recombination and making it difficult to detect the event. 3. The probability of multiple crossover events occurring on a single chromosome pair decreases with the distance between them due to interference - the non-random distribution of crossovers along the chromosome.
04

Conclusion

Double-crossover events are expected less frequently than single-crossover events due to the increased complexity, precision required by the chromosomes' alignment and interaction, and interference between crossover events in close proximity. This makes double-crossover events less likely to occur and be detected in comparison to single-crossover events, which directly contribute to genetic diversity through recombination.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Understanding Single-Crossover Events
Single-crossover events are a cornerstone of genetic recombination, which occurs during a phase of cell division called meiosis. Think of homologous chromosomes as two similar but not identical storylines. During meiosis, these storylines come together, and a crossover event is akin to swapping pages between them, creating a new, mixed narrative. This exchange happens at points called chiasmata (plural for chiasma).

Imagine two dancers representing the chromosomes. They hold hands (align), spin around (cross over), and in the process, exchange colored bracelets (alleles). A single-crossover alters the pattern of these bracelets on their arms, leading to new combinations. This way, each resulting gamete — a sex cell like sperm or egg — has its unique blueprint, enhancing the diversity of offspring.
Exploring Genetic Recombination
Genetic recombination is nature's method to shuffle the deck of genes, ensuring each 'hand'—or new organism—is different. It's a natural copy-paste error with a beneficial twist. During meiosis, when homologous chromosomes swap sections of DNA, this shuffling generates a rich tapestry of genetic variations. Imagine every offspring as a unique painting, with recombination acting as an artist who blends colors in unpredictable, yet fascinating ways.

Thanks to recombination, siblings share traits with their parents, yet each stands out as a distinctive individual. Recombination also offers an evolutionary advantage by enabling populations to adapt to changing environments, as new gene combinations might be better suited for survival.
Homologous Chromosomes
Homologous chromosomes are like two sets of encyclopedias from different years. They cover the same topics (genes) but may present updated information (different alleles). These paired structures—one from each parent—line up during meiosis, like librarians arranging books by subject. Together, they hold the full story of an organism's genetic potential, ready to be remixed during recombination.

The dance of homologous chromosomes during meiosis ensures genetic diversity but within a framework of organized similarity. Just as newer editions update encyclopedias, crossover events between homologs update the genetic information, preparing the next generation for the world that awaits them.
Chiasma Formation
Chiasma formation is the physical manifestation of a crossover event. Envision a group of people exchanging gifts; a chiasma is the moment hands overlap and gifts switch owners. During meiosis, chromosomes form a cross-like structure called a chiasma at points where they exchange DNA.

These are not arbitrary events — cells control where and how chiasmata form. They ensure genetic exchange is precise, like matching puzzle pieces, so the new combination is viable. The chiasmata hold the chromosomes together briefly, like a well-timed handshake, before releasing them to set the stage for new genetic possibilities in the resulting cells.
Genetic Diversity Through Recombination
Genetic diversity is the backbone of a species' resilience. Just as having a diversified investment portfolio is key for financial stability, a broad genetic makeup enables populations to survive various challenges, from diseases to environmental changes. Recombination during meiosis is akin to constantly updating a community's skill set, by creating offspring with unique combinations of traits ready to face the world's unpredictability.

Without genetic diversity, a population might be vulnerable to a single threat. But with each single-crossover event serving as a new draft of the genetic blueprint, nature ensures that life is adaptable, robust, and perpetually innovative.

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Most popular questions from this chapter

Why is a 50 percent recovery of single-crossover products the upper limit, even when crossing over always occurs between two linked genes?

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

What three essential criteria must be met in order to execute a successful mapping cross?

Describe the cytological observation that suggests that crossing over occurs during the first meiotic prophase.

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 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 testross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained: colored, green 88 colored, yellow 12 colorless, green 8 colorless, yellow 92 Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise association of the two genes on the homologs (i.e., the arrangement).
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