Cite this Article - IdmcrackfreedownloadInfo

Cite this Article

Crossing Over Errors during meiosis

sciencing

What Is Crossing Over in Genetics?

By Marni Wolfe; Updated September 18, 2018

Crossing over occurs frequently during meiosis, the process forming sperm and egg cells.

sperm and egg 4 image by chrisharvey from Fotolia.com

A gene is defined as a unit of heredity that is passed along from one generation to the next, according to MedicineNet.com. Genes comprised short sequences of deoxyribonucleic acid, or DNA, and are arranged along chromosomes. Chromosomes are simply long sequences of DNA made up of many genes. Geneticists define “crossing over” as a process by which a pair of chromosomes aligns closely to each other and swap segments of DNA-containing genes during replication. Crossing over is also known as genetic recombination.

DNA Replication

There are two different DNA replication processes that occur in plants and animals. The first, mitosis, occurs when a cell replicates to form two copies of itself. The second replication process is called meiosis and occurs only in the creation of sperm or egg cells. Meiosis starts with one cell containing pairs of chromosomes and ends with two cells containing single copies of each chromosome. When a sperm and an egg combine to form a zygote, as an embryo is called in its early stages, they form chromosome pairs. Crossing over occurs during both mitosis and meiosis, though the frequency is much higher in meiosis, according to Science Gateway.

Crossing Over Biology: Alleles

The number of chromosomes an organism has varies among species; humans have 23 pairs, or 46 chromosomes in total. The pairs consist of two copies of each chromosome; however, the copies may not be identical as they often contain different alleles. An allele is an alternative form of a gene, according to Access Science. For example, a DNA segment on each chromosome section may code for eye color, although one chromosome may code for brown eyes and the other for blue eyes. Which eye color is expressed will depend on which gene is dominant. Crossing over occurs most often between different alleles coding for the same gene.

Sciencing Video Vault

Mechanics of Crossing Over

Chromosomes normally exist in a compacted, super-coiled state. During mitosis and meiosis, they must be unwound to allow for replication to occur. This happens when enzymes make breaks at several points along the chromosomes, allowing them to unwind and be copied. Following replication, another set of enzymes reattaches the broken fragments of DNA. Chromosome pairs line up in close proximity to one another during these processes. In the unwound and fragmented phase, DNA segments of equal sizes may be swapped and then re-glued, forming a chromosome with a different combination of alleles than it started with, according to Science Gateway.

Frequency of Crossing Over: Meiosis

According to the Origins of Sex, the frequency of crossing over within an individual’s genome is not consistent. There are hot spots, so named as they cross over with greater frequency than average, as well as cold spots that rarely recombine. In humans, a difference has also been noted among sexes, with the average male having crossover events approximately 57 times during meiosis, while in females it is estimated to occur 75 times during the same phase.

Genetic Diversity

A benefit of crossing over is that it maintains genetic diversity within a population, allowing for millions of different genetic combinations to be passed from parents to offspring. Genetic variability is very important to the long-term survival of a species. Without crossing over, meiosis and mitosis cannot produce the genetic diversity necessary for populations to survive adverse conditions, such as drought or disease.

References

  • MedicineNet: Definition of a gene
  • Science Gateway: Meiosis and Genetic Recombination
  • Access Science: Crossing Over (Genetics)
  • BookRags: Crossing Over

About the Author

Marni Wolfe began writing professionally in 2009. She has been published in the scientific journals “Brain Research” and “Endocrine,” and in various online publications. Wolfe worked for more than 10 years in the pharmaceutical and nutraceutical industries before leaving to write about health and science. Wolfe holds a Bachelor of Science in genetics from the University of Western Ontario.

More Articles

  • How to Find Recombinant Offspring
  • Physical Structure of Chromosomes
  • What Are the Steps in Meiosis That Increase Variability?
  • The Difference Between Orthologous & Paralogous Genes
  • What Is Rearrangement in Meiosis?
  • What Is the Role of the Y Chromosome in Sex Determination in Humans?
A tool to create a citation to reference this article Cite this Article

Choose Citation Style

Wolfe, Marni. “What Is Crossing Over in Genetics?” Sciencing, https://sciencing.com/crossing-over-genetics-6628252.html. 18 September 2018.
Wolfe, Marni. (2018, September 18). What Is Crossing Over in Genetics? Sciencing. Retrieved from https://sciencing.com/crossing-over-genetics-6628252.html
Wolfe, Marni. “What Is Crossing Over in Genetics?” last modified September 18, 2018. https://sciencing.com/crossing-over-genetics-6628252.html

Note: Depending on which text editor you’re pasting into, you might have to add the italics to the site name.

Related Content

  • This 9-Course Training will Teach You Java, Python, and More

  • What Is a Homologous Allele?

  • How to Calculate Centimorgans

  • Why Are There 61 Anticodons?

Copyright 2018 Leaf Group Ltd. All Rights Reserved. // Leaf Group Education
AdChoices

  • About Us
  • Terms of Use
  • Privacy Policy
  • Copyright Policy

Principles of Biology

Adapted by Dr. Angela Hodgson


39 Meiosis and Sexual Reproduction

Mechanisms that Increase Genetic Variation

The evolution of life on planet Earth is a dynamic process that is a direct result of genetic variation. Mutations in an organism’s DNA produce changes in genes. When these changes occur, they may provide either a beneficial phenotypic change or a nonbeneficial one. Over time, processes such as natural selection favor organisms with mutations that are beneficial. If these changes provide enough of a benefit in the sexual reproduction of an organism, the mutations will become more prevalent in the population as a whole.

This photograph shows a sea anemone, which has pale green noodle-like tentacles with purple ends. The sea anemone lives in an ocean environment and reproduces asexually. Without meiosis, the opportunity for introducing genetic variation into a population is very limited.

Figure 12: The sea anemone (order Actiniaria) reproduces via asexual reproduction.
In organisms like the sea anemone that reproduce without meiosis, the opportunity for introducing genetic variation into a population is very limited.
Courtesy of Laszlo Ilyes. Some rights reserved. View Terms of Use

One significant advantage for genetic variation produced by sexual reproduction over the consistency of asexual reproduction is seen with viral disease. In many species that produce asexually, such as the sea anemone, a single virus may have devastating effects on a population (Figure 12). In sexually reproducing organisms with a varied gene pool, a virus will likely have a less detrimental effect because some of the genetic variations that arise may provide some degree of resistance to the virus. In humans, for example, gene variants that confer resistance to viruses have probably been favored by natural selection. For example, a study in 2010 from the University of Milan analyzed 52 populations worldwide and identified 139 human genes that modulate susceptibility to viral infections.

Independent assortment produces new combinations of alleles.

How is genetic variation generated? There are several points during sexual reproduction at which genetic variation can increase. In meiosis I, crossing over during prophase and independent assortment during anaphase creates sets of chromosomes with new combinations of alleles. Genetic variation is also introduced by random fertilization of the gametes produced by meiosis. Any of the genetically unique sperm generated by a male may fertilize the genetically unique egg produced by a female.
During metaphase I, the homologous pairs of chromosomes are aligned along the metaphase plate. The orientation of the homologous pairs is random and is different for every cell that undergoes meiosis. In humans, this arrangement involves 23 different tetrads. Each tetrad contains one maternal and one paternal pair of sister chromatids. In one cell, for example, the tetrad corresponding to human chromosome 1 may align so that its paternal sister chromatids face toward one pole while the maternal sister chromatids are facing toward the other pole. And there is a 50% chance that the opposite orientation will occur during metaphase I in another cell.

Bioskill

Calculating the Probability of Genetic Variation

The number of different gamete types resulting from independent assortment of the homologs is calculated using the formula 2n where n = the number of chromosomes in a haploid cell of a given species. How many different combinations are possible in a fruit fly with 4 different chromosomes and a human with 23 different chromosomes?
In the fruit fly n = 4, so there are 24 different assortments possible = 2 x 2 x 2 x 2 = 16 possible chromosome combinations in the gametes.
In humans, however, where n is a much larger 23, there are far more possibilities. In fact, there are almost 8.4 million (223) different ways that maternal and paternal chromosomes could be assorted in gametes. This results in a massive amount of variation in the gametes that form at the end of meiosis II.

Bioskill

Crossing-over involves switching sections of DNA between two non-sister chromatids.

Recombinant chromosomes are made of DNA that has been randomly transferred between two non-sister chromatids of two homologous chromosomes. Crossing-over occurs early during prophase I while the homologous chromosome pairs begin to loosely bind to each other. While they are connected, the two non-sister chromatids switch sections of DNA at specific points. Usually one to three crossover events occur per chromosome depending on the size and species (Figure 13).

Figure 13: Crossing over during meiosis.

Click on the forward and back arrows to step through meiosis, selecting ‘with recombination’ or ‘without recombination’ to view the effect of recombination on the assortment of genetic information.
© 2011 Nature Education All rights reserved. View Terms of Use

Transcript

Random fertilization increases genetic diversity.

When a male gamete and a female gamete finally meet, each is the result of an immense number of genetic possibilities created during independent assortment and crossing over. Human diploid cells have 23 pairs of chromosomes. Because of independent assortment during meiosis I, there are 223, or 8.4 million possible gametes that may be created even if crossing over didn’t occur. In humans, there are about 2 to 3 crossovers per chromosome, and different crossovers occur in each meiotic division, so there is the potential to produce an enormous number of unique gametes.

Errors during meiosis.

The process of meiosis is highly regulated. Many different molecules and proteins are responsible for regulating the steps of meiosis as they occur. If any mistakes occur during the replication of chromosomes, it may have drastically detrimental effects on the resulting offspring. Errors may also occur during chromosome segregation. The term nondisjunction is used to describe the abnormal separation of chromosomes, resulting in the wrong number of chromosomes going to each gamete. There are many different human disorders that are the direct result of the incorrect separation and movement of homologous chromosome pairs. Disorders that are a direct result of nondisjunction include Down syndrome, Klinefelter syndrome, and XYY syndrome.

 

  • Sign In
  • Tutor Bios
  • Test Prep

    HIGH SCHOOL
    • ACT Tutoring
    • SAT Tutoring
    • PSAT Tutoring
    • ASPIRE Tutoring
    • SHSAT Tutoring
    • STAAR Tutoring
    GRADUATE SCHOOL
    • MCAT Tutoring
    • GRE Tutoring
    • LSAT Tutoring
    • GMAT Tutoring
    K-8
    • AIMS Tutoring
    • HSPT Tutoring
    • ISEE Tutoring
    • ISAT Tutoring
    • SSAT Tutoring
    • STAAR Tutoring
    Search 50+ Tests


    Loading Page

  • Academic Tutoring

    math tutoring
    • Algebra
    • Calculus
    • Elementary Math
    • Geometry
    • Pre-Calculus
    • Statistics
    • Trigonometry
    science tutoring
    • Anatomy
    • Biology
    • Chemistry
    • Physical Chemistry
    • Physics
    foreign languages
    • French
    • German
    • Latin
    • Mandarin Chinese
    • Spanish
    elementary tutoring
    • Reading
    • Phonics
    • Elementary Math
    other
    • Accounting
    • Computer Science
    • Economics
    • English
    • Finance
    • History
    • Writing
    • Summer
    Search 350+ Subjects


    Loading Page

  • About

    • Video Overview
    • Tutor Selection Process
    • Online Tutoring
    • Mobile Tutoring
    • Instant Tutoring
    • How We Operate
    • Our Guarantee
    • Impact of Tutoring
    • Video Reviews & Testimonials
    • Media Coverage
    • About Varsity Tutors

Call Now to Set Up Tutoring:

(888) 888-0446

High School Biology : Understanding Crossing Over

Study concepts, example questions & explanations for High School Biology

varsity tutors app store

varsity tutors android store

CREATE AN ACCOUNT
Create Tests & Flashcards

Home


Embed







All High School Biology Resources

10 Diagnostic Tests

248 Practice Tests

Question of the Day

Flashcards

Learn by Concept

Example Questions

High School Biology Help »
Cell Biology »
Cell Division »
Meiosis »
Understanding Crossing Over

Example Question #1 : Understanding Crossing Over

Crossing over occurs during which stage of meiosis?

Possible Answers:

Metaphase I

Prophase I

Prophase II

Anaphase I

Correct answer:

Prophase I

Explanation:

During prophase I homologous chromosomes will line up with one another, forming tetrads. During this lining up, DNA sequences can be exchanged between the homologous chromosomes. This type of genetic recombination is called crossing over, and allows the daughter cells of meiosis to be genetically unique from one another.

Crossing over can only occur between homologous chromosomes. Cells become haploid after meiosis I, and can no longer perform crossing over.

Report an Error

Example Question #1 : Understanding Crossing Over

What is the evolutionary purpose of cells that undergo crossing over?

Possible Answers:

To keep the redundancy of the cell high

To increase genetic diversity

To produce two cells instead of one

To keep mutations from forming

To produce gametes that are genetically identical

Correct answer:

To increase genetic diversity

Explanation:

Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material. Gametes gain the ability to be genetically different from their neighboring gametes after crossing over occurs. This allows for genetic diversity, which will help cells participate in survival of the fittest and evolution.

Report an Error

Example Question #1 : Understanding Crossing Over

During which step of cell division does crossing over occur?

Possible Answers:

Prophase I

Prophase II

Metaphase II

Metaphase I

Correct answer:

Prophase I

Explanation:

When chromatids “cross over,” homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I.

By meiosis II, only sister chromatids remain and homologous chromosomes have been moved to separate cells. Recall that the point of crossing over is to increase genetic diversity. If crossing over did not occur until sometime during meiosis II, sister chromatids, which are identical, would be exchanging alleles. Since these chromatids are identical, this swap of material would not actually change the alleles of the chromatids.

Report an Error

Example Question #1 : Understanding Crossing Over

What structures exchange genetic material during crossing over?

Possible Answers:

Sister chromatids

Non-homologous chromosomes

Egg and sperm chromosomes

Nonsister chromatids

Correct answer:

Nonsister chromatids

Explanation:

During crossing over, homologous chromosomes come together in order to form a tetrad. This close contact allows the nonsister chromatids from homolgous chromosomes to attach to one another and exchange nucleotide sequences. The word “nonsister” implies that the chromatids have the same genes, but are not exact copies of one another, as they come from separate chromosomes.

Report an Error

Example Question #1 : Understanding Crossing Over

Crossover of homologous chromosomes in meiosis occurs during which phase?

Possible Answers:

Anaphase I of meiosis

Prophase I of meiosis

Anaphase II of meiosis

Prophase II of meiosis

Correct answer:

Prophase I of meiosis

Explanation:

The crossing over of homologous chromosomes occurs in prophase I of meiosis. Prophase I of meiosis is characterized by the lining up of homologous chromosomes close together to form a structure known as a tetrad. A tetrad is composed of four chromatids.

Anaphase I is marked by the separation of homologous chromosomes, whereas in anaphase II there is the separation of sister chromatids. In anaphase I sister chromatids are still intact and connected at the centromere. Prophase II is similar to prophase in mitosis in that there is the break down of the nuclear membrane and the formation of spindle fibers in preparation for the separation of sister chromatids.

Report an Error

Example Question #1 : Understanding Crossing Over

During crossing over, two homologous chromosomes pair to form which of the following choices?

Possible Answers:

Base Pair

Mitotic Bond

Chromatid

Tetrad

None of these

Correct answer:

Tetrad

Explanation:

The tetrad, which divides into non-sister chromatids, exchanges genetic information in order to make the genetic pool more variant, and result in combinations of phenotypic traits that can occur outside of linked genotypic coding.

Report an Error

Example Question #211 : Cell Biology

Chromosomal crossover occurs in which phase of meiosis?

Possible Answers:

Anaphase II

Anaphase I

Prophase II

Metaphase I

Prophase I

Correct answer:

Prophase I

Explanation:

During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover. The exchange occurs in segments over a small region of homology (similarity in sequence, ie., the same alleles). The new combinations of DNA created during crossover provide a significant source of genetic variation.  

Report an Error

Example Question #2 : Understanding Crossing Over

Crossing over is a phenomenon that happens during Meiosis I in the attempt to create genetic diversity. Crossing over typically occurs between which of the following structures?

Possible Answers:

Homologous chromosomes

Tetrads

Sister chromatids

Chromatin

Correct answer:

Homologous chromosomes

Explanation:

Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information.

Report an Error

Example Question #1 : Understanding Crossing Over

When in meiosis does crossing over occur?

Possible Answers:

Telophase I

Prophase I

Metaphase I

Anaphase I

Interphase

Correct answer:

Prophase I

Explanation:

Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.

Report an Error
Copyright Notice
Display vt optimized

View Tutors

Anna
Certified Tutor
Kansas State University, Bachelor of Science, Microbiology. University of Minnesota-Twin Cities, Master of Science, Biology, …

Display vt optimized

View Tutors

Cristina
Certified Tutor
Rutgers University-New Brunswick, Bachelors, Microbial Biotechnology.

Display vt optimized

View Tutors

Nneka
Certified Tutor
Temple University, Bachelor of Science, Neuroscience.

All High School Biology Resources

10 Diagnostic Tests

248 Practice Tests

Question of the Day

Flashcards

Learn by Concept

Report an issue with this question

If you’ve found an issue with this question, please let us know. With the help of the community we can continue to
improve our educational resources.

DMCA Complaint

If you believe that content available by means of the Website (as defined in our Terms of Service) infringes one
or more of your copyrights, please notify us by providing a written notice (“Infringement Notice”) containing
the
information described below to the designated agent listed below. If Varsity Tutors takes action in response to
an
Infringement Notice, it will make a good faith attempt to contact the party that made such content available by
means of the most recent email address, if any, provided by such party to Varsity Tutors.

Your Infringement Notice may be forwarded to the party that made the content available or to third parties such
as
ChillingEffects.org.

Please be advised that you will be liable for damages (including costs and attorneys’ fees) if you materially
misrepresent that a product or activity is infringing your copyrights. Thus, if you are not sure content located
on or linked-to by the Website infringes your copyright, you should consider first contacting an attorney.

Please follow these steps to file a notice:

You must include the following:

A physical or electronic signature of the copyright owner or a person authorized to act on their behalf;
An identification of the copyright claimed to have been infringed;
A description of the nature and exact location of the content that you claim to infringe your copyright, in \
sufficient detail to permit Varsity Tutors to find and positively identify that content; for example we require
a
link to the specific question (not just the name of the question) that contains the content and a description of
which specific portion of the question – an image, a link, the text, etc – your complaint refers to;
Your name, address, telephone number and email address; and
A statement by you: (a) that you believe in good faith that the use of the content that you claim to infringe
your copyright is not authorized by law, or by the copyright owner or such owner’s agent; (b) that all of the
information contained in your Infringement Notice is accurate, and (c) under penalty of perjury, that you are
either the copyright owner or a person authorized to act on their behalf.

Send your complaint to our designated agent at:

Charles Cohn
Varsity Tutors LLC
101 S. Hanley Rd, Suite 300
St. Louis, MO 63105

Or fill out the form below:

 

Learning Tools by Varsity Tutors

FIND THE BEST TUTORS

Track your scores, create tests, and take your learning to the next level!

Create Account – It’s FREE

Our Guarantee
Online Tutoring
Mobile Tutoring App
Instant Tutoring
Reviews & Testimonials

How We Operate

Press Coverage

Top Subjects

ACT Tutors
Algebra Tutors
Biology Tutors
Calculus Tutors
Chemistry Tutors
French Tutors

 

Geometry Tutors
German Tutors
GMAT Tutors
Grammar Tutors
GRE Tutors
ISEE Tutors

 

LSAT Tutors
MCAT Tutors
Math Tutors
Physics Tutors
PSAT Tutors
Reading Tutors

 

SAT Tutors
Spanish Tutors
SSAT Tutors
Statistics Tutors
Test Prep Tutors
Writing Tutors

Top Locations

Atlanta Tutoring
Boston Tutoring
Brooklyn Tutoring
Chicago Tutoring
Dallas Tutoring
Denver Tutoring

 

Houston Tutoring
Kansas City Tutoring
Los Angeles Tutoring
Miami Tutoring
New York City Tutoring
Philadelphia Tutoring

 

Phoenix Tutoring
San Diego Tutoring
San Francisco Tutoring
Seattle Tutoring
St. Louis Tutoring
Washington DC Tutoring

Our Company

About Us
Honor Code
Partnerships

Free Resources

Tests, Problems & Flashcards
Classroom Assessment Tools
Mobile Applications

College Scholarship
Admissions Blog
Test Prep Books
Web English Teacher
Early America
Hotmath
Aplusmath
Jobs

Tutoring Jobs
Careers

Varsity Tutors. © 2007-2018 All Rights Reserved

Privacy Policy

Terms of Use

Sitemap

Sign In

 Provide Feedback

disclaimer

Clicky

About the Author: admin