Jack Cassidy, Jessica Matthews, Keith Murphy, Mike Nickelsburg
Genetic Recombination http://library.thinkquest.org/19037/genome3.html

Overview
• Who, What, When, Where, Why
• Basics
• Example
• Summary
21 Apr 2002 2

Who Does Crossing-Over
Effect?
Eukaryotes
Limited in Prokaryotes

What is Crossing Over?
The exchange of chromosomal segments between two nonsister chromatids

Crossing Over
21 Apr 2002
Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml
5

When Does it Happen?
During Prophase I of Meiosis
Remember the differences between
Mitosis and Meiosis?
Meiosis increases genetic diversity in a species
Mitosis creates genetically identical daughter cells

Where does Crossing Over
Occur?
Genetic swapping occurs between paired homologous chromosomes in our sex cells—
The Egg and Sperm

Egg and Sperm
• Chromosomes pair
• Chromosomes may swap genetic material
• New genetic material has been acquired from the other homologue
• NOTE : Genes that have a tendency to remain together during crossing over are said to be linked —we will talk about this concept during Crossing Over Basics
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Homologous Chromosomes Exchanging DNA by Crossing Over
21 Apr 2002
From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over
9

Why Does Crossing Over
Occur?
To provide genetic variation during meiosis

Its Why You and I Don’t
Look Alike
Crossing Over ensures a combination of the maternal and paternal genes we inherited

The History of Crossing
Over

Linkage
• Gregor Mendel, 1823-1884
– Patterns of Inheritance
• Carl Correns, 1900s
– Gene Linkage
• Alfred Sturtevant http://zygote.swarthmore.edu/cleave1.html
http://www.library.villanova.edu/html2/blueprints/feb01index.html
http://www.oeaw.ac.at/biblio/Archiv/Mendel/mendel.html
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Crossing Over Basics

Crossing Over Basics
• Occurs at One or More Points Along
Adjacent Homologues during Synapsis
• Points contact each other
• DNA is Exchanged http://waynesword.palomar.edu/images/cross3.jpg
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Ref: Access Excellence http://www.accessexcellence.org/AB/GG/crossing.html
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Crossing Over Basics
• Gene Mapping
– Tracking crossing over helps determine where genes are located on the chromosome
– Genes that are far apart have a GREATER chance of crossing over
– Genes that are closer have a LESS LIKELY chance of crossing over
• Genes that stay together are said to be LINKED
• One gene can be identified as a MARKER that can infer the presence of the other gene
– This can be used in identifying disease predisposition
21 Apr 2002 17

Crossing Over Basics
• Gene Mapping
– Tracking crossing over helps determine where genes are located on the chromosome
– Genes that are far apart have a
GREATER chance of crossing over
– Genes that are closer have a LESS
LIKELY chance of crossing over
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Genes get shuffled when chromosomes exchange pieces
Watch an animation of crossing over with an explanation of how the concept was discovered at http://www.dnaftb.org/dnaftb/11/concept/index.html
This web site was produced by the
Dolan DNA Learning Center, Cold Spring Harbor Laboratory
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Summary
• Who, What, When, Where, Why
• History
• Basics
• Example
• Summary
From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over
20 21 Apr 2002