Crossing over occurs during prophase I of meiosis in eukaryotes. It involves the exchange of genetic material between paired homologous chromosomes, resulting in genetic variation. Tracking crossing over helped scientists determine that genes located farther apart on a chromosome have a greater chance of being exchanged than genes closer together, establishing the concept of genetic linkage. Crossing over ensures the combination of maternal and paternal genes in offspring.

1. Crossing Over
Genetic Recombination

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Overview
• Who, What, When, Where, Why
• Basics
• Example
• Summary

3. Who Does Crossing-Over
Effect?
Eukaryotes
Limited in Prokaryotes

4. What is Crossing Over?
The exchange of chromosomal
segments between two non-
sister chromatids

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Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml
Crossing Over

6. 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

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

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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
From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over

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

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

12. The History of Crossing
Over

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

14. Crossing Over Basics

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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

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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

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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