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CROSSING OVER
GENETIC RECOMBINATION
WHAT IS CROSSING OVER?
THE EXCHANGE OF CHROMOSOMAL
SEGMENTS BETWEEN TWO NON- SISTER
CHROMATIDS
WHEN DOES IT HAPPEN?
• Crossing over only happens during
Prophase 1 of Meiosis
• This ensures the creation of 4
genetically different gametes.
WHERE DOES CROSSING OVER OCCUR?
Genetic swapping occurs between paired
homologous chromosomes in our sex cells Egg and Sperm
EGG AND SPERM = GAMETES
• Gametes have half the number of
Chromosomes (23 vs. 46)
• Each gamete is unique due to crossing over
• Gametes are produced through meiosis
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21 Apr 2002
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Homologous Chromosomes Exchanging DNA
by Crossing Over
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WHY DOES CROSSING OVER OCCUR?
● To provide genetic variation in meiosis,
thus in all species.
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
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CROSSING OVER BASICS
CROSSING OVER BASICS
● Nonsister chromatids join at a chiasma (plural,
chiasmata), the site of attachment and crossing over
● Corresponding amounts of genetic material are
exchanged between maternal and paternal (nonsister)
chromatids
CROSSING OVER BASICS
• Occurs at One or More Points
Along Adjacent Homologues
during Synapsis
• Synapsis is the pairing of two
homologous chromosomes.
• Points contact each other
• DNA is Exchanged
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Tetrad
Chiasma
Centromere
Coat-color
genes
C
Eye-color
genes
E
c
e
1
Breakage of homologous chromatids
C
E
c
e
2
C
Tetrad
(homologous pair of
chromosomes in synapsis)
Joining of homologous chromatids
E
Chiasma
c
e
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
e
4
C
Separation of chromatids at
anaphase II and
completion of meiosis
E
Parental type of chromosome
C
e
c
E
c
e
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
Coat-color
genes
C
Eye-color
genes
E
c
e
1
Breakage of homologous chromatids
C
E
c
e
2
C
Tetrad
(homologous pair of
chromosomes in synapsis)
Joining of homologous chromatids
E
Chiasma
c
e
C
E
Chiasma
e
c
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
4
C
e
Separation of chromatids at
anaphase II and
completion of meiosis
E
Parental type of chromosome
C
e
c
E
c
e
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
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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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CROSSING OVER BASICS
• 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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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
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