1. GENETIC RECOMBINATION
CROSSING OVER
GENETIC RECOMBINATION 1

2. WHAT IS CROSSING OVER?
THE EXCHANGE OF CHROMOSOMAL SEGMENTS BETWEEN TWO NON- SISTER CHROMATIDS

3. WHEN DOES IT HAPPEN?
Crossing over only happens during Prophase 1 of Meiosis
This ensures the creation of 4 genetically different gametes.

4. WHERE DOES CROSSING OVER OCCUR?
Genetic swapping occurs between paired homologous chromosomes in our sex cells - Egg and Sperm

5. Gametes have half the number of Chromosomes (23 vs. 46)
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 5

6.   
21 Apr 2002 6

7. Homologous Chromosomes Exchanging DNA
by Crossing Over 7

8. WHY DOES CROSSING OVER OCCUR?
To provide genetic variation in meiosis, thus in all species.

9. ITS WHY YOU AND I DON’T LOOK ALIKE
CROSSING OVER ENSURES A COMBINATION OF THE MATERNAL AND PATERNAL GENES WE INHERITED

10. THE HISTORY OF CROSSING OVER

11. Patterns of Inheritance Carl Correns, 1900s Gene Linkage
Gregor Mendel,
Patterns of Inheritance
Carl Correns, 1900s
Gene Linkage
Alfred Sturtevant 11

12. CROSSING OVER BASICS

13. 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

14. Occurs at One or More Points Along Adjacent Homologues during Synapsis
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 14

15. Tetrad
Chiasma
Figure 8.18A Chiasmata.
Centromere *

16. Figure 8.18B How crossing over leads to genetic recombination.
Coat-color
genes
Eye-color
genes C E
Tetrad
(homologous pair of
chromosomes in synapsis) c e 1
Breakage of homologous chromatids C E c e 2
Joining of homologous chromatids C E
Chiasma c e
Separation of homologous
chromosomes at anaphase I 3 C E C e
Figure 8.18B How crossing over leads to genetic recombination.
This figure shows the outcome of crossing over between the coat and eye color genes for the mice introduced in module One of the parents contributes the genes for brown coat with black eyes, while the other parent donates the genes for white coat with pink eyes. Four genetically different chromosomes are produced, two parental versions carrying genes for brown coat with black eyes and white coat with pink eyes, and two recombinant versions carrying genes for brown coat with pink eyes and white coat with black eyes. c E c e
Separation of chromatids at
anaphase II and
completion of meiosis 4 C E
Parental type of chromosome C e
Recombinant chromosome c E
Recombinant chromosome c e
Parental type of chromosome
Gametes of four genetic types *

17. Coat-color genes Eye-color genes C E c e C E c e C E c e
Tetrad
(homologous pair of
chromosomes in synapsis) c e
Breakage of homologous chromatids 1 C E c e
Figure 8.18B How crossing over leads to genetic recombination.
This figure shows the outcome of crossing over between the coat and eye color genes for the mice introduced in module One of the parents contributes the genes for brown coat with black eyes, while the other parent donates the genes for white coat with pink eyes. Four genetically different chromosomes are produced, two parental versions carrying genes for brown coat with black eyes and white coat with pink eyes, and two recombinant versions carrying genes for brown coat with pink eyes and white coat with black eyes.
Joining of homologous chromatids 2 C E
Chiasma c e *

18. Figure 8.18B How crossing over leads to genetic recombination.
Chiasma c e
Separation of homologous
chromosomes at anaphase I 3 C E C e c E c e
Separation of chromatids at
anaphase II and
completion of meiosis 4
Figure 8.18B How crossing over leads to genetic recombination.
This figure shows the outcome of crossing over between the coat and eye color genes for the mice introduced in module One of the parents contributes the genes for brown coat with black eyes, while the other parent donates the genes for white coat with pink eyes. Four genetically different chromosomes are produced, two parental versions carrying genes for brown coat with black eyes and white coat with pink eyes, and two recombinant versions carrying genes for brown coat with pink eyes and white coat with black eyes. C E
Parental type of chromosome C e
Recombinant chromosome c E
Recombinant chromosome c e
Parental type of chromosome
Gametes of four genetic types *

19. 19

20. 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 20

21. 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 21

22. GENES GET SHUFFLED WHEN CHROMOSOMES EXCHANGE PIECES
Watch an animation of crossing over with an explanation
of how the concept was discovered at 22