GENETIC RECOMBINATION CROSSING OVER GENETIC RECOMBINATION 1

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

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

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Homologous Chromosomes Exchanging DNA by Crossing Over 7

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

Patterns of Inheritance Carl Correns, 1900s Gene Linkage Gregor Mendel, 1823-1884 Patterns of Inheritance Carl Correns, 1900s Gene Linkage Alfred Sturtevant 11

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

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

Tetrad Chiasma Figure 8.18A Chiasmata. Centromere *

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 8.17. 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 *

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 8.17. 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 *

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 8.17. 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 *

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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 20

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

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 22