Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that...
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Transcript of Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that...
![Page 1: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/1.jpg)
Fig. 4-1
Chapter 4overview
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Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different fromthe combinations received from parents.
• Independent assortment of homologous chromosomes (Anaphase I). Genes on non- homologous chromosomes (unlinked genes) assort independently.
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Fig. 4-6
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Fig. 4-7
Using a testcrossto distinguish gamete genotypes
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Fig. 4-8
50% = independent assortment(genes are not linked)
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Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different fromthe combinations received from parents.
• Independent assortment of homologous chromosomes (Anaphase I). Genes on non- homologous chromosomes (unlinked genes) assort independently.
• Crossing over (recombination among linked genes)
![Page 7: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/7.jpg)
Fig. 4-2
cis linked: both dominant alleles on the same homolog
trans linked: dominant alleles on different homologs
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Fig. 4-3
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Crossing over
• Physical exchanges among non-sister chromatids; visualized cytologically as chiasmata
• Typically, several crossing over events occur within each tetrad in each meiosis (chiasmata physically hold homologous chromosome together and assure proper segregation at Anaphase I)
p. 115
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Fig. 4-4
Crossing over occurs at the four-strand stage(pre-meiotic G2 or very early prophase I)
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Fig. 4-5
Crossing over can involve 2, 3, or 4 chromatids in a single meiosis
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Crossing over
• Physical exchanges among non-sister chromatids; visualized cytologically as chiasmata
• Typically, several crossing over events occur within each tetrad in each meiosis (chiasmata physically hold homologous chromosome together and assure proper segregation at Anaphase I)
• The sites at which crossing over occur are random
• The likelihood that a crossover occurs between any two particular sites (genes) is a function of the physical distance between those two sites
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Fig. 4-9
Crossing over usually affects a minority of chromatids in a collection of meioses – recombinants are typically a
minority of products
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Fig. 4-10
<50% = linked genes
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A.H. Sturtevant (1911-3): frequency of crossing over between two genes is a function of their distance apart on the chromosome; created the first genetic map
number of recombinants
Recombination frequency = total number of progeny
One map unit = one centimorgan = 1% recombinants
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Fig. 4-11
Rationales:• Crossover events are random• Greater separation, greater likelihood that crossover will occur• Map distance should be sum of smaller intervals
• Construct entire chromosome maps by mapping intervals• Linear map correlates with linear chromosome
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Markers used in trihybrid testcross in Drosophila
v = vermilion eyes (red eyes; v+ are red-brown)
cv = crossveinless (cv+ wings have crossveins)
ct = cut wing (ct+ wings have regular margins)
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Data from three-point testcross
v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester)
Progeny phenotypes
v cv+ ct+ 580v+ cv ct 592v cv ct+ 45v+ cv+ ct 40v cv ct 89v+ cv+ ct+ 94v cv+ ct 3v+ cv ct+ 5
1448
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Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
![Page 20: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/20.jpg)
Data from three-point testcross
v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester)
Progeny phenotypes
v cv+ ct+ 580v+ cv ct 592v cv ct+ 45v+ cv+ ct 40v cv ct 89v+ cv+ ct+ 94v cv+ ct 3v+ cv ct+ 5
1448
![Page 21: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/21.jpg)
Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
3. Identify parental types as the most frequent pair of products
4. Identify double crossover products as least frequent pair of products
![Page 22: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/22.jpg)
Data from three-point testcross
v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester)
Progeny phenotypes
v cv+ ct+ 580v+ cv ct 592v cv ct+ 45v+ cv+ ct 40v cv ct 89v+ cv+ ct+ 94v cv+ ct 3v+ cv ct+ 5
1448
Parental types - nco
dco
sco
sco
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Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
3. Identify parental types as the most frequent pair of products
4. Identify double crossover products as least frequent pair of products
5. Compare the parental and double crossover products to deduce the order of the three gene loci
![Page 24: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/24.jpg)
Fig. 4-12
In dco products, the central marker is displacedrelative to the parental types
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Fig. 4-13
![Page 26: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/26.jpg)
Steps in solving three-point testcross problem
1. Anticipate and identify eight types of products (23)
2. Identify pairs of reciprocal products
3. Identify parental types as the most frequent pair of products
4. Identify double crossover products as least frequent pair of products
5. Compare the parental and double crossover products to deduce the order of the three gene loci
6. Compute map distances by breaking down the results for each interval
![Page 27: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/27.jpg)
85 + 8 1448
(0.064)
183 + 8 1448
(0.132)
RF =
Fig. 4-12
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85 + 8 1448
(0.064)
183 + 8 1448
(0.132)
RF =
13.2 m.u. 6.4 m.u.
Fig. 4-12
v ct cv
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Interference: crossing over in one region interferes with simultaneous crossing over in adjacent regions
Expected frequency of dco = product of frequency crossovers in two regions
0.132 X 0.064 = 0.0084
0.084 X 1448 = 12 expected (if two sco are independent events)
![Page 30: Fig. 4-1 Chapter 4 overview. Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different.](https://reader030.fdocuments.us/reader030/viewer/2022032702/56649cda5503460f949a4193/html5/thumbnails/30.jpg)
Interference: crossing over in one region interferes with simultaneous crossing over in adjacent regions
Expected frequency of dco = product of frequency crossovers in two regions
0.132 X 0.064 = 0.0084
0.084 X 1448 = 12 expected (if two sco are independent events)
Coefficient of coincidence = observed dco / expected dco
8 / 12 = 0.667
Interference = 1 – coefficient of coincidence
1 – 0.667 = 0.333
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Fig. 4-14
Tomato karyotype (n=12)
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Fig. 4-14
Tomato linkage map(1952)
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p. 136
Typical phenotypic ratios for a variety of crosses(complete allele dominance)
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