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Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or disBaidu Nhomakorabealay
Designations of “parental” and “recombinant” relate to past history.
5-3
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Independent assortment
Genes on different chromosomes
A A a a B B b b
Transmission of gametes is based on chance events.
Deviations from 1:1:1:1 ratios can represent chance events OR linkage. Ratios alone will never allow you to determine if observed data are significantly different from predicted values. The larger your sample, the closer your observed values are expected to match the predicted values.
yellow (y+) – yellow body color white (w+) – white eye color
5-7
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
A A a a B B b b
x
Gametes Parental Recombinant
A B a b
a
B
A
b
5-6
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Some genes on the same chromosome assort together more often than not.
Mapping
The frequency with which genes become separated reflects the physical distance between them.
Mitotic recombination
Rarely, recombination occurs during meiosis. In eukaryotes mitotic recombination produces genetic mosaics.
5-2
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Outline of Linkage, Recombination, and the Mapping of Genes on Chromosomes
In dihybrid crosses, departures from a 1:1:1:1 ratio of F1 gametes indicate that the two genes are on the same chromosome. How we determine if two genes are on the same chromosome can be demonstrated by the white and yellow genes on the X chromosome of Drosophila.
Gametes
A B a B
A b
a
b
5-4
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage
Two genes on same chromosome segregate together.
Chi square test measures “goodness of fit” between observed and expected (predicted) results.
Accounts for sample size, or the size of the experimental population
5-9
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage in an autosomal gene
• Genotypes of F1 female revealed by test cross • Parental class outnumbers recombinant class demonstrating linkage.
A A a a B B b b
Gametes
A B A B
a
b
a
b
5-5
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Crossing over and linkage
Lead to separation of linked genes
Linkage at a sex-linked gene
• Deviation from 1:1:1:1 ratio of phenotypes for males • Draw traits on chromosomes and work through the cross
Fig. 5.2
5-8
Chi square table of critical values
5-15
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Recombination results when crossingcrossingover during meiosis separates linked genes.
Applying the chi square test to a linkage study
Genotype AB ab Ab AB Total Experiment 1 17 14 8 11 50 Experiment 2 34 28 16 22 100
Observed/Expected Observed/Expected Class Parentals 31 25 62 50 Recombination 19 25 38 50
5-1
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage, Recombination, and the Mapping of Genes on Chromosomes
5-13
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Chi Square – Experiment 1 & 2
χ2 = Σ (observed – expected)2 number expected
Powerpoint to accompany
Genetics: From Genes to Genomes
Third Edition
Hartwell ● Hood ● Goldberg ● Reynolds ● Silver ● Veres
Chapter 5
Prepared by Malcolm D. Schug University of North Carolina Greensboro
Experiment 1 χ2 = Σ (31 – 25)2 + (19 – 25)2 25 25 χ2 = Σ (62 – 50)2 + (38 – 50)2 50 50 = 2.88
Experiment 2
= 5.76
5-14
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage and meiotic recombination
Genes linked together on the same chromosome usually assort together. Linked genes may become separated through recombination.
5-11
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Applying the chi square test
Framing a hypothesis
Null hypothesis – observed values are not different from the expected values
Fig. 5.4
5-10
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Chi square test pinpoints the probability that ratios are evidence of linkage.
1909 – Frans Janssens observed chiasmata, chiasmata, regions in which nonsister chromatids of homologous chromosomes cross over each other. Thomas Hunt Morgan suggested these were sites of chromosome breakage and change resulting in genetic recombination.
For linkage studies – no linkage is null hypothesis Expect a 1:1:1:1 ratio of gametes.
Alternative hypothesis – observed values are different from expected values
Parental and recombinant classes are opposite of one another in these two crosses. Similar percentages of recombinant and parental types show that the frequency of recombination is independent of the arrangement of alleles.
For linkage studies – genes are linked. Expect significant deviation from 1:1:1:1 ratio.
5-12
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Designations of “parental” and “recombinant” relate to past history.
5-3
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Independent assortment
Genes on different chromosomes
A A a a B B b b
Transmission of gametes is based on chance events.
Deviations from 1:1:1:1 ratios can represent chance events OR linkage. Ratios alone will never allow you to determine if observed data are significantly different from predicted values. The larger your sample, the closer your observed values are expected to match the predicted values.
yellow (y+) – yellow body color white (w+) – white eye color
5-7
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
A A a a B B b b
x
Gametes Parental Recombinant
A B a b
a
B
A
b
5-6
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Some genes on the same chromosome assort together more often than not.
Mapping
The frequency with which genes become separated reflects the physical distance between them.
Mitotic recombination
Rarely, recombination occurs during meiosis. In eukaryotes mitotic recombination produces genetic mosaics.
5-2
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Outline of Linkage, Recombination, and the Mapping of Genes on Chromosomes
In dihybrid crosses, departures from a 1:1:1:1 ratio of F1 gametes indicate that the two genes are on the same chromosome. How we determine if two genes are on the same chromosome can be demonstrated by the white and yellow genes on the X chromosome of Drosophila.
Gametes
A B a B
A b
a
b
5-4
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage
Two genes on same chromosome segregate together.
Chi square test measures “goodness of fit” between observed and expected (predicted) results.
Accounts for sample size, or the size of the experimental population
5-9
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage in an autosomal gene
• Genotypes of F1 female revealed by test cross • Parental class outnumbers recombinant class demonstrating linkage.
A A a a B B b b
Gametes
A B A B
a
b
a
b
5-5
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Crossing over and linkage
Lead to separation of linked genes
Linkage at a sex-linked gene
• Deviation from 1:1:1:1 ratio of phenotypes for males • Draw traits on chromosomes and work through the cross
Fig. 5.2
5-8
Chi square table of critical values
5-15
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Recombination results when crossingcrossingover during meiosis separates linked genes.
Applying the chi square test to a linkage study
Genotype AB ab Ab AB Total Experiment 1 17 14 8 11 50 Experiment 2 34 28 16 22 100
Observed/Expected Observed/Expected Class Parentals 31 25 62 50 Recombination 19 25 38 50
5-1
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage, Recombination, and the Mapping of Genes on Chromosomes
5-13
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Chi Square – Experiment 1 & 2
χ2 = Σ (observed – expected)2 number expected
Powerpoint to accompany
Genetics: From Genes to Genomes
Third Edition
Hartwell ● Hood ● Goldberg ● Reynolds ● Silver ● Veres
Chapter 5
Prepared by Malcolm D. Schug University of North Carolina Greensboro
Experiment 1 χ2 = Σ (31 – 25)2 + (19 – 25)2 25 25 χ2 = Σ (62 – 50)2 + (38 – 50)2 50 50 = 2.88
Experiment 2
= 5.76
5-14
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage and meiotic recombination
Genes linked together on the same chromosome usually assort together. Linked genes may become separated through recombination.
5-11
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Applying the chi square test
Framing a hypothesis
Null hypothesis – observed values are not different from the expected values
Fig. 5.4
5-10
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Chi square test pinpoints the probability that ratios are evidence of linkage.
1909 – Frans Janssens observed chiasmata, chiasmata, regions in which nonsister chromatids of homologous chromosomes cross over each other. Thomas Hunt Morgan suggested these were sites of chromosome breakage and change resulting in genetic recombination.
For linkage studies – no linkage is null hypothesis Expect a 1:1:1:1 ratio of gametes.
Alternative hypothesis – observed values are different from expected values
Parental and recombinant classes are opposite of one another in these two crosses. Similar percentages of recombinant and parental types show that the frequency of recombination is independent of the arrangement of alleles.
For linkage studies – genes are linked. Expect significant deviation from 1:1:1:1 ratio.
5-12
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display