chapter05_lecture
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5-3
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Independent assortment
Genes on different chromosomes
A
B
A
B
a
b
a
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.
Linkage and meiotic recombination
Genes linked together on the same chromosome usually assort together.
Linked genes may become separated through recombination.
b
Gametes
A
a
B
B
A
a
b
b
5-4
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage
Two genes on same chromosome segregate together.
genes.
1909 – Frans Janssens observed 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.
yellow (y+) – yellow body color white (w+) – white eye color
5-7
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
Chapter 5
5-1
Linkage, Recombination, and the Mapping of Genes on Chromosomes
5-2
Outline of Linkage, Recombination, and the Mapping of Genes on Chromosomes
A
B
A
B
a
b
a
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
• Genotypes of F1 female revealed by test cross • Parental class outnumbers recombinant class demonstrating linkage.
Fig. 5.4
Βιβλιοθήκη Baidu5-10
Recombination results when crossingover during meiosis separates linked
Fig. 5.2
5-8
Designations of “parental” and “recombinant” relate to past history.
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
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.
recombination is independent of the arrangement of alleles.
5-9
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage in an autosomal gene
A
B
Ax
B
a
b
a
b
Gametes
Parental
A
B
a
b
Recombinant
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.
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Independent assortment
Genes on different chromosomes
A
B
A
B
a
b
a
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.
Linkage and meiotic recombination
Genes linked together on the same chromosome usually assort together.
Linked genes may become separated through recombination.
b
Gametes
A
a
B
B
A
a
b
b
5-4
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage
Two genes on same chromosome segregate together.
genes.
1909 – Frans Janssens observed 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.
yellow (y+) – yellow body color white (w+) – white eye color
5-7
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
Chapter 5
5-1
Linkage, Recombination, and the Mapping of Genes on Chromosomes
5-2
Outline of Linkage, Recombination, and the Mapping of Genes on Chromosomes
A
B
A
B
a
b
a
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
• Genotypes of F1 female revealed by test cross • Parental class outnumbers recombinant class demonstrating linkage.
Fig. 5.4
Βιβλιοθήκη Baidu5-10
Recombination results when crossingover during meiosis separates linked
Fig. 5.2
5-8
Designations of “parental” and “recombinant” relate to past history.
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
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.
recombination is independent of the arrangement of alleles.
5-9
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
Linkage in an autosomal gene
A
B
Ax
B
a
b
a
b
Gametes
Parental
A
B
a
b
Recombinant
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.