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Chromosome Theory of Inheritance
Linkage, Crossing Over
Punjab EDUSAT Society{PES}
By – Surinder KaurDIET Ropar
Surinder-1998@ yahoo.in
Mob No 9988530775
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1902: Walter Sutton and Theodor Boveri
Chromosome theory of inheritance:
1. Independently recognized that the transmission of chromosomes from one generation to the next parallels inheritance
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generation to the next parallels inheritance of Mendelian factors.
2. Mendelian factors (genes) are located on chromosomes.
3. Support for theory derived from study of sex chromosomes.
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Sex chromosomes:
Chromosomes or group of chromosomes in eukaryotes in which the sexes are represented differently.
Typically designated X and Y (sometimes
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Typically designated X and Y (sometimes W and Z)
Autosomes:
All of the other chromosomes.
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Linkage
• Tendency of genes of same chromosome to remain together .
• Such genes are called – linked genes.
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• Such genes are called – linked genes.
• Linked genes present only parental types in progeny.
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A
B
C
D
EF
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C F
A, B, C – LINKED GENES A & D – UNLINKED GENES
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Why Linkage
• Linkage refers to packaging genes onto chromosomes
• Chromosomes (and therefore linkage) are for organizing genes for their safe coordinated transmission from cell to cell, parent to
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transmission from cell to cell, parent to offspring
• Simply put, what’s easier to handle:
� 30000 quarters (i.e., genes) OR
� 23 bags of quarters (i.e., chromosomes)?
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Some Definitions
• Complete linkage – Genes that are linked but never crossover
�Incomplete linkage – Genes that are linked and can crossover
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• Parentals – Offspring that are like the parents
�Recombinants – Offspring with arrangements different from the parents
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Unlinked genes
• Parents AABB X aabb
• Gametes AB, AB ab, ab
ABA B
a ab b
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• Gametes AB, AB ab, ab
• F1progeny AaBb
• T.cross AaBb X aabb
• Progeny AaBb , Aabb , aaBb , aabb (1:1:1:1)
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Linked Genes__ __ __ __
Parents AB / AB ab / ab
Gametes
F1 Progeny
__ __ __ __
X
__ __
AB / ab
AB abAB
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__ __ __ __
Test Cross AB / ab ab / ab
Gametes
Test Cross
Progeny AB Test Ratio 1:1
ab AB / ab
X
ab
AB / ab
ab abAB
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FACTORS AFFECTING LINKAGE
1. Age
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1. Age
2. Temperature
3. X-Rays
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Significance of Linkage
• Keeps parental / racial specific traits together.
• Valuable traits of new variety maintained.
• Disallows desirable mixing of traits.
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• Disallows desirable mixing of traits.
• No. of linkage groups = No. of chromosome
pairs.
This shows genes are located on chromosomes.
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• Linked genes are not inherited together every time.
• Chromosomes exchange homologous genes
during meiosis.
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Crossing- Over
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Steps of Crossing- over
• Synapsis of homologous chromosomes – Zygotene
• Tetrad formation- Pachytene
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• Tetrad formation- Pachytene
• Crossing over- Pachytene
• Disjunction
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FACTORS AFFECTING CROSS OVER
1. Temrperature
2. X-Rays
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3. Chemicals
4. Age
5. Interference
6. Sex
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Significance of Crossing-over
• Produces new combinations of traits.
• Forms raw material for evolution.
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• Establishes concept of linear arrangement of genes.
• Helps to determine loci of genes in the chromosomes.