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This document discusses chromosome mapping and genetic maps. It explains that Sturtevant created the first genetic map by mapping 5 genes on the X chromosome of Drosophila. Genetic maps show the relative distances and linear order of linked genes on a chromosome based on crossover frequencies during meiosis. The number of linkage groups equals the number of chromosomes plus sex chromosomes in an organism. Genetic maps can be used to predict results of dihybrid and trihybrid crosses.

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2K views22 pages

Document Encoding Error

This document discusses chromosome mapping and genetic maps. It explains that Sturtevant created the first genetic map by mapping 5 genes on the X chromosome of Drosophila. Genetic maps show the relative distances and linear order of linked genes on a chromosome based on crossover frequencies during meiosis. The number of linkage groups equals the number of chromosomes plus sex chromosomes in an organism. Genetic maps can be used to predict results of dihybrid and trihybrid crosses.

Uploaded by

Vipin
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PPT, PDF, TXT or read online on Scribd
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Chromosome Mapping

 In1903 Sutton and Boveri came up with


the Chromosomal Theory of Inheritance.
 Evidences accumulated pointing that
genes make up chromosomes.
 Each chromosome has many loci.
 Genes are arranged in a linear fashion on
the chromosomes.: Sturtevant.
Sturtevant’s map included 5 genes on the X
chromosome of Drosophila
 Alleliccombinations would be permanent if
the loci are locked.
 At meiosis, crossing over allows the
associated alleles to show a degree of
independence.
 Loci carried on the same chromosome are
said to be linked.
 Number of linkage groups (l) is equal to
the number of haploid set of
chromosomes plus the sex chromosomes
in an organism.
 Drosophila (2n = 8) has l = 3 autosomes
+ x + y = 5 linkage groups.
Diagrammatic graphical
representation of relative distances
between linked genes of a
chromosomes is called Linkage or
Genetic map.
Determination of linkage groups
 Hybridisationexperiments between wild
and mutant strains.
 Linked phenotypic traits can be
determined.
Map Distance
 Calculated depending on the percentage
of crossing over.
 Crossing over frequency is directly
proportional to the distance between
genes.
 Map unit / morgan / centi morgan
Two point test cross
Three point cross
Gene order
 After
determining the relative distances
between the genes of a linkage group, it
becomes easy to place genes in their
proper linear order.
Combining map segments
 Thedifferent segments of maps of a
complete chromosome are combined to
form a complete genetic map.
Chromosome, cytological &
physical mapping
 Specialcytological techniques are used to
determine the physical locations of a gene
in a chromosome.
Differences between genetic &
chromosome maps
 The frequency of crossing over varies in
different segments of the chromosome.
 Frequency of crossing over is predictable
between any two gene loci.
 Distance between linked genes bears no
direct relation to the map distances
calculated on the basis of cross over
percentages.
 Thelinear order of linked genes is
identical in both the maps.
Uses of genetic maps
 Displays the exact location, arrangement
& combination of genes in a linkage group
or chromosome.
 Predicts the results of di-hybrid and tri-
hybrid crosses
References
 Robert Tamarin: Principles of genetics, 7th
edition.
 Griffith et al: Genetics
 B.D. Singh: Fundamentals of genetics

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