Download Genetic Modification Regulations and Procedures

Genes in Chromosome
and
Genetic Mapping
Thomas Hunt Morgan
 Birthday : 1866
 Death : 1945
 Nationality :
American
 Achievement :
Winner of the 1933
Nobel Prize in
Medicine
 Thomas Hunt
Morgan was the first
to associate a
specific gene with a
specific chromosome.
 Finished the first
genetic mapping
Where are genes located?
 Since T. H. Morgan it is known to be on
chromosomes, at least in Drosophila (light
microscopy)
 Assigning genes to individual chromosomes
- chromosomal aneuploidy
- sex-linked traits (white-eyed in Drosophila)
- somatic cell hybrids – selective human
chromosome retention in mouse/human
hybrids (TK gene on chrom. 17)
- G-banding / deletion mapping (DMD-gene)
Double helix
cell
Chromosome
DNA
strand
a gene
nucleus
chromatid
Morgan traced a gene to a
specific chromosome
•Morgan choice an experimental animal,
Drosophila melanogaster
•fruit fly species that eats fungi on fruit
•prolific breeders
•generation time of two weeks.
•Fruit flies have three pairs of autosomes and a pair of sex
chromosomes (XX in females, XY in males). 3+1
•easier to see mutation.
•displayed a clear genetic variation
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
 The normal character phenotype is the wild type (red).
 Alternative traits are mutant phenotypes.
female
male
Fig. 15.2
Genes located on a sex chromosome are called sex-linked genes.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Linked genes tend to be inherited together because they
are located on the same chromosome
 Results of crosses with linked genes deviate from those
expected according to independent assortment.
• Morgan reasoned that
body color and wing
shape are usually
inherited together
because their genes
are on the same
chromosome.
(b+vg+
Or b vg+)
(b vg)
Fig. 15.4
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Geneticists can use recombination
data to map a chromosome’s
genetic loci
 One of Morgan’s students, Alfred Sturtevant,
used crossing over of linked genes to
develop a method for constructing a
chromosome map.
 This map is an ordered list of the genetic loci
along a particular chromosome.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Sturtevant hypothesized :
that the frequency of recombinant offspring reflected the
distances between genes on a chromosome.

The farther apart two genes are, the higher the
probability that a crossover will occur between
them and therefore a higher recombination
frequency.
Sturtevant used recombination frequencies
from fruit fly crosses to map the relative
position of genes along chromosomes, a
linkage map.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Using recombination frequencies to construct a genetic map
One map unit (sometimes called a centimorgan) is
equivalent to a 1% recombination frequency.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 15.6
 Some genes on a chromosome are so far
apart, the frequency of recombination
reaches is its maximum value of 50% and
the genes act as if found on separate
chromosomes and are inherited
independently.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
More than one gene can
affect
a given phenotypic
characteristic
(eye color)
Partial genetic map
Fig. 15.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
 Map units (linkage map) indicate relative
distance and order, not precise locations of
genes.  The frequency of crossing over is not actually
uniform over the length of a chromosome.
•Combined with other methods like
chromosomal banding, geneticists can develop
cytological maps.  the positions of genes with
respect to chromosomal features.
•More recent techniques show the absolute
distances between gene loci in DNA
nucleotides.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
VARIATIONS IN
RECOMBINATION FRACTIONS
 The frequency of
recombination is not
absolute but depends
on (i) sex of gametes
(is higher during
oogenesis)
VARIATIONS IN
RECOMBINATION FRACTIONS
 The frequency of
recombination is also
dependent on
(ii) relative position
along the chromosom
(is higher near chromosome ends)