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Gene Mapping
Morgan & the Fruit Fly
• Morgan’s studies of the fruit fly
and mutant gene for white eye
proved Sutton’s ideas of
chromosomal inheritance true
–Also gave rise to interesting idea
of linkage
A
B
a
b
a
B
A B
a
b
Tetrad
A
b
Crossing over
Gametes
Figure 9.19A, B
Linkage Groups
• Genes located on different chromosomes
segregate independently & form new
combinations (recombinants)
• Genes on same chromosome may be
inherited together – “linked” – patterns
remain similar to parental types
– ***The further apart genes are, the more they
act like they are on separate chromosomes***
crossover data & gene mapping
• Crossing over is more likely to occur between
genes that are farther apart
– Recombination allows for a new mix of traits
Chromosome
g
c
l
17%
9%
9.5%
Figure 9.20B
Genetic Maps
• Frequency of crossing over (exchange of
chromosomes) can be used to “map” the
position of genes –Recombination map
• One map unit (centimorgan) is the
frequency of crossing over (1 in 100
gametes) or Recombination
• A partial genetic map of a fruit fly chromosome
Mutant phenotypes
Short
aristae
Black
body
(g)
Long aristae
(appendages
on head)
Gray
body
(G)
Cinnabar
eyes
(c)
Red
eyes
(C)
Vestigial
wings
(l)
Brown
eyes
Normal
wings
(L)
Red
eyes
Wild-type phenotypes
Figure 9.20C
Human Genome Project
• Purpose was to map all 30,000 genes
on our 23 pairs of chromosomes
• Project basically completed in 2002
• Why would this be valuable??