Transcript The Chromosome Theory of Inheritance

The Chromosome Theory of
Inheritance
Thomas Morgan & Fruit Flies
• Thomas Morgan
was one of the early
geneticist, who after
Mendel’s work was
rediscovered in
1900, wanted to
further extend our
understanding of
inheritance
Fruit Flies
• Small Organisms
• Easy to Keep
• Short Generation
Time (28 days)
• Few Highly Visible
Chromosomes (8)
Morgan’s Experiment
• In an effort to replicate Mendel’s twotrait crosses he complete a cross
between flies that were heterozygous
for Body Color and Wing Size (GgNn)
and flies that were homozygous
recessive for Body Color and Wing Size
(ggnn)
Independent Assortment or
Mendelian Meiosis
• Mendilian Assortment produces all the
possible gametes, having the alleles
GN, Gn, gN, and gn
Mendelian Punnett Square
GN
Gn
gN
gn
gn
GgNn
Ggnn
ggNn
ggnn
gn
GgNn
Ggnn
ggNn
ggnn
gn
GgNn
Ggnn
ggNn
ggnn
gn
GgNn
Ggnn
ggNn
ggnn
Mendelian Expectations
Gray Body and Normal Wings
25%
Gray Body and Short Wings
25%
Black Body and Normal Wings25%
Black Body and Short Wings
25%
Morgan’s Actual Results
Gray Body and Normal Wings
42%
Gray Body and Short Wings
8%
Black Body and Normal Wings 8%
Black Body and Short Wings
42%
Walter Sutton
• In the study of the production of sperm
in grasshoppers, Walter Sutton had
noticed that the factor (or alleles) that
Mendel had discovered acted just like
the chromosomes as they separated
during the process of meiosis
• Thus, he linked Mendelian Inheritance
with Chromosomes
Gene Linkage
• Knowing the work of
Sutton, Morgan
wondered if the
genes for Body
Color and Wing Size
were on the same
chromosome
• Would this explain
his actual results?
Gene Linkage Meiosis
• Gene linkage
produces the
gametes having the
alleles GN and gn
only, and can’t
produce Gn and gN
Predicted Results
Gray Body and Normal Wings
50%
Gray Body and Short Wings
0%
Black Body and Normal Wings 0%
Black Body and Short Wings
50%
Gene Linkage &
Crossing Over
• Next, he wondered if
the events of
Crossing Over may
help to explain his
results
Crossing Over Meiosis
Predicted Results
Gray Body and Normal Wings
25%
Gray Body and Short Wings
25%
Black Body and Normal Wings25%
Black Body and Short Wings
25%
The Final Solution
• The previous results assume that
Crossing Over occurs 100% of the time
• In this case, Crossing Over occurs only
32% of the time (the Cross Over
Percentage is determined by adding the
two smaller phenotypic percentages
and multiplying by two)
Gene Mapping
• Morgan’s results lead to the mapping of
genes on chromosomes.
• If two genes are on separate chromosomes,
then you expect Mendelian Results
• If two genes are on the same chromosome,
then the smaller the cross over percentage
the closer they are together
Non-Disjunction
Meiosis Errors