The
Chromosome
Theory of
Inheritance
Chpt. 15
.
1866 Mendel’s paper
Putting the
1875 process of mitosis revealed
pieces
1890 process of meiosis revealed
together
1902 realization that chromosomes were Mendel’s
“factors”
Sex
chromosomes-
Autosomes
Sex
chromosomes-
vary within
species -->
-->
-->
Sex
chromosomes-
In 1902 Sutton suggested that
"the association of paternal and
maternal chromosomes in pairs
and their subsequent separation
during the reduction
division...may constitute the
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basis of the Mendelian
law of heredity." His "The
Chromosomes in Heredity" was
published in 1903.
Walter Sutton 1902
Genes have specific locations (loci)
on chromosomes
chromosomes undergo segregation
and independent assortment.
The Chromosome Theory of Inheritance
tracking 2
genes on
different
chromosomes
in their
homologous
pairs.
Thomas
Hunt Morgan
st
1
to
associate
specific genes
on specific
chromosomes
Thomas Hunt Morgan
Utilized:Drosophila
melanogaster
•
fruit fly
• 3 pairs autosomes
• 1 pair sex
chromosomes
Thomas Hunt Morgan
Utilized:
Drosophila
melanogaster
• can reproduce a
generation in
two weeks.
Fruit Fly Chromosomes
Genetic Symbols
• Mendel - use of uppercase or
lowercase letters.
T = tall
t = short
• Morgan: symbol from the
mutant phenotype (for fruitflies).
+ = wild phenotype
(predominant)
Examples
• Recessive mutant:
w = white eyes
w+ = red eyes
or + = red eyes
Examples
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• “Mutant” =
Cy = Curly wings
Wild Type =
Cy+ = Normal wings
Evidence for
Sex Linkage
female red eyes
X
male white eyes
F1 --> 4:0 RED
You would expect this with
Mendel… would you not?
Evidence for
Sex Linkage
female red eyes
X
male red eyes
F2 --> 3:1
BUT ONLY males
had white eyes??
F2
eye color
linked to
gender
Examples
• Recessive mutant:
w = white eyes
w+ = red eyes
or + = red eyes
eye color
linked to gender
50% males had
red eyes.
50% males had
white eyes.
0% females had
white eyes.
Evidence for
Sex Linkage
50% males had
red eyes.
50% males had
white eyes.
0% females had
white eyes.
Fruit Fly Chromosomes
Linked Genes
(not sex-linked genes)
A
B
•Genes are on the
same chromosome,
therefore passed along as a
“unit”
these DEVIATE
from Mendel’s
“norm”
Ratios
mimic
monohybrid
crosses.. But it is a
dihybrid cross.
Linked Genes (not x-linked)
1:1:1:1
5:5:1:1
IF genes are linked, we would
NOT expect any offspring to
have different characters from
that of the parents…
THAT was not the case
though, the offspring were, in
fact different
“Well now,
THAT’s
confusing
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Linked Genes (not x-linked)
Something’s
goofy
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Recombination
due to
crossing
over
in linked
genes
“YES!!! I
remember
this stuff!!”
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Crossing-Over
Breaks
up linkages
and creates new ones.
Recombinant
offspring formed that
doesn't match the
parental types.
Alfred Sturtevant
Recombination
Frequency=
X
100
# of recombinants
total offspring
•gives a rough estimate of
how often recombination
occurs.
Recombination
Frequency=
•The farther apart two genes,
the higher the probability that
a X-over will occur.
•The higher the
“YES!!! I
remember
this stuff!!”
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b - vg = 17
r - vg = 9.5
b - r = 7.5
Map of 3 genes relative to each
other (genes: b, r, vg)
b - vg = 17
b - r = 7.5
r - vg = 9.5
b
vg
17%
7.5
Map of 3 genes relative to each
(genes:
other b, r, vg)
b - vg = 17
b - r = 7.5
r - vg = 9.5
b
r
vg
17%
7.5
Map of 3 genes relative to each
(genes:
other b, r, vg)
b - vg = 17
b - r = 7.5
r - vg = 9.5
b
r
vg
17%
7.5
9.5
Map of 3 genes relative to each
(genes:
other b, r, vg)
Map of 3 genes relative to each
other
Map of 3 genes relative to each
other
Gene Map -
•Relative
position using
map units.
•Obtained
from
Recombination
Frequency Data
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Calico Cats.
Calico Cats.
Result
- body
cells are a mosaic
of X types.