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Chapter 10
10.1 The Chromosome Theory
of Heredity
Chromosomes are located in the
nucleus
Factors (genes) are found on
chromosomes
Sutton discovered that genes are on
chromosomes in 1902
Chromosome Theory of
Heredity
States that genes are located on
chromosomes and each gene occupies
a specific place on a chromosome
Only one allele is on a chromosome
Independent Assortment
Gene Linkage
Genes on a chromosome are linked
together
Inherited together – THEREFORE they
do not undergo independent
assortment
Linked Genes- genes on the
same chromosome – inherited
as a package
A
Height Gene
B
Flower color gene
C
Flower position gene
Thomas Hunt Morgan
Studied fruit flies – Drosophilia
melanogaster
Fruit Flies are excellent for
genetic studies because:
Reproduce quickly
Easy to raise
Many mutations
Have 8 chromosomes (n=4)
Fruit Fly Mutations
Thomas Hunt Morgan began
to carry out experiments with
Morgan looked at TWO traits
Gray bodies – G
Normal Wings - W
Black bodies – g
Small wings – w
The flies mated….
The female laid eggs
P1
F1
GGWW
x
GgWw
100%
ggww
Morgan then mated the F1 back
to the recessive parent
GgWw
x
ggww
Expected ratio – 1:1:1:1
25% GgWw
25% ggWw
25% Ggww
25% ggww
Morgan’s Actual Results
41.5%
41.5%
8.5 %
8.5%
gray normal
black small
black normal
gray small
Conclusion
Gene for body size and wing color were
somehow connected or linked
Can’t undergo independent assortment
Linkage Groups
Package of genes that are always
inherited together
Chromosome
One linkage group for each homologous
pair
Fruit flies – 4 linkage groups
Humans – 23 linkage groups
Corn – 10 linkage groups
So linkage groups explain the
high percentages (41.5%) but
What about the
8.5%??????
17% had new combinations
The combinations that were
expected would be:
Gray normal – GW
or
Black small - gw
P1
G
G
g
g
W
W
w
w
Mom
Dad
F1
G
g
W
w
G
g
g
g
W
w
w
w
F1
F1
F1
Heterozygous
X
Recessive
Fruit Fly
The Offspring of the Cross
g
G
g
g
w
w
and
W
w
F1
F1
41.5 %
41.5 %
Genes of the Heterozygous Parent
G
G
g
g
W
W
w
w
The homologous pair copied
The homolgous pairs pair up in Prophase
and form a tetrad
G
G
g
g
W
W
w
w
When they are lined up they
can become twisted and
switch genes
Crossing
Over
So you could then have …..
G
G
g
g
W
w
W
w
switch
The other offspring of the cross
g
G
g
g
W
w
and
w
w
F1
F1
8.5 %
8.5 %
The 17% that had new
combinations are known as
Recombinants – individuals with new
combinations of genes
Crossing Over – gives rise to new
combinations – Prophase I
Gene Mapping
Sturtevant – associate of Morgan
Crossing over occurs at random
The distance between two genes
determines how often they cross over
Genes that are close do not crossover
often
Genes that are far apart – cross over
often
So……
If you know the frequency with which
crossing over occurs then you can use
that to map the position of the genes
on the chromosome
Frequency of crossover exchange...
is
GREATER the FARTHER apart 2 genes are
is proportional to relative distance
between 2 linked genes
Relative distance is established as...
1% crossover frequency =
1 map unit of map distance
1% CrossOver Freq
=
1 centiMorgan
Sex Linkage
Stevens – made observations of meal
worm chromosomes
Sex Chromosomes
One pair
Female – XX
Male – XY
Autosomes
All the chromosomes except the sex
chromosomes
Sex Determination
Genes on Sex Chromosomes
Sex chromosomes determine a person’s
sex
Sex chromosomes also contain genes
Sex Linked
A gene located on a sex chromosome
Usually X
Example – Fruit Fly Eye Color
So the gene for
eye color is on
the X chromosome
and not the Y
Fruit Fly Sex Chromosomes
X
X
X
Y
Males
Females
XRXR
XRY
Red Eyed
XRXr
XrXr
White Eyed
XrY
Mutations
A change in the DNA of an organism
Can involve an entire chromosome or a
single DNA nucleotide and they may
take place in any cell
Germ Cell Mutation
Occur in an organism’s germ cells
(gametes)- can only affect offpsring
Somatic Mutations
Take place in an organisms body cells
and can affect the organism
Lethal Mutation
Cause death, often before birth
Good Mutations
Some mutations can be beneficial –
these organisms have a better chance
to reproduce and therefore have an
evolutionary advantage
Provide the variation on which natural
selection acts
Chromosome Mutations
Are either changes in the structure of a
chromosome or the loss of an entire
chromosome or an addition
Four Types (duplication, deletion,
inversion and translocation)
Duplication – segment of a
chromosome is repeated
Deletion – the loss of a chromosome or
part due to chromosomal breakage –
that information is lost
Inversion – a chromosomal segment
breaks off and then reattached in
reverse orientation to the same
chromosome
Translocation – a chromosome breaks
off and reattaches to another
nonhomologous chromosome
Nondisjunction
Some chromosome mutations alter the
number of chromosomes found in a cell
Nondisjunction – the failure of a
chromosome to separate from its
homologue during meiosis
Gene Mutations
May involve large segments of DNA or a
single nucleotide within a codon
Involve individual genes
Point Mutations – 3 types
The substitution, addition or removal
of a single nucleotide
1. Substitution – a point mutation where
one nucleotide in a codon is replaced
with a different nucleotide, resulting in
a new codon
Ex. Sickle Cell Anemia – sub. Of A for
T in a single codon
2 & 3. Insertion and Deletions – one or
more nucleotides is lost or added –
have more serious effects
Frameshift Mutation
When a nucleotide is lost or added so
that the remaining codons are grouped
incorrectly
Insertions and deletions are frameshift
mutations
THE FAT CAT ATE THE RAT
Polyploidy
Condition in which an organism has an
extra set of chromosomes
3N, 4N
Usually fatal in animals
Plants – usually more robust
Caused by - Nondisjunction
10-3 Regulation of Gene
Expression
As biologists have intensified their
studies of gene activity, it has become
clear that interactions between different
genes and between genes and their
environment are critically important
Gene Interactions
Gene – piece of DNA – DNA codes for
proteins
In many cases the dominant allele
codes for a protein that works and the
recessive allele codes for a protein that
does not work
Incomplete Dominance
When offspring have a phenotype that
is in-between the two parents
Occurs when two or more alleles
influence the phenotype
Example – flowers – four o’ clocks,
snapdragons
Alleles – R/R’, R/r, R/W, FR F r
Red
Flower
White
Flower
Pink
Flower
Red mixed with white makes pink
Incomplete Dominance
Example #2
Incomplete dominance is a half way between point.
Halfway to dark blue is light blue.
Incomplete Dominance
is not a blending.
RR
rr
Rr
Phenotypic Ratio:
1:2:1
Genotypic Ratio:
1:2:1
Codominace
Occurs when both alleles for a gene are
expressed in a heterozygous offspring
Neither allele is dominant or recessive
Example – horse coat color
Horse Coat Color
Red – HR HR
White – HWHW
Roan – HR HW
Roan – red and white hairs
Blue roan - The coat has white
hairs and blue hairs
Polygenic Inheritance
Traits controlled by two or more genes
Examples – height, skin color, coat
patterns
Phenotypes are seen in a range
Polygenic Inheritance
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Gene Expression in
Prokaryotes
Genes serve as a pattern for the
production of mRNA
mRNA serves as the instructions to
make a protein
All the genes of an organism can’t be
active all the time
When a cell needs a product it must be
able to make it fast
When the product of a gene is being
made we say the gene is being
expressed
Genes are:
Rarely expressed
Constantly expressed
Turn on and off
The Operon
Genes that work together are clustered
together
Some genes in the cluster do not code
for proteins instead they are involved in
regulation and expression
Operon
Genes that work together
Operator
Promotor
There is slight overlap between the
operator and the promotor
Inducer
Molecule that causes the production of
a protein
To Make a Protein
RNA polymerase must attach to the
promoter (“Start here”)
Moves along to the genes  mRNA
The Repressor
Special protein
Attaches itself to the RNA between the
promotor and the genes
Does not let RNA polymerase make a
protein
Turns off genes
Each repressor has a special shape that
allows it to attach to a specific piece of
RNA
Gene Activation
When an inducer enters a cell it binds
to the repressor
The repressor changes shape and can
no longer bond
RNA polymerase can then attach
Proteins  eats up the inducer 
repressor attaches again
Ex. Lactose – sugar – food for bacteria
Gene Expression in Eukaryotes
More complex than Eukaryotes
More DNA in a nucleus
1976 – Sharp and Berget
Discovered mRNA produced during
transcription may be altered before it is
used to make a protein
DNA  mRNA  not an exact copy as
was thought – not complementary
Exons
Sequences that code for a protein
Expressed sequences
Introns
Segments that do not code for a protein
Intervening sequences
IN the way