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Genes and Chromosomes
Gene Linkage, Crossing Over,
Codominance and Incomplete Dominance
Mendel and Chromosomes
Mendel never asked “Where in the cell are
the factors that control heredity?”
i.e. where are the genes
He couldn’t have answered the question
though
His promotion to abbot kept him too busy
Where are they?
By 1900 cell biologists had discovered
most organelles and the process of
mitosis and meiosis
Nucleus was logical place
Why the Nucleus?
Central Location
Large size
Lots of activity during mitosis and meiosis
Chromosome Theory of Inheritance
Walter Sutton – Columbia University
Genes are located on chromosomes
Each gene occupies a specific place on a
chromosome
A gene may exist in several forms, or alleles
Each chromosome contains one allele for each
gene
Thomas Morgan
Embryologist at Columbia University
Worked with fruit flies
Fruit Flies?! …Yuck!
Drosophila melanogaster
Prolific reproducers:
a single cross will
produce 100s of offspring
4 pairs of chromosomes
Larva hatch
within
two weeks
Genes on a Chromosome
Each chromosome has
hundreds or thousands
of genes
Genes on the same
chromosome tend to be
inherited together
Remember: a
chromosome is passed
along as a single unit
during meiosis
Thomas Morgan
First to associate a specific gene with a specific
chromosome
Crossed gray, normal winged homozygous
parents with black, vestigial wing parents.
– GGNN x ggnn = GgNn
– Then test crossed GgNn x ggnn
Let’s do the Punnet Square:
Expected Punnet 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
Expected:
25% Gray, Normal wing
25% Gray, Vestigial wing
25% Black, Normal wing
25% Black, Vestigial wing
Actual Phenotypes:
41.5 % Gray, Normal
41.5% Black, Vestigial
8.5% Gray, Vestigial
8.5% Black, Normal
If they are linked…
Let’s let G = G and N
Let’s let g = g and n
So GG would be GGNN and …
Gg would be GgNn and …
gg would be ggnn.
If they are linked…
G
g
g
Gg
gg
g
Gg
gg
If they are linked…
Don’t forget that Gg is GgNn and gg is ggnn
So we have 50% Gray bodied, Normal wing
And 50% Black bodied, Vestigial wing
This explains the variation from the original
expected.
Morgan’s Hypothesis
Body color (gray or black) and wing
shape (normal or vestigial) are
usually inherited together in a
specific combination due to GENE
LINKAGE
Gene Linkage
Genes located on the same
chromosome tend to be inherited
together because they are part of a
single chromosome that is passed
along as a unit.
Morgan’s Crosses
83% of Morgan’s flies had
characteristics of their parents.
(gray/normal wings or black/
vestigial wings.)
(41.5% gray/normal + 41.5%
black/vestigial =83%)
17% of the flies had either
a)Gray, vestigial wings
b)Black, normal wings
(8.5% gray, vestigial + 8.5% black,
normal = 17%)
Morgan and Sturtevant (his associate)
hypothesized that linkages could
sometimes be broken
Gray/normal and black/vestigial genes
can occasionally be separated
INTERESTING QUESTION:
If body color and wing shape
genes are linked, how come all of
the offspring were not like their
parents (gray/normal and
black/vestigial)?????????????
CROSSING OVER IS TO
BLAME!!!!!
CROSSING OVER
DURING MEIOSIS,
HOMOLOGOUS
CHROMOSOMES EXCHANGE
PORTIONS OF THEIR
CHROMATIDS
CROSSING OVER
GENE MAPPING
Distance between chromosomes
determines how often crossing over
occurs
Further away = more crossing over
Frequency of crossing over allows
mapping of position on chromosomes
Gene Mapping Diagram
Genes A & B are very
close together
C & D are farther apart
A random crossover
event is much more likely
to separate genes C & D
than A and B (which are
closer together).
Sex Linkage
1 exception to rule that every
chromosome has a corresponding
homologous chromosome …
1905 American – Nettie Stevens –
mealworms
– Female contains 20 large
chromosomes
– male – 19 and one small
These are sex chromosomes
– Female: XX
– Male: XY
Because X and Y determine sex, genes
located on one of these chromosomes are
called “Sex-Linked genes”
HERE’S AN EXAMPLE:
Genes associated with vision in humans
are located on the X chromosome.
– Males only have ONE X chromosome!!!!
This can be a problem if the X
chromosome given to him by Mom has a
genetic disorder!
Red-Green Color Blind?
Red-Green Color Blind?
Colorblindness:
Your mother unknowingly has the
C
c
genotype X X . Dad is not
colorblind and therefore has the
genotype XCY.
Will any of the children be
colorblind??????
XC
Xc
XC
XCXC
XCXc
Y
XCY
XcY
Answer: Yes!
One of the sons
will be colorblind
because he will
have the
genotype XcY.
Revenge against Men!
Males have just one X chromosome.
Therefore, if the mom gives them a
recessive allele on her X chromosome,
it will be expressed in the son EVEN
though the mom’s phenotype did not
express the allele!
INCOMPLETE DOMINANCE
Josef Kolreuter – crossed white
carnations (rr) with Red (RR)
carnations.
Got all Pink!!! (Rr)
INCOMPLETE DOMINANCE
Did red and
pink blend
together????
DID THE COLORS BLEND????
NO!!!!!!! How do we know????
– When crossed Rr x Rr (F1 x F1)…….
– ¼ Red
– ½ Pink
– ¼ White
Let’s do the Punnett Square
PUNNET SQUARE
25% RR: Red
50% Rr: Pink
25% rr: White
R
r
R
RR
Rr
r
Rr
rr
INCOMPLETE DOMINANCE



When mom makes a birthday cake and
wants to frost it, how does she do it?
White frosting
Wants to make red frosting
 no red food color makes white
 little red food color makes pink
 lots of red food color makes red
INCOMPETE DOMINANCE
Same with genes
Genes code for a protein
If the protein is a pigment, like carnations:
– no dose = white
– single dose = pink
– double dose = red
CODOMINANCE
What does dominant mean?
In codominance, both alleles are expressed
CODOMINANCE EXAMPLE
On sports teams, often have two
captains. Do both captains have
equal representation?
Sure they do!!!!
CODOMINANCE
The same is true for codominant
alleles…both are equally expressed within
a phenotype.
Written as a capital letter with a superscript
HR or HW means the trait is hair color and
red and white are both dominant.
CATTLE:
Cattle – Red hair (HR) and White hair
(HW) are codominant
– HRHR= Red hair
– HW HW= White hair
– HR HW = roan or combination – rust
colored (Red and white)
Try This!
What would you get if you crossed
a Roan with a White haired cow?
Do the punnett square:
PUNNET SQUARE
HR
HW
HW
HR HW
HWHW
HW
HR HW
HWHW
25% ROAN
50% WHITE
NONDISJUNCTION
Whole chromosomes or sets of
chromosomes fail to separate normally
during meiosis
Example: Down’s syndrome
When all chromosomes fail to separate,
can cause 3N or 4N organisms.