Genes and Chromosomes
Gene Linkage, Crossing Over,
Codominance and Incomplete Dominance
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.