Extensions to Mendel
Unit 4
Genetics
Color Blindness
• X-Linked Recessive Condition
X
Xc
X
XX
XXc
Y
XY
XcY
Physiology of Color Blindness
• Defective Cone Cells which detect color
• Shift the wavelengths of light so the signal sent
to the brain is the wrong color.
• L, M and S wavelengths (Long, Medium and Short)
• Mixture of abnormalities with cones (missing…)
Most Common- M Cone
The eye
• Rods
and
Cones
are on
the cells
of the
retina
Are You
Color Blind?
Everyone can see these shapes
People who are colorblind cannot see the
circle.
Example: Find the Red Crayon
Key
Questions
1) Can a female inherit colorblindness from dad?
Yes - nearly 100%
2) Does this mean that she will be colorblind?
No - need to factor in the other X chromosome from
mom
3) Why?
It’s a recessive XcX-Normal vision XcXc-colorblind
4) Can a male inherit color blindness from dad?
No, a male only inherits the Y chromosome from dad
Exceptions to Mendelian ratios:
1 Incomplete or Codominance - Two or more
alleles exist, but none is dominant to the other/s
2 Multiple alleles for a single gene
3 Epistasis - interactions b/t more than one gene
4 Sex-linkage - locus of a gene is on a sex
chromosome
5 Sex influenced or limited expression is
influenced or limited by gender (hormones…)
Incomplete Or Codominance
• Incomplete or codominance - Two or more
alleles exist, but none are dominant to the
other
• Incomplete dominance results in blending of
the parental traits
• Example: In some flowers red crossed with
white results in pink F1 generation
RED +
Incomplete Or Codominance
In the F2 generation a 1:2:1 ratio results of
red to pink to white
• F2 results shown below
CRCR CWCW
F2 Generation
1:2:
C R CW
CR
CW
CR CR CR CR CW
CW CRCW CWCW
Epistasis
1
2
A
B
C
 pathway
produces a red pigment, C, in
flowers and that A is a colorless precursor and
B is a yellow intermediate
A
 If
X
1
B
2
C
the gene for enzyme 1 was knocked out, the
flower would be colorless
Epistasis (cont.)
A
 If
1
B
X
2
C
the gene for enzyme 2 was knocked out, the
flowers would be yellow
Epistasis
 If
both genes were knocked out, the flowers
would be colorless
A
X
1
B
X
2
C
1F1n2F2n X 1F1n2F2n
Confusing
1F2F
1F2n
1n2F
1n2n
F1F2F2F1F1F2F2n1F1n2F2F1F1n2F2n
F
F
1
1 2
1F2n 1F1F2F2n1F1F2n2n1F1n2F2n1F1n2n2n
1n2F 1F1n2F2F1F1n2F2n1n1n2F2F1n1n2F2n
F1n2F2n1F1n2n2n1n1n2F2n1n1n2n2n
n
n
1
1 2
1F1n2F2n X 1F1n2F2n
Mendelian- 9:3:3:1
1F2F
1F2n
1n2F
1n2n
F1F2F2F1F1F2F2n1F1n2F2F1F1n2F2n
F
F
1
1 2
1F2n 1F1F2F2n1F1F2n2n1F1n2F2n1F1n2n2n
1n2F 1F1n2F2F1F1n2F2n1n1n2F2F1n1n2F2n
F1n2F2n1F1n2n2n1n1n2F2n1n1n2n2n
n
n
1
1 2
1F1n2F2n X 1F1n2F2n
Epistatic
1F2F
1F2n
1n2F
1n2n
F1F2F2F1F1F2F2n1F1n2F2F1F1n2F2n
F
F
1
1 2
1F2n 1F1F2F2n1F1F2n2n1F1n2F2n1F1n2n2n
1n2F 1F1n2F2F1F1n2F2n1n1n2F2F1n1n2F2n
F1n2F2n1F1n2n2n1n1n2F2n1n1n2n2n
n
n
1
1 2
A 9:4:3 Ratio

A biochemical pathway like this one = 9:4:3 ratio

as long as there are two alleles each of which behaves in a
simple dominant/recessive way
The 9:4:3 ratio is really a 9:(3+1):3 ratio
 Other possible phenotypic ratios for a dihybrid cross
involving epistasis are below:

– 9:7 = 9:(3+3+1)
– 12:3:1 = (9+3):3:1
– 12:4 =(9+3):(3+1)

– 10:3:3 = (9+1):3:3
– 10:6 = (9+1):(3+3)
– 13:3 = (9+1+3):3
A ratio made up of some combination of 9:3:3:1 is
generally a good hint that epistasis is at work