Independent Assortment &
Non-Mendelian Genetics
EQ: What are the other outcomes of a
cross?
Learning Goal - Genetics
4 – I can predict the offspring of different crosses
of parents and explain how the traits were
inherited. I can apply monohybrid crosses to a
dihybrid cross (Ex: RRYY x rryy)
3 – I can usually find the probability of the offspring
and I understand meiosis
2 – I can do basic Punnett squares but still need
help setting it up
1 – I need to review genetics
I. Mendelian Genetics:
A. Dominant & Recessive Review
One allele is DOMINANT over the other
(because the dominant allele can “mask” the
recessive allele)
genotype: PP
genotype: pp genotype: Pp
phenotype: purple phenotype: white phenotype: purple
Review Problem:
A. Dominant & Recessive
In pea plants, purple flowers (P) are dominant over
white flowers (p). Show the cross between two
heterozygous plants.
GENOTYPES:
- PP (25%)
Pp (50%)
pp (25%)
- ratio 1:2:1
PHENOTYPES:
- purple (75%)
white (25%)
- ratio 3:1
P
p
P
PP
Pp
p
Pp
pp
B. Two Cross/Dihybrid Cross
• Mendel wondered if one allele for one
gene would influence another allele for a
separate gene
• RRYY x rryy
• (R) is round, (r) is wrinkled
• (Y) is yellow, (y) is green
– He found the two factor/dihybrid cross
Example 1: F1 RRYY x rryy
Example 2: F2 RrYy x RrYy
9:3:3:1
What did Mendel find?
In a two factor cross, the alleles would
separate independently (not linked).
Ratio for the F2 generation was 9:3:3:1
Review Dihybrid Crosses
Tall = TT
Tall = Tt
Short = tt
Red = RR
Red = Rr
Yellow = rr
• Cross two heterozygous plants
for both traits.
TtRr X TtRr
TtRr X TtRr
TR
Tr
tR
tr
TR TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
TtRr X TtRr
NNDD x NnDd
II. Non-Mendelian Genetics
A. Incomplete Dominance
B. Codominance
C. Multiple Alleles
D. Polygenic Traits
E. Sex-Linked Traits
A. Incomplete Dominance
a third (new) phenotype
appears in the heterozygous
condition as a BLEND of the
dominant and recessive
phenotypes.
Ex - Dominant Red (R) + Recessive White (r) = Hybrid
Pink (Rr)
RR = red
rr = white
Rr = pink
Example 3:
Incomplete Dominance
Show the cross between a red and a white flower.
GENOTYPES:
- RR (0%)
Rr (100%)
rr (0%)
- ratio 4:0
R
R
r
Rr
Rr
r
Rr
Rr
PHENOTYPES:
- pink (100%); white (0%);
red (0%)
Example 4:
Incomplete Dominance
Show the cross between a pink and a white flower.
GENOTYPES:
- RR (0%)
Rr (50%)
rr (50%)
- ratio 1:1
R
r
r
Rr
rr
r
Rr
rr
PHENOTYPES:
- pink (50%); white (50%)
- ratio 1:1
B. Codominance
in the heterozygous condition, both alleles are expressed
equally with NO blending! Represented by using two
DIFFERENT capital letters.
Example:
Dominant Black (B) + Dominant White (W) =
Speckled Black and White Phenotype (BW)
Codominance Example:
Speckled Chickens
BB = black feathers
WW = white feathers
BW = black & white speckled feathers
Notice –
NO GRAY!
NO BLEND!
Each feather is
either black or white
Codominance
Example 5:
Rhodedendron
R = allele for red flowers
W = allele for white flowers
Cross a homozygous red
flower with a homozygous
white flower.
Note that it’s not blended or light pink
Codominance Example:
Roan cattle
cattle can be
red
(RR – all red hairs)
white
(WW – all white hairs)
roan
(RW – red and white hairs together)
Codominance Example 6:
Appaloosa horses
Gray horses (GG) are codominant to white horses
(WW). The heterozygous horse (GW) is an Appaloosa
(a white horse with gray spots).
Cross a white horse with an appaloosa horse.
W
W
G
GW
GW
W
WW
WW
Codominance Example #7
Sickle Cell Anemia – blood disorder commonly found in
Africans (1 in 500)
NN =
normal cells
SS = sickle cells
NS = some of
each
Example 7: Sickle Cell
Codominance
Show the cross between an individual with
sickle-cell anemia and another who is a carrier
but not sick.
N
S
GENOTYPES:
- NS (50%)
SS (50%)
- ratio 1:1
S
NS
SS
PHENOTYPES:
- carrier (50%)
sick (50%)
- ratio 1:1
S
NS
SS
C. Multiple Alleles
there are more than two alleles for a gene.
Ex – blood type consists of two dominant
and one recessive allele options.
Allele A & B are
dominant over
Allele O (i)
Multiple Alleles:
Blood Types (A, B, AB, O)
Rules for Blood Types:
A and B are co-dominant (Both show)
AA or IAIA = type A
BB or IBIB = type B
AB or IAIB = type AB
A and B are dominant over O (Regular dom/rec)
AO or IAi = type A
BO or IBi = type B
OO or ii = type O
Multiple Alleles:
Blood Types (A, B, AB, O)
Allele
Can
Can
(antigen) Donate Receive
Possible
on RBC Blood
Blood
Phenotype Genotype(s) surface
To
From
A
IAi
IAIA
A
A, AB
A, O
B
IBi
IBIB
B
B, AB
B, O
AB
AB
A, B,
AB, O
O
A, B,
AB, O
O
AB
O
IAIB
ii
Example 8:
Multiple Alleles
Show the cross between a mother who has type
O blood and a father who has type AB blood.
GENOTYPES:
- Ai (50%)
Bi (50%)
- ratio 1:1
PHENOTYPES:
- type A (50%)
type B (50%)
- ratio 1:1
i
i
A
Ai
Ai
B
Bi
Bi
Example 9:
Multiple Alleles
Show the cross between a mother who is heterozygous for
type B blood and a father who is heterozygous for type A
blood.
GENOTYPES:
-AB (25%); Bi (25%);
Ai (25%); ii (25%)
- ratio 1:1:1:1
A
i
B
AB
Bi
PHENOTYPES:
i
-type AB (25%); type B (25%)
type A (25%); type O (25%)
- ratio 1:1:1:1
Ai
ii
Multiple Alleles:
Lab Mouse Fur Colors
Fur colors (determined by 4 alleles):
black
agouti
yellow & (white)
Multiple Alleles:
Rabbit Fur Colors
Fur colors (determined by 4 alleles):
full, chinchilla, himalayan, albino
D. Polygenic Traits
traits produced by multiple genes
example: skin color, tallness, eye color
E. Sex-Linked Traits
Gene is attached to the X
chromosome only, not
found on the Y
chromosome at all.
(women have XX, men
have XY chromosomes).
These disorders are more
common in boys.
Examples:
red-green colorblindness
Hemophilia
baldness
Sex-Linked Traits
in males, there is
no second X
chromosome to
“mask” a recessive
gene. If they get an
X with the disorder,
they have it. Girls
must inherit
defective X’s from
both parents.
Sex-Linked Traits
A: 29, B: 45, C: --, D: 26
 Normal vision
A: 70, B: --, C: 5, D: - Red-green color blind
A: 70, B: --, C: 5, D: 6
 Red color blind
A: 70, B: --, C: 5, D: 2
 Green color blind
Chromosomal Disorders
When too
many/few
chromosomes
produced from
meiosis
Trisomy 21 – Down Syndrome
Other Chromosomal Abnormalities
• http://learn.genetics.utah.edu/content/disor
ders/chromosomal/