Multiple Alleles
• For each trait observed by Mendel….
• Only 2 alleles existed (T & t, for example)
• Many traits are controlled by multiple
alleles
• 2 examples include
–Eye color in Drosophila (fruit
flies)
–Blood types in humans
•
•
•
•
•
Eye Color in
Drosophila
4 alleles control eye color
E1 = Wild
E2 = Apricot
E3 = Honey
E4 = White
1
•E
>
2
E
>
3
E
>
4
E
Normal Fruit Fly
Mutant Fruit Flies
Short winged fly
Normal fruit fly
Curly winged fly
Eye Color in Drosophila
• Phenotypes Genotypes
• Wild Type
E1E4
• Apricot
• Honey
• White
E1E1, E1E2, E1E3,
E2E2, E2E3, E2E4
E3E3, E3E4
E4E4
Predict the phenotypic outcome if a honey colored
heterozygous fly is crossed with a white fly!
E3
E4
E4
E3E4 E4E4
E4
E3E4 E4E4
Phenotypes
2 Honey : 2 White
Genotypes
2 E3E4 : 2 E4E4
What are the possible phenotypes of the parents in
a cross which produces the following offspring?
?
?
?
E1E2 E1E3
?
E2E3 E3E3
Genotpes
E1E3 : E2E3
Phenotypes
Wild & Apricot
Another Example of Multiple Alleles
Possible
alleles
C
cch
ch
c
Antigen type
Determines
Blood type
Blood
Types
Person with
A A
blood type A A
A
A
Has antigen
A
A
A
A
on RBC’s
A A
&
Antibody B
(floating in plasma)
Blood Typing in
Humans
• Another example of multiple alleles
• Controlled by 3 alleles
• IA, IB and i
• IA and IB are said to be codominant
• If a person inherits both of them, then
antigen A and antigen B are on the RBC’s
•i is recessive to
B
I
A
I
and
Blood Components
Antigens A, B or Rh
may be present
on RBC surface
Genotypes and
Phenotypes
for ABO Blood Types
• Genotype Phenotype
•
•
•
•
IAIA, IAi
IBIB, IBi
IAIB
ii
A antigen
B antigen
A & B antigen
neither antigen
Blood Type
A
B
AB
0
Predict the phenotypic outcome if a person with
blood type AB mates with a person with type O!
IA
IB
i
IAi
IBi
i
IAi
IBi
Phenotypes
1 Type A : 1 Type B
Genotypes
2 IAi : 2 IBi
Blood Typing Problem
1
Mom
Blood Type A
Dad
Unknown
Blood Type
IA IA
or
IA i
Baby
Blood Type B
IBIB or IBi
Must carry
IB__
I BI B
IBi
IA IB
Blood Typing Problem 1
Dad
Unknown
Blood Type
Mom
Blood Type B
Must carry
IB IB
or
IB i
Baby
Blood Type O
i__
IBi
IA i
ii
ii
Punnet Squares for Codominance
Calculate the probability of producing a son who
is blood type A from the following cross!
Must
Use
Product
Rule
IA
IB
i
IAi
IBi
i
IAi
IBi
Probability of
producing a son
= 0.5
Answer
0.5 X 0.5 =
0.25 or
25%
Prob. Of
producing Type A
= 0.5
Blood Typing and Gender
• Calculate the probability of producing a
daughter who is blood type A from a cross
between a Type B mother and a Type A
father, given that each of these individuals
have a parent with Type O.
• Since parents are ii, both must be carriers
for i
• Mother must be IBi, Father must be IAi
• Prob. of producing Type A = 0.25
• Prob. of producing a daughter = 0.50
• Therefore prob of producing Type A
daughter must be found with the product
rule……..
• 0.25 ( 0.5) = 0.125
or 1/8 or 13%
• Diploma questions ask for answer
rounded to 2 decimal places or to 2
significant digits
Genetics Problem Set
www.biology.arizona
The Product Rule
Incomplete
Dominance
Intermediate
Inheritance
Snapdragons
Red X White =
Pink!!
Blending occurs
Codominance
Cattle
Red X White = Roan
Both genes
expressed
White and red
hair present
Red X White =
Pink
Gene for red (R) is incompletely dominant
to gene for white (W)
Incomplete Dominance: Blending/3 distinct phenotypes
Predict the phenotypic outcome if a Red (RR)
snapdragon is crossed with a White (WW) sd!
R
R
W
RW
RW
W
RW
RW
Phenotypes
All pink!
Capital Letters
used for
both alleles
since
they are equally
Dominant
Genotypes
All RW
Predict the phenotypic outcome if a red (RR)
snapdragon is crossed with a pink (RW) sd!
R
R
R
RR
RR
W
RW
RW
Phenotypes
2 Red : 2 pink
Genotypes
2 RR : 2 RW
Intermediate?
Or
Co-dominant?
Ask
yourself
does
blending
occur??
Predict the
genotypes of
the parents!
Rr & Rr