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Multiple Alleles and Polygenic
Traits
Multiple Alleles:
when more than two different alleles
exist for the same trait.
 (Remember: each individual will only have two alleles for
a trait but there are several alleles to choose from.)
In rabbits, coat color is controlled by
multiple alleles. Full color (C), white
(c), light-gray or chinchilla (cch) and
white with black points or a Himalayan
(ch). Full color is dominant to all the
other alleles. Chinchilla is dominant to
Himalayan and white.
Cross a heterozygous Chinchilla
(cchc) with a heterozygous full color
with himalayan (Cch).
cchc x Cch
ch
c
C
ch
c
ch
c
C
ch
ch
Cc
c
C
ch
ch
c
c
ch
Cc
Cc
ch
c
c
C
ch
Cc
Cc
ch
cchch
ch
c
c
C
ch
Cc
Cc
ch
cchch
chc
Genotypic Ratios:
1 Ccch: 1Cc : 1 cchch: 1 chc
Phenotypic Ratios:
2 full color: 1 Chinchilla: 1
Himalayan
In humans, blood type has multiple
alleles. IA and IB are codominant
and i (type O blood) is recessive.
Blood Type
Type A
Type B
Type AB
Type O
Type of Antigen
Possible Genotypes
Blood Type
Type A
Type B
Type AB
Type O
Type of Antigen
A antigen
Possible Genotypes
Blood Type
Type A
Type B
Type AB
Type O
Type of Antigen
Possible Genotypes
A antigen
IAIA, IAi
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
Type AB
Type O
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
Type O
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
Type O
A and B antigens
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
A and B antigens
IAIB
Type AB
Type O
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
A and B antigens
IAIB
Type AB
Type O
No antigens
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
A and B antigens
IAIB
No antigens
ii
Type AB
Type O
Cross a heterozygous type A with a
heterozygous type B.
IAi x ___
Cross a heterozygous type A with a
heterozygous type B.
IAi x IBi
A
I
B
I
i
i
A
I
B
I
i
A
B
I I
i
B
I
i
A
I
i
A
B
I I
B
I i
A
I
i
B
I
A
B
I I
B
I i
i
IAi
A
I
i
B
I
A
B
I I
B
I i
i
IAi
ii
Genotypic Ratios:
1 IAi : 1 IAIB : 1 IBi : 1 ii
Phenotypic Ratios:
1 Type A: 1 Type AB: 1 Type B:
1Type O
Cross a person with type AB blood
with another person with type O
blood.
IAIB x ___
Cross a person with type AB blood
with another person with type O
blood.
IAIB x ii
A
I
i
i
B
I
A
I
i
i
A
I i
B
I
i
i
A
I
B
I
A
I i
B
I i
A
I
B
I
i
A
I i
B
I i
i
IAi
A
I
B
I
i
A
I i
B
I i
i
IAi
IBi
Genotypic Ratios:
2 IAi : 2 IBi
Phenotypic Ratios:
2 Type A: 0 Type AB: 2 Type B: 0
Type O
Polygenic Inheritance:
the inheritance pattern is controlled
by two or more genes each with
two alleles.
In humans, eye color is the result of polygenic
inheritance. At the present, three gene pairs controlling
human eye color are known. Two of the gene pairs
occur on chromosome pair 15 and one occurs on
chromosome pair 19. The bey 2 gene, on chromosome
15, has a brown and a blue allele. A second gene,
located on chromosome 19 (the gey gene) has a blue
and a green allele. A third gene, bey 1, located on
chromosome 15, is a central brown eye color gene.
The brown allele is always dominant over the blue allele
so even if a person is heterozygous (one brown and
one blue allele) for the bey 2 gene on chromosome 15
the brown allele will be expressed. The gey gene also
has two alleles, one green and one blue. The green
allele is dominant to the blue allele on either
chromosome but is recessive to the brown allele on
chromosome 15. This means that there is a dominance
order among the two gene pairs. If a person has a
brown allele on chromosome 15 and all other alleles
are blue or green the person will have brown eyes. If
there is a green allele on chromosome 19 and the rest
of the alleles are blue, eye color will be green. Blue
eyes will occur only if all four alleles are for blue
eyes.
Polygenic inheritance usually
shows up as a range of variation
such has height, skin color or hair
color.
The results of an average
population will result in a bellshaped curve.