MULTIPLE ALLELIC
Sometimes a gene has more than two alleles. A gene with more than two alleles is
said to have multiple alleles or is described as multiple allelic. The inheritance of the
human ABO blood groups is an example.
ABO BLOOD GROUPS
There are three alleles associated with the immunoglobin gene (I) determining an
individual’s blood type. These alleles lead to the production of different antigens on
the surface membrane of red blood cells as follows:
Allele IA leads to the production of antigen A
Allele IB leads to the production of antigen B
Allele IO does not lead to the production of any antigen
The alleles IA and IB are codominant, whereas the allele IO is recessive to both.
Example 1
Determine the genotypes and phenotypes of offspring expected when a man with
type AB blood has children a woman with type O blood.
PARENTAL PHENOTYPE: blood group AB X blood group
O
PARENTAL GENOTYPE: IA IB X IO IO
GAMETES: IA IB X IO IO
Male Gametes
Female Gametes IA IB
IO IA IO – blood group A IB IO – blood group B
IO IA IO – blood group A IB IO – blood group B
F1 OFFSPRING GENOTYPES: IA IO , IA IO, IB IO , IB IO
F1 OFFSPRING PHENOTYPES: blood type A, blood type A, blood type B, blood
type B.
Example 2
A man with type B blood, whose mother had type O blood, has a child with his wife
who has type AB blood. What is the probability that this child has type B blood?
PARENTAL PHENOTYPE: blood group B X blood group AB
PARENTAL GENOTYPE: IOIB X IAIB
GAMETES: IO IB X IA IB
Male Gametes
Female Gametes IO IB
IA IO IA – blood group A IA IB – blood group AB
IB IO IB – blood group B IB IB – blood group B
F1 OFFSPRING GENOTYPES: IO IA, IA IB, IO IB, IB IB
FI OFFSPRING PHENOTYPES: blood group A, blood group AB, blood group B,
blood group B
PROBERBILITY OF BLOOD TYPE B: 50%
MULTIPLE ALLELES AND A DOMINANCE HIERARCHY
In blood groups IA and IB are codominant. Sometimes, however, the alleles are
arranged in a hierarchy with each allele being dominant to those below it and
recessive to those above it. One example is coat colour in rabbits. The gene for
coat colour has four alleles as follows:
CA – agouti coat
Ca – albino coat
Ch – Himalayan coat
Cg – chinchilla coat
The allele for agouti coat is dominant to all of the other 3 alleles. The allele for albino
coat is recessive to the other alleles. The allele for chinchilla is dominant to the
allele for Himalayan coat.
Example 1
a. Write down the ten possible genotypes for coat colour and their phenotypes.
Do this in the form of a table.
b. Draw genetic diagrams to explain each of the following:
i. An albino rabbit is crossed with a chinchilla rabbit producing offspring which are all
chinchilla. Two of these chinchilla offspring are then crossed producing 4 chinchilla
and 2 albino offspring.
ii. An agouti rabbit is crossed with a Himalayan rabbit producing 3 agouti offspring
and 3 Himalayan offspring
iii. Two agouti rabbits produce a litter of 5 young, three of whom are agouti and two
chinchilla. The two chinchilla young are then crossed, producing 4 chinchilla
offspring and 1 Himalayan.
a. Write down the ten possible genotypes for coat colour and their phenotypes.
Do this in the form of a table.
Genotypes Phenotypes
CACA agouti
CACg agouti
CACh agouti
CACa agouti
CgCg chinchilla
CgCh chinchilla
CgCa chinchilla
ChCh Himalayan
ChCa Himalayan
CaCa albino
b i. An albino rabbit is crossed with a chinchilla rabbit producing offspring which are
all chinchilla. Two of these chinchilla offspring are then crossed producing 4
chinchilla and 2 albino offspring.
Parental phenotype: albino X chinchilla
Parental genotype: CaCa X CgCg
Gametes: Ca X Cg
F1 genotypes: CaCg
F1 phenotype: chinchilla
F1 self-crossed: CaCg X CaCg
Gametes: Ca Cg X Ca Cg
Male gametes
Female gametes Cg Ca
Cg CgCg - chinchilla CgCa - chinchilla
Ca CgCa -chinchilla CaCa - albino
Phenotype ratio: 3 chinchilla: 1 albino
With this expected ratio it is therefore possible to produce a litter with 2 albino and 4
chinchilla.
ii. An agouti rabbit is crossed with a Himalayan rabbit producing 3 agouti offspring
and 3 Himalayan offspring.
Parental phenotypes: Agouti X Himalayan
Parental genotypes: CACh X ChCh
Gametes: CA Ch X Ch Ch
Male gametes
Female gametes CA Ch
Ch CACh - agouti ChCh – Himalayan
Ch CACh - agouti ChCh- Himalayan
F1 phenotype ratio: 1 agouti: 1 Himalayan
With this expected ratio it is therefore possible to produce a litter with 3 chinchilla
and 3 Himalayan.
iii. Two agouti rabbits produce a litter of 5 young, three of whom are agouti and two
chinchilla. The two chinchilla young are then crossed, producing 4 chinchilla
offspring and 1 Himalayan.
Parental phenotype: agouti X agouti
Parental genotype: CACh X CACg
Gametes: CA Ch X CA Cg
Male gametes
Female gametes CA Ch
CA CACA -agouti CACh - agouti
Cg CACg - agouti ChCg - chinchilla
Chinchilla F1 self-crossed: ChCg X ChCg
Gametes: Ch Cg X Ch Cg
Male gametes
Female gametes Ch Cg
Ch ChCh - Himalayan CgCh - chinchilla
Cg CgCh - chinchilla CgCg - chinchilla
F2 phenotype ratio: 3 chinchilla: 1 Himalayan
With this expected ratio it is therefore possible to produce a litter with 4 chinchilla
and 1 Himalayan.
