DIHYBRID CROSSES
AND BLOOD TYPING
DIHYBRID CROSSES
• Have space provided for 4 gamete alleles from
two parents.
YYRR Pure Dominant
yyrr
Pure
Recessive
YR
YR
YR
YR
yr
YyRr
YyRr
YyRr
YyRr
yr
YyRr
YyRr
YyRr
YyRr
yr
YyRr
YyRr
YyRr
YyRr
yr
YyRr
YyRr
YyRr
YyRr
Y – yellow R – round seeds
F1 – 100 % 4:0 dominant for both traits
Phenotype – 100% Yellow and Round
F1 HYBRID CROSS
YyRr
YyRr
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
Phenotypes: Yellow and Round 9
Yellow and Wrinkled 3
Green and Round 3
Green and Wrinkled 1
Phenotype 9:3:3:1 but each trait by itself is still 3:1
YR 9:3 GR
GR 3:1 GW
TEST CROSS
• An individual showing a dominant trait is crossed
with a recessive.
• If any offspring show the recessive trait, the test
individual must be a hybrid.
• This is done by breeders because it is not possible to
tell from appearance alone whether an individual
showing a dominant trait is pure for the trait
(homozygous) or hybrid (heterozygous).
• Breeders of plants and animals often need this
information about their parent stock (and keep
detailed breeding records).
INCOMPLETE DOMINANCE
• In some organisms both alleles contribute to the
phenotype of a heterozygous individual. This is also
known as blending inheritance.
• In a hybrid you will get a new colour which is a
blend of the dominant trait a recessive trait.
CO-DOMINANCE
• Two dominant alleles are expressed at the same time.
Because both alleles are expressed there is no blending
of traits.
• RR- red shorthorn cattle
• WW – white shorthorn cattle
• RW – roan coat ( a mixture of both red hairs and white
hairs).
• Capital letters with superscripts are often used to
represent genotypes in co-dominance.
• CR red coat CW white coat
• CRCR homozygous red CWCW homozygous white
• CRCW roan coat
• Co-dominance occurs in human heredity. Sickle
cell anemia is controlled by co-dominance.
• Blood type inheritance is another example od codominance.
MULTIPLE ALLELES
• For some traits more than two alleles exist in the
species. These are referred to as multiple alleles.
• An individual cannot have more than two alleles
for each trait, but different individuals can have
different pairs of alleles when multiples exist.
• The alleles for human blood type is an example of
multiple alleles.
ABO BLOOD GROUPS
• There are four major types of human blood.
• There are antigens (a substance that can cause a
response of the immune system) on the surface of red
blood cells.
1) A antigens (type A blood)
2) B antigens (type B blood)
3) Both a and B antigens (type AB blood)
4) Neither A nor B antigens (type O blood)
There are 3 alleles that control blood type: A,B and O.
• O Is recessive. A and B are both dominant over O but
neither is dominant over the other making them codominant.
• The usual way to represent alleles in a multiple allele
system is to use the capital letter I to represent a codominant allele and a lower case i to represent a
recessive allele.
• A superscript letter then identifies each particular
co-dominant allele.
IA represents dominant allele A
IB represents dominant allele B
i represents recessive allele O
Since there are 3 alleles, there are 6 possible
genotypes:
1) IAIA
2) IAIB
3) IAi
4) IBIB
5) IBi
6) ii
• The possible human phenotypes for blood group
are type A, type B, type AB, and type O. Type A
and B individuals can be either homozygous (IAIA or
IBIB, respectively), or heterozygous (IAi or IBi,
respectively).
• A woman with type A blood and a man with type B
blood could potentially have offspring with which of
the following blood types?
BLOOD TYPE AND GENOTYPE
The relationship between blood
type (phenotype) and genotype
is shown in the table to the left.
The mother (blood type A) and
father (blood type B) could be
either homozygous or heterozygous.
TYPE A AND TYPE B CROSS
• Four different genetic
crosses are possible.
All four crosses must be
considered to determine
all potential offspring.
HOW ARE BLOOD TYPES RELATED TO
THE SIX GENOTYPES?
• A blood test is used to determine whether the A and/or B
characteristics are present in a blood sample. It is not possible to
determine the exact genotype from a blood test result of either type
A or type B.
• If someone has blood type A, they must have at least one copy of
the A allele, but they could have two copies. Their genotype is either
AA or AO. Similarly, someone who is blood type B could have a
genotype of either BB or BO.
Blood Type
Possible Genotypes
A
AA
AO
B
BB
BO
• A blood test of either type AB or type O is more
informative. Someone with blood type AB must
have both the A and B alleles. The genotype must
be AB. Someone with blood type O has neither the
A nor the B allele. The genotype must be OO.
Blood Type
Possible Genotypes
AB
AB
O
OO
BLOOD TRANSFUSIONS
• Rule: Match the antigen of the donor with the
antibodies of the recipient.
Blood Type
Can Donate To
Can Receive
From
A
A, AB
A, O
B
B,AB
B, O
AB
AB
AB, A, B, O
O
O, A, B, AB
O
Blood Type
A
In Anti-A
Serum
clumping
In Anti-B
Serum
No
clumping
B
AB
O
No
clumping
clumping
No
clumping
clumping
clumping
No
clumping