dihybrid crosses and blood typing n.
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Dihybrid Crosses and Blood Typing. Dihybrid Crosses. Have space provided for 4 gamete alleles from two parents. YYRR Pure Dominant y yrr Pure Recessive Y – yellow R – round seeds F1 – 100 % 4:0 dominant for both traits Phenotype – 100% Yellow and Round. F1 Hybrid Cross. YyRr

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dihybrid crosses
Dihybrid Crosses
  • Have space provided for 4 gamete alleles from two parents.

YYRR Pure Dominant




Y – yellow R – round seeds

F1 – 100 % 4:0 dominant for both traits

Phenotype – 100% Yellow and Round

f1 hybrid cross
F1 Hybrid Cross



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
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
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 co-dominance.
multiple alleles
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
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 co-dominant.

The usual way to represent alleles in a multiple allele system is to use the capital letter I to represent a co-dominant 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
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
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
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.

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 transfusions
Blood Transfusions
  • Rule: Match the antigen of the donor with the antibodies of the recipient.