Codominance and Beyond: Comprehensive Blood Type Genetics & Inheritance Patterns

Codominance

• Codominance: Patterns of inheritance not mentioned by Mendel but still follow his rules.
  • If an offspring has two different alleles, both will be expressed.

Examples of Codominance

• ABO Blood Groups (Lanztiger Blood Groups):
  • If you inherit an A allele from one parent and a B allele from the other, you get AB blood type.
  • Both A and B are expressed on red blood cells.

ABO Blood Groups: Phenotypes

• Possible blood groups (phenotypes):
  • A
  • B
  • AB
  • O

ABO Blood Groups: Genotypes

• Capital I: Stands for isoglutinogen (antigen on the surface of red blood cells).
  • $I^A$: Allele for A blood type.
  • $I^B$: Allele for B blood type.
  • i: Allele for O blood type (recessive).
• A and B are codominant and dominant to O.
  • If you inherit $I^A$ and i, you have A blood type (A dominates).
  • If you inherit $I^B$ and i, you have B blood type (B dominates).
  • If you inherit $I^A$ and $I^B$, you have AB blood type.
  • The only way to have O blood type is to inherit two recessive i alleles.

Genotypes for Each Blood Type

• A blood type:
  • $I^A I^A$
  • $I^A i$
• B blood type:
  • $I^B I^B$
  • $I^B i$
• AB blood type:
  • $I^A I^B$ (only one possibility)
• O blood type:
  • ii (only one possibility)

Explanation:

• For A or B blood types, there are two possibilities because the allele can be paired with a recessive i allele.
• Example: A child with B blood type can have a B allele from the father and an i allele from the mother.

Antigens on Red Blood Cells

• Antigens: Sugars on the surface of red blood cells.

Blood Type A

• Has A sugars (galactosamine) on the surface.

Blood Type B

• Has B sugars (galactose) on the surface.

Blood Type AB

• Has both A and B sugars on the surface (codominance).

Blood Type O

• Has no A or B sugars on the surface.

Antibodies and Blood Transfusions

• Antibodies: Response to what you DON'T have on the surface.
  • If you have an antigen, you tolerate it and don't make antibodies against it.
  • If you don't have an antigen, you make antibodies against it.

O Blood Type

• Makes anti-A and anti-B antibodies.
• Cannot receive A or B blood.

A Blood Type

• Makes anti-B antibodies.

B Blood Type

• Makes anti-A antibodies.

AB Blood Type

• Makes no antibodies against A or B.

Blood Transfusion Compatibility

• O can receive from O only (universal donor).
• A can receive from A and O.
• B can receive from B and O.
• AB can receive from A, B, AB, and O (universal recipient).

Rh Factor

• Second way of typing blood (after ABO).
  • Rh positive or Rh negative.

Distribution (United States)

• 85% Rh positive
• 15% Rh negative
• Rh positive is dominant; Rh negative is recessive.

Antigen D

• Used to denote the Rh factor.

Genotypes for Rh Factor

• Rh positive:
  • DD (two dominant genes)
  • Dd (dominant and recessive)
• Rh negative:
  • dd (two recessive genes)

Antigens in Rh Factor

• Rh positive: Has Rh antigen (D antigen) on the surface of red blood cells.
• Rh negative: Does not have the Rh antigen on the surface.

Antibodies in Rh Factor

• Rh positive: Does not make antibodies against Rh factor because it has it.
• Rh negative: Makes anti-Rh antibodies because it does not have it.

Blood Type Combinations

• A positive: Has A antigen and Rh antigen.
• A negative: Has A antigen and no Rh antigen.

Rh Factor: Blood Transfusion Compatibility

• Rh positive can receive from positive and negative.
• Rh negative can receive from negative only.

Universal Donor and Recipient

• Universal donor: O negative
• Universal recipient: AB positive

Blood Incompatibility During Pregnancy

• Occurs when an Rh negative mother is pregnant with an Rh positive baby.
  • Fetal red blood cells can cross the placenta and enter the mother's body.
  • The mother's immune system recognizes Rh positive blood as foreign and attacks it.

First Pregnancy

• The first encounter takes time to build the attack.
• The baby may develop jaundice or need a blood transfusion but can usually be saved.

Subsequent Pregnancies

• The mother has memory cells and mounts a worse attack.
• Subsequent Rh positive babies may not survive.

Remedy: RhoGAM

• Rh factor gamma globulins or antibodies are injected into the mother.
  • These antibodies destroy any fetal red blood cells that enter her body.
  • This prevents her from developing memory cells.

Statistics

• 15% of the US population is Rh negative, and about half of those are female.

Erythroblastosis Fetalis (Hemolytic Disease of the Fetus and Newborn)

• Occurs when a Rh positive baby is born to a Rh negative mother.
• Red blood cells are ruptured in the fetus.

Genetic Crosses Examples

Cross 1: A blood type x B blood type -> O blood type child:

• Possible with heterozygous parents ($I^A i$ x $I^B i$).

Cross 2: AB blood type x O blood type -> O blood type offspring:

• Not possible because the offspring HAS to have an A or B allele from the AB parent.

Cross 3: O blood type x O blood type -> A blood type offspring:

• Not possible because O blood type is ii, they can only pass on the i allele.

Cross 4: Rh positive x Rh positive -> Rh negative offspring:

• Possible with heterozygous parents (Dd x Dd).

Other Examples of Codominance

Cattle

• Red hair x white hair -> roan (red and white spots).

Calico Cats

• Orange and black coloration on the X chromosome.
• Mostly female because they need two X chromosomes. (XXY males are rare).

Pleiotropy

• One gene controls multiple outcomes.

Example: Waardenburg Syndrome

• White forelock of hair, pale irises, and deafness are all controlled by one gene.

Epistasis

• One gene masks the expression of another gene.

Example: Albinism in Mice

• Black coat (B) is dominant to brown coat (b).
• Pigment (C) is required for coat color.
• Albino mice have the genotype cc, which blocks the expression of coat color.

Crossing Albino Mice

• If two albino mice (cc) are crossed, all offspring will be albino.

Example: Labrador Retrievers

• Black coat (B) is dominant to chocolate coat (b).
• Yellow labs: Recessive genes for pigment (ee) mask the coat color.

Crossing Yellow and Black Labs

• You'll need to consider the genotypes of both and do a Punnett square.
• There are many different ways of crossing them.

Polygenic Inheritance

• Traits with varying degrees such as height, skin color, etc.
• Traits controlled by more than just a pair of genes

Matching Review:

• Red snapdragon crossed with white snapdragon produces a pink offspring: Incomplete dominance
• Red carnation crossed with white carnation forms offspring that are red and white: Codominance
• A single gene that affects many traits: Plyotropy
• Many genes that affect one trait: Polygenic
• One gene that modifies expression of another: Epistasis