UNIT 2 - Lesson 2.12 - Variations in Heredity (Incomplete Dominance and Codominance) + Blood Types

Beyond Mendelian Genetics

• Mendel's experiments involved crossing "typical" (homozygous dominant) with "atypical" (homozygous recessive) plants.

• Mendel discovered complete dominance.

• Complete Dominance: Only one allele is expressed, even if the other allele is present.

  

Incomplete Dominance

• Incomplete Dominance: Neither allele dominates, and both influence the individual, resulting in a partial expression of both traits.

  

• Example: Snapdragon flower color

  • $C^RC^R$ = homozygous RED

  • $C^WC^W$ = homozygous WHITE

  • $C^RC^W$ = heterozygous PINK

    

• Crossing a red ($C^RC^R$) and a white ($C^WC^W$) snapdragon produces pink ($C^RC^W$) offspring in the F₁ generation.

  

• Incomplete dominance results in a blend of the two homozygous phenotypes in the heterozygous phenotype.

• Example Problem:

  • Wildcats in northeast Kansas come in blue, red, and purple colors, which is controlled by a single locus gene with incomplete dominance.

  • $BB$ = blue

  • $RR$ = red

  • $BR$ = purple

Codominance

• Codominance: Both alleles are fully expressed to produce offspring with a third phenotype.

• Neither allele masks the other; both influence the final phenotype.

• Example: Shorthorn cattle

  

  • $C^WC^W$ = White

  • $C^RC^R$ = Red

  • $C^RC^W$ = Roan (intermingled red and white hairs)

• In codominance, the "recessive" and "dominant" traits appear together in the phenotype of heterozygous organisms.

  

  A cross between organisms with two different phenotypes

  • produces offspring with a third phenotype in which both parental traits appear together.

    

• Cattle example: Red (RR), white (WW), and roan (RW = red & white hairs together) phenotypes.

  

• Black cat (BB) crossed with a tan cat (TT) produces a tabby pattern (black & tan fur together) (BT).

BLOOD TYPES

Multiple Alleles

• Multiple Alleles: More than two alleles code for a specific trait.

• Example: Human blood type (A, B, and O alleles)

• Human blood type is both Codominant & Dominant genetic trait.

  • Phenotypes:

    • A: $I^AI^A$ or $I^Ai$

    • B: $I^BI^B$ or $I^Bi$

    • AB: $I^AI^B$ (Example of codominance)

    • O: $ii$

  • If "i" is paired with $I^A$, $I^A$ (type A) is dominant and will be expressed.

  • If "i" is paired with $I^B$, $I^B$ (type B) is dominant and will be expressed.

    

Blood Composition

• Blood components:

  • Plasma (55%): Contains salts, minerals, water, and proteins.

  • Buffy Coat (<1%): Leukocytes (white blood cells) and platelets.

  • Erythrocytes (45%): Red blood cells.

• Red Blood Cells:

  • Contain hemoglobin

  • binds oxygen and transports it to tissues, removing carbon dioxide.

• White Blood Cells:

  • Fight infection by producing antibodies to destroy foreign material.

• Platelets: Help the blood to clot.

  

Blood Types

• Eight different blood types:

  • A Rh+, A Rh-

  • B Rh+, B Rh-

  • AB Rh+, AB Rh-

  • O Rh+, or O Rh-

• Blood types differ due to combinations of antigens on the surface of red blood cells.

  • A and B antigens are sugars, and Rh antigens are proteins.

  • The antigens expressed in the red blood cells determine an individual's blood type.

• You can have A and/or B and/or Rh antigens or none of them.

• You can have A and/or B and/or Rh antibodies or none of them.

• Blood type genotypes:

  • A: $I^AI^A$, $I^Ai$

  • AB: $I^AI^B$

  • B: $I^BI^B$, $I^Bi$

  • O: $ii$

Blood Type Antigens and Antibodies

• A Rh-:

  • A antigens, B antibodies, no Rh antigens.

• A Rh+:

  • A and Rh antigens, B antibodies.

• B Rh-:

  • B antigens, A antibodies, no Rh antigens.

• B Rh+:

  • B and Rh antigens, A antibodies.

• AB Rh-:

  • A and B antigens, Rh antibodies, no Rh antigens.

• AB Rh+:

  • A, B, and Rh antigens.

• O Rh-:

  • No A, B, or Rh antigens, A, B, and Rh antibodies.

• O Rh+:

  • Rh antigen, A and B antibodies, no A or B antigens.