ABO Blood Type Inheritance

ABO Blood Type

• ABO blood type follows both codominant and dominant inheritance patterns
• The A and B alleles are codominant with each other, while both are dominant over the O allele
• The O allele is recessive

ABO Blood Type Genes and Alleles

• The blood type gene is represented by the letter I
• The I gene has three alleles: IA, IB, and i
• The alleles are expressed in heterozygous individuals, resulting in different blood types
• The possible blood types are:
  • A: IAIA or IAi
  • B: IBIB or IBi
  • AB: IAIB
  • O: ii

Mendel's Luck

• Mendel's experiments were successful because he studied traits in garden peas that behaved simply
• These traits were not linked or lethal, had only two alleles, and displayed complete dominance
• Other scientists, including Darwin, were conducting similar experiments, but they were using species with more complex patterns of heredity

Sex-linked Traits

• Traits are located on the sex chromosomes, X and Y
• Females have an XX genotype, while males have an XY genotype
• Many sex-linked traits are carried on the X chromosome

Hemophilia

• Hemophilia is a recessive gene on the X chromosome that causes a bleeding disorder
• There are approximately 20,000 hemophilia patients in the United States
• The severity of hemophilia is related to the amount of clotting factor in the blood
• About 70% of hemophilia patients have severe hemophilia, with less than 1% of the normal amount of clotting factor

Color Blindness

• Color blindness is a recessive sex-linked condition caused by a lack of color receptors in the eye
• It results in an inability to see some colors correctly
• Color blindness is more common in males
• Individuals with color blindness are unable to distinguish between certain shades of red and green

Exceptions to Mendel's Laws

• Incomplete dominance: alleles appear blended in the phenotype
• Codominance: both alleles are expressed equally
• Multiple alleles: some genes have more than two alleles

Incomplete Dominance

• Incomplete dominance occurs when neither allele is fully dominant over the other
• The two alleles are represented as two capital letters, one with an apostrophe to indicate the different allele
• Example: snapdragons (flower)
• F1 hybrids have an appearance that is a mixture of the two parental varieties

Codominance

• Codominance occurs when both alleles of a gene are dominant and the heterozygous phenotype has both traits equally expressed
• The two alleles are represented as two capital letters: the first letter of one trait and the first letter of the other trait
• Example: ABO blood type

Multiple Alleles

• Blood type is an example of multiple alleles, with three alleles: A, B, and O
• Each person has two of these alleles in their DNA