Overview
This lecture explains how genetics determines human blood groups, focusing on antigens, genetic dominance, codominance, inheritance patterns, and limitations of paternity testing with blood types.
Blood Group Basics
- Blood groups are classified based on antigens (proteins) on the surface of red blood cells.
- There are four main blood groups: A, B, AB, and O.
- Type A has A antigens, type B has B antigens, type AB has both, and type O has none.
- Antigen presence determines compatibility for blood donation and transfusion.
- Blood group O is the universal donor; AB is the universal recipient.
Genetics of Blood Groups
- Blood group genes use specific notation: capital "I" with superscript "A" or "B" for A/B alleles and lowercase "i" for O.
- A and B alleles are codominant, meaning both are equally expressed in AB individuals (genotype: IAIB).
- A and B alleles are dominant over O (i), so genotypes IAi and IBi result in phenotypes A and B, respectively.
- Blood group O requires two recessive alleles (ii).
Genetic Crosses and Inheritance
- Exam questions may ask for possible blood groups of children based on parental blood groups.
- Use a genetic cross (monohybrid cross) to determine possible allele combinations and resulting blood types.
- For example, parents with genotypes IAi (A) x IAi (A) can produce children with A or O blood groups.
Paternity Testing and Blood Groups
- Blood group testing can only exclude a man as a biological father, not confirm him.
- Multiple unrelated individuals can share common blood groups, so matching blood types is not proof of paternity.
- Example: A father with AB blood cannot have a child with O blood type.
Key Terms & Definitions
- Antigen — protein on cell surfaces that determines blood group and triggers immune response.
- Blood group (A, B, AB, O) — classification based on presence/absence of A and B antigens.
- Allele — alternative forms of a gene (IA, IB, i for blood groups).
- Complete dominance — when one allele masks the effect of a recessive allele.
- Codominance — both alleles are equally expressed in phenotype (e.g., AB blood type).
- Phenotype — observable physical characteristic resulting from genotype.
- Genotype — genetic makeup of an organism (e.g., IAi).
- Paternity testing — analysis to determine if a man is a biological father.
Action Items / Next Steps
- Make flashcards for key blood group terms and concepts.
- Practice genetic crosses for different parent blood group scenarios.
- Review linked videos on types of dominance and monohybrid crosses.