Mendelian Genetics and Phenotypic Expression

Predicting Phenotype and Genotype

  • We can predict the offspring's phenotype based on the known genotype of the parents.

Mendel's Laws

Law of Segregation
  • States that alleles at the same genetic locus segregate independently during meiosis.
  • In heterozygous individuals, gametes have an equal probability of carrying the dominant or recessive allele.
  • Applies to the segregation of two different alleles for a single gene.
Law of Independent Assortment
  • Pertains to the segregation of alleles of different genes at different loci.
  • States that the segregation of alleles from one locus does not affect the segregation of alleles from another locus.
  • Example: The segregation of allele for $e$ does not influence the segregation of allele for $h$.

Exceptions to Mendel's Laws

Incomplete Dominance
  • In heterozygous individuals, the phenotype is an intermediate blend of dominant and recessive traits.
  • Commonly observed in plants (e.g., snapdragons).
    • Example: True breeding red flower ($RR$) crossed with true breeding white flower ($rr$) results in heterozygous pink flowers ($Rr$).
Codominance
  • Both traits are fully and equally expressed in heterozygous individuals.
  • Example: Human blood groups.
    • IA allele (Type A) and IB allele (Type B) are codominant; both are expressed in IAIB (Type AB).
    • The little i allele is recessive and does not express any blood type when homozygous.
Incomplete Penetrance
  • Occurs when individuals with a particular genotype do not express the expected phenotype.
  • Example: Osteogenesis Imperfecta (OI) - an autosomal dominant condition affecting collagen.
    • Penetrance of OI is around 70%; 30% of those with the genotype do not express OI phenotype.
Variable Expressivity
  • The trait is always expressed in individuals with the genotype but may vary in severity.
  • Example: Marfan syndrome - an autosomal dominant disorder affecting connective tissue.
    • Symptoms range from tall stature with minimal complications to severe heart and lung issues.

Summary

  • Mendelian genetics involves laws governing allele segregation and exceptions like incomplete dominance, codominance, incomplete penetrance, and variable expressivity that illustrate the complexity of genetic expression in traits.