Genetics: Dihybrid Cross and Genetic Exceptions

Dihybrid Cross

Genotypes of Parents

• Both parents are heterozygous for both traits: widows peak and being tall.
• Genotypes:
  • Parent 1: WwTt
  • Parent 2: WwTt

Gamete Formation

• Use the FOIL method (First, Outer, Inner, Last) to determine the alleles each parent can donate:
  • Possible combinations for each parent:
    • WT
    • Wt
    • wT
    • wt

Exceptions in Mendelian Genetics

Incomplete Dominance

• Definition: Neither allele is completely dominant, leading to a blend of traits in the heterozygous phenotype.
• Example: Snapdragons
  • Red flowers (RR) x White flowers (rr) = Pink flowers (Rr)
  • Notation:
    • RR = Red flowers
    • rr = White flowers
    • Rr = Pink flowers (heterozygous, intermediate phenotype)

Codominance

• Definition: Both alleles are expressed equally in the phenotype.
• Example: AB Blood Type
  • IA and IB alleles are co-dominant.
  • Genotype: IAIB = AB blood type.

Multiple Alleles

• Definition: Some traits have more than two allelic forms.
• Example: ABO Blood Group System
  • Involves three alleles: IA, IB, and i.
  • IA and IB are dominant; i is recessive.
  • Blood types:
    • IAIA or IAi = Type A
    • IBIB or IBi = Type B
    • IAIB = Type AB (co-dominant)
    • ii = Type O (recessive)

Polygenic Inheritance

• Definition: Traits controlled by multiple genes, often with a range of phenotypes.
• Example: Human Skin Color
  • Determined by multiple genes contributing to the phenotype.
  • More dominant alleles result in darker skin color.

These genetic concepts illustrate that inheritance can be more complex than simple Mendelian genetics, involving interactions of multiple alleles and genes contributing to a variety of phenotypes.