3.2 Multiple Alleles and Pleiotropy

Multiple alleles and pleiotropy add complexity to genetic inheritance. Instead of simple dominant-recessive patterns, genes can have several versions or affect multiple traits. This diversity allows for a wider range of phenotypes and adaptations in populations.

ABO blood types and rabbit coat colors showcase multiple alleles in action. Meanwhile, conditions like phenylketonuria and sickle cell anemia demonstrate how a single gene can influence various traits. These concepts are crucial for understanding genetic diversity and evolution.

Multiple Alleles

Multiple alleles and trait examples

• Multiple alleles occur when a single gene has more than two allelic variations
  • Most genes have two alleles (dominant and recessive) but some have multiple alleles
• Traits controlled by multiple alleles:
  • ABO blood type system in humans
    • Determined by three alleles: IA, IB, and i
    • IA and IB are codominant while i is recessive
    • Genotypes and phenotypes:
      • IAIA and IAi: Type A
      • IBIB and IBi: Type B
      • IAIB: Type AB
      • ii: Type O
  • Coat color in rabbits
    • Controlled by the C gene with four alleles: C, cch, ch, and c
    • Genotypes and phenotypes:
      • CC: Full color
      • Ccch: Chinchilla
      • Cch: Himalayan
      • Cc: Full color
      • cchcch: Chinchilla
      • cchch: Himalayan
      • cchc: Himalayan
      • chch: Himalayan
      • chc: Himalayan
      • cc: Albino

Pedigree analysis for multiple alleles

• Assign alleles to individuals based on phenotypes
• Use Punnett squares to determine possible offspring genotypes
• Consider relationships (parents, siblings) to infer genotypes
• Genetic crosses with multiple alleles:
  1. Set up Punnett square with all possible allele combinations
  2. Determine genotypic and phenotypic ratios of offspring
  • Example cross between type A (IAi) and type B (IBi):
    • Punnett square:

      	IA	i
      IB	IAIB (AB)	IBi (B)
      i	IAi (A)	ii (O)

    • Genotypic ratio: 1 IAIB : 1 IBi : 1 IAi : 1 ii
    • Phenotypic ratio: 1 AB : 1 B : 1 A : 1 O

Pleiotropy

Pleiotropy and phenotypic influence

• Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits
  • Happens when a gene has multiple functions or affects multiple biochemical pathways
• Examples of pleiotropy:
  • Phenylketonuria (PKU) in humans
    • Caused by mutation in PAH gene which encodes phenylalanine hydroxylase enzyme
    • Affects cognitive development, pigmentation, and behavior
  • Sickle cell anemia
    • Caused by mutation in HBB gene which encodes beta-globin protein
    • Affects red blood cell shape, oxygen transport, and malaria resistance

Genetic diversity from allelic variation

• Multiple alleles increase genetic diversity within populations
  • More alleles for a gene means more possible genotypes and phenotypes
  • Diversity allows for wider range of traits and adaptations to environments
• Pleiotropy can influence trait evolution
  • If a pleiotropic gene affects multiple traits, selection pressure on one trait can indirectly affect others
  • Balancing selection may maintain multiple alleles if they have both beneficial and detrimental effects on different traits
• Multiple alleles and pleiotropy are important for:
  • Predicting inheritance patterns of traits
  • Assessing genetic basis of diseases and disorders
  • Developing targeted therapies and interventions
  • Studying evolution and adaptation of populations