Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 14.3 Inheritance Patterns Are Often More Complex Than Predicted by Simple Mendelian Genetics
While Mendel’s laws provide the foundation of genetics, many traits do not follow his simple patterns. Various inheritance mechanisms—such as incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance—expand our understanding of how genes influence traits.
Extensions of Mendelian Genetics for a Single Gene
Incomplete dominance: Heterozygotes show a phenotype intermediate between the two homozygotes
Example: red × white snapdragons → pink F₁ flowers (with 1:2:1 phenotype ratio in F₂)
Codominance: Both alleles are fully expressed in heterozygotes
Example: MN blood type expresses both M and N molecules on red blood cells
Dominance vs. phenotype:
Dominance depends on the level of phenotype examined (organismal, biochemical, molecular)
Example: Tay-Sachs disease appears recessive at the organism level, incompletely dominant at the biochemical level, and codominant at the molecular level
Frequency of dominance: A dominant allele is not necessarily more common in a population
Example: polydactyly is caused by a dominant allele but is rare
Multiple Alleles
Most genes exist in more than two allelic forms
Example: ABO blood groups in humans involve three alleles (IA, IB, i)
IA and IB are codominant
Blood types: A, B, AB, or O depending on allele combination
Important for safe blood transfusions
Pleiotropy
A single gene affects multiple traits
Seen in many human genetic disorders like sickle-cell disease and cystic fibrosis
Example: one pea gene affects both flower color and seed coat color
Extensions for Two or More Genes
Epistasis: One gene affects the expression of another
Example: Labrador retriever coat color
B/b gene determines black or brown
E/e gene controls pigment deposition
If ee genotype → yellow coat regardless of B/b alleles
Modified 9:3:3:1 ratio to 9:3:4 in F₂ generation
Polygenic inheritance: Multiple genes contribute additively to one trait
Results in continuous variation (quantitative traits)
Examples: skin color, height, eye color
Skin color is influenced by at least 378 genes, but can be simplified using 3 genes for learning purposes
Environmental Influence on Phenotype
The environment affects gene expression
Example: hydrangea flower color depends on soil pH and aluminum content
Human traits like height, skin color, and intelligence are influenced by both genetics and environment
Genotype provides potential; phenotype is shaped by both genes and experience
Traits influenced by many genes and environment are called multifactorial traits
A Broader View of Heredity
The terms "phenotype" and "genotype" can apply to an organism as a whole
Most traits result from interactions between multiple genes and environmental factors
Mendel’s principles still apply and explain many genetic patterns, but must be integrated with modern understanding of gene interactions and complexity
In a Nutshell
Mendelian genetics forms the core of inheritance theory, but many traits show more complex patterns. These include multiple alleles, varying dominance relationships, gene interactions, and environmental effects—together creating the diversity we observe in the natural world.