Chapter 14.3: Patterns of Inheritance

Module 2 Learning Outcomes

• Understanding Mendel’s Laws: Students should be able to apply Mendel’s two laws of inheritance (Law of Segregation and Law of Independent Assortment) to solve Punnett squares and calculate genotype and phenotype probabilities after a mating event.

• Environmental Effects on Phenotype: Students should recognize, explain, and provide examples of how environmental factors can influence phenotype.

Extending Mendelian Genetics: Single Gene Inheritance

Types of Inheritance Beyond Simple Dominance

While Mendel’s laws describe inheritance patterns for traits controlled by single genes with two alleles, many traits exhibit more complex patterns. These include incomplete dominance, codominance, multiple alleles, and pleiotropy.

• Incomplete Dominance: Neither allele is completely dominant over the other. The heterozygote displays an intermediate phenotype between the two homozygotes. Example: Snapdragon flowers—crossing red (CRCR) and white (CWCW) produces pink (CRCW) offspring. Genotype-Phenotype Relationship: (red), (white), (pink)

• Codominance: Both alleles are fully expressed in the heterozygote, resulting in a phenotype that shows both traits simultaneously. Example: ABO blood group—individuals with genotype express both A and B antigens. Genotype-Phenotype Relationship: (type A), (type B), (type AB)

• Multiple Alleles: More than two alleles exist for a gene in the population, though each individual only inherits two. Example: ABO blood group system—three alleles (, , ) determine blood type. Genotype-Phenotype Relationship: or (type A), or (type B), (type AB), (type O)

• Pleiotropy: A single gene affects multiple, seemingly unrelated phenotypic traits. Example: Sickle cell disease—the sickle cell allele affects red blood cell shape, anemia, and resistance to malaria.

Extending Mendelian Genetics: Two or More Genes

Complex Inheritance Patterns Involving Multiple Genes

Some traits are influenced by interactions between two or more genes, leading to patterns such as epistasis and polygenic inheritance.

• Epistasis: The expression of one gene is affected by another gene at a different locus. One gene can mask or modify the effect of another. Example: Coat color in Labrador retrievers—one gene determines pigment color (black or brown), another gene determines whether pigment is deposited in fur. Genotype-Phenotype Relationship: If the second gene prevents pigment deposition, the dog will be yellow regardless of the first gene.

• Polygenic Inheritance: Multiple genes contribute additively to a single trait, resulting in continuous variation. Example: Human skin color, height—these traits show a range of phenotypes due to the additive effects of several genes. Genotype-Phenotype Relationship: The more dominant alleles present, the more pronounced the trait (e.g., darker skin, greater height).

Influence of the Environment on Phenotype

Gene-Environment Interactions

The phenotype of an organism is determined by the interaction between its genotype and environmental factors. Environmental conditions can influence the expression of genetic traits.

• Definition: Phenotype = Genotype + Environment

• Examples:

  • Himalayan rabbits—fur color is affected by temperature; cooler body parts (ears, nose, feet) become darker.

  • Hydrangea flowers—flower color depends on soil pH; acidic soil produces blue flowers, alkaline soil produces pink flowers.

  • Human traits—nutrition and sunlight can affect height and skin color.

Punnett Squares and Probability Calculations

Solving Genetic Crosses

Punnett squares are used to predict the probability of offspring genotypes and phenotypes from parental crosses. They are essential tools for understanding Mendelian and non-Mendelian inheritance.

• Steps to Solve:

  1. Determine parental genotypes.

  2. List possible gametes from each parent.

  3. Fill in the Punnett square to show all possible offspring combinations.

  4. Calculate genotype and phenotype ratios.

• Example Equation:

Summary Table: Types of Inheritance

Key Takeaways

• Mendelian genetics provides a foundation for understanding inheritance, but many traits follow more complex patterns.

• Environmental factors can significantly influence phenotype, sometimes overriding genetic predisposition.

• Punnett squares and probability calculations are essential tools for predicting genetic outcomes.

Additional info: Some examples and definitions have been expanded for clarity and completeness. The summary table is inferred and organized for study purposes.