Chapter 3- Extension to Mendel's Law

Mendel's Laws and Extensions

• Mendel's Law of Independent Assortment:

  • Parental Generation (P): involves pure-breeding lines.

  • First Filial Generation (F1): All yellow peas (yellow = dominant).

  • Second Filial Generation (F2): Generated through self-fertilization, can show different phenotypes.

• More Complex Traits: Difficult to fit into Mendelian genetics.

Key Extensions to Mendel's Laws

• Dominance Patterns:

  • Not always simple (complete dominance).

  • Types:

  • Complete Dominance: Hybrid resembles one parent.

  • Incomplete Dominance: Hybrid shows a blend of traits.

  • Codominance: Traits from both parents are fully expressed.

• Example - Snapdragons:

  • Incomplete dominance observed with flower colors.

  • Genotypes:

  • Red (A1A1): Red color.

  • Pink (A1A2): Incomplete color due to one normal and one non-functional allele.

  • White (A2A2): No functional pigmentation.

• Hierarchies of Dominance:

  • Can exist in populations (e.g., lentils showing multiple colors and patterns).

Pleiotropy and Gene Interaction

• Pleiotropy: One gene affects multiple traits.

  • Example: Mice with differing coat colors due to the agouti gene.

  • The AY allele:

  • Dominant over A for hair color, but lethal when homozygous (AYAY).

• Gene Interactions:

  • Epistasis: One gene can mask or modify the expression of another.

  • Example: Labrador coat color determined by B and E genes.

    • Genotype bb masks color irrespective of B allele presence, yielding yellow fur.

    • Dominant epistasis patterns affect resulting color ratios in offspring.

Ratios and Examples of Gene Interactions

• Observed ratios based on interaction type (from dihybrid crosses):

  • 9:3:3:1: No epistasis.

  • 9:7: Complementary gene action necessary for phenotype.

  • 9:3:4: Recessive epistasis.

  • 12:3:1: Dominant epistasis I.

  • 13:3: Dominant epistasis II.

  • 15:1: Redundant gene action.

Continuous Traits and Polygenic Inheritance

• Continuous traits (e.g., human height) result from the interaction of multiple genes (polygenic traits).

  • Traits appear to blend due to several genes contributing to phenotypic expression.

  • Distribution of traits becomes continuous with increasing gene involvement influencing the trait.

• Penetrance and Expressivity:

  • Penetrance: Proportion of individuals with a genotype expressing the phenotype.

  • Expressivity: Variation in phenotype expression among individuals with a given genotype.

Conclusion

• Complex traits may still adhere to Mendel's fundamental principles, illustrating the probabilistic nature of genetics in predicting inheritance.

Mendel's Laws and Extensions

Mendel's Law of Independent Assortment:

Parental Generation (P): This generation involves the crossing of pure-breeding lines. Each parent contributes gametes that carry distinct alleles, leading to heterozygous offspring.

First Filial Generation (F1): In this generation, all offspring exhibit a specific phenotype, frequently represented by a dominant trait. For instance, in pea plants, all F1 generations may display yellow peas when the yellow allele is dominant.

Second Filial Generation (F2): Generated through self-fertilization or interbreeding of the F1 hybrids, the F2 generation can reveal different phenotypes and help in understanding inheritance patterns. Typically, phenotypic ratios such as 3:1 for dominant to recessive traits can be observed in traits conforming to simple Mendelian inheritance.

More Complex Traits: Some traits cannot be accurately captured by Mendelian genetics due to their multifactorial nature. These traits often involve interactions among multiple genes (polygenic traits) or are influenced by environmental factors, complicating predictions.

Key Extensions to Mendel's Laws

Dominance Patterns:

Dominance is not always straightforward, and various dominance patterns exist:

• Complete Dominance: The hybrid resembles one of the parents, expressing only the dominant phenotype.

• Incomplete Dominance: The phenotype of the hybrid shows a blend of traits from both parents, leading to intermediate characteristics.

• Codominance: Both traits from the parents are fully and independently expressed in the offspring without blending.

Example - Snapdragons:

A classic example of incomplete dominance is found in snapdragons, where red (A1A1) and white (A2A2) varieties produce pink offspring (A1A2) when crossed. This illustrates how the resulting phenotype can be intermediary rather than completely resembling one of the parents.

Genotypes:

• Red (A1A1): Produces a vibrant red flower color.

• Pink (A1A2): Results from the blending of alleles, with one normal and one non-functional allele.

• White (A2A2): Exhibits no functional pigmentation due to the absence of the functional allele.

Hierarchies of Dominance:

Complex hierarchies of dominance can appear in populations. For example, in certain lentil species, various color morphs are prevalent, indicating that multiple alleles may coexist that contribute to different expressions of traits.

Pleiotropy and Gene Interaction

Pleiotropy:

Pleiotropy occurs when one gene influences multiple phenotypic traits. For instance, different coat colors in mice can be attributed to variations in the agouti gene, which affects pigmentation and distribution patterns in fur.

The AY allele:

• Dominates over the A allele concerning hair color, demarcating the color phenotype in mice.

• However, the AY allele is lethal when homozygous (AYAY), emphasizing the importance of gene dosage and its implications in inheritance and survival.

Gene Interactions:

Gene interactions involve the interplay between multiple genes affecting a single trait or phenotype:

• Epistasis: This phenomenon occurs when one gene masks or alters the expression of another gene. A prime example of this is seen in Labrador retriever coat color, which is determined by two genes, B and E. The genotype bb masks the phenotypic expression of coat color regardless of the presence of dominant B alleles, resulting in yellow fur.

Dominant epistasis patterns can affect color ratios among offspring, highlighting the need for multifaceted models in genetic predictions.

Ratios and Examples of Gene Interactions

Dihybrid crosses lead to observed ratios that depend on the type of gene interaction:

• 9:3:3:1: Indicates no epistasis among the traits.

• 9:7: Represents a scenario wherein complementary gene action is essential for expressing a phenotype.

• 9:3:4: Denotes recessive epistasis, where one recessive allele suppresses the effects of another.

• 12:3:1: Illustrates dominant epistasis I, where a dominant allele of one gene masks another trait.

• 13:3: Reflects dominant epistasis II, with a different masking scenario.

• 15:1: Showcases redundant gene action, where multiple genes contribute to a single phenotype without showing dominance over one another.

Continuous Traits and Polygenic Inheritance

Continuous traits, such as human height, result from the cumulative effects of multiple genes, demonstrating polygenic inheritance. Traits affected by several genes often exhibit a range of phenotypes rather than discrete categories.

As multiple genes contribute to such traits, they tend to distribute continuously, forming a bell curve in populations. More alleles contributing to a trait typically produce greater phenotypic variety.

Penetrance and Expressivity:

• Penetrance: Defined as the proportion of individuals with a particular genotype who actually express the corresponding phenotype. Full penetrance means that all individuals with the genotype exhibit the trait, while incomplete penetrance means some do not.

• Expressivity: Refers to the degree or intensity to which a phenotype is expressed among individuals with the same genotype. Variations of this expression can be influenced by environmental factors, developmental timing, and interaction with other genes.

Conclusion

Complex traits, while rooted in Mendelian principles, often extend beyond these basic rules, highlighting the probabilistic nature of genetics in predicting inheritance patterns. Understanding these intricate relationships furthers the comprehension of both simple and complex genetic traits across diverse organisms.