BICD 100 Notes Lecture 4

Introduction to Incomplete Dominance and Codominance

  • Explanation of hybridization experiments with flower color traits.

  • Trait of interest: Flower color phenotype in two strains.

    • Parent 1: Red flowers (phenotype).

    • Parent 2: White flowers (phenotype).

  • Hybridization experiment resulted in all pink F1 generation.

  • Observations of ratios in the F2 generation.

Mendelian Genetics Overview

  • Reference to Mendel's principles of heredity.

  • Importance of self-cross experiments to understand inheritance patterns.

Observation of F2 Generation

  • F2 Generation Results:

    • Ratios of colors observed: 1 Red : 2 Pink : 1 White.

    • Misinterpretation possibility: Blending of traits leading to intermediate phenotype.

    • Confirmation that both red and white alleles remain intact, leading to phenotypic presence of red, pink, and white flowers.

Modified Mendelian Ratio and Incomplete Dominance

  • Modified Mendelian ratio: 1 Red : 2 Pink : 1 White.

  • Incomplete dominance concept:

    • Definition: Form of inheritance where the phenotype of heterozygote is an intermediate of both homozygous phenotypes.

    • Example: Red (RR) + White (rr) = Pink (Rr).

  • Discussion of symbols for alleles in this genetic model.

    • Symbol selection for alleles in genetic studies is flexible, as long as they are clearly defined.

Genotypic and Phenotypic Ratios

  • Explanation of genotypic results:

    • Big R Big R (Red) : Big R Little r (Pink) : Little r Little r (White).

    • Expectation of gametes in F2 generation based on alleles.

Key Concepts in Genetics

  • Understanding the relationship between phenotype and genotype.

  • Clarification of differences between phenotypic ratio and genotypic ratio.

  • Importance of creating a Punnett square to predict offspring ratios.

Test Cross Example

  • Test cross using F1 pink (Rr) crossed with homozygous recessive (rr).

    • Prediction: Expectation that half of the progeny will be pink, confirming hypothesis.

Additional Example: Chicken Feather Color

  • Birds as another example of modified dominance.

  • Experimental crosses from true breeding white chickens (q1) to true breeding black chickens (q2).

    • Result: Speckled black and white chickens (neither parent phenotype).

  • Application of genetic symbols to describe these traits:

    • Assigning Big W (black) and little w (white).

  • Anticipated genotypic ratio in the offspring.

    • 1 Big W Big W (black) : 2 Big W little w (white speckled) : 1 little w little w (white).

Codominance Concept Introduction

  • Definition of codominance: Both traits are fully expressed simultaneously in the phenotype.

    • Example: An organism with both black and white feathers (not blended, no intermediate colors).

Blood Type Genetics Example

  • Transition to human blood types.

  • Parent with AB blood type leading to three possible offspring (A, B, AB).

    • Observed offspring ratios in blood types from AB x O parents.

    • Modeling genetic ratios based on given alleles:

    • A (phenotype AA or AO) and B (phenotype BB or BO).

Codominance in Blood Types

  • Exploration of codominance within human blood:

    • A and B blood type alleles acting as codominant.

    • Introduction of type O (neither A nor B).

Exploring Offspring Phenotypes

  • Examination of unions between type A and type O parents:

    • Managed breeding, resulting offspring showing type A phenotype due to A allele presence.

    • Understanding patterns of inheritance based upon phenotypic outcomes.

Genetic Complexity in Blood Types

  • Acknowledgment that it's common for genes to have multiple alleles.

  • Example of dominant relationships between alleles:

    • A is dominant over type O, while A and B are codominant.

  • The concept of allelic series presented, emphasizing the complexity of inheritance patterns.

Example Scenarios and Predictive Analysis

  • Discussing potential offspring outcomes when breeding different blood types:

    • Analysis based on blood type data, slices of potential outcomes.

  • Structuring a genetic problem statement:

    • Example involving a phenotypic A x B cross to derive offspring ratios.

Summary Concepts

  • Review key concepts:

    • Inheritance involves both dominance and recessive relationships between alleles.

    • Two alleles maintain integrity while expressing unique ratios.

    • Phenotypic ratios calculated from the underlying genotypes produced by environmental mating.

    • Emphasis on continued study of inherited traits and their ratios using fundamental Mendelian principles.

Conclusion

  • Reinforcement of Mendelian principles and their applications in various genetic scenarios.

  • Note the challenges of incorporating sex-linked traits in discussions but affirm that these scenarios can be analyzed.

  • Closing remarks and encouragement for further exploration within genetic principles before next session.