Genetics Lecture 9/29/25

Genetics Overview

Warm-Up Question

• Question posed about flower color genetics based on Chapter 4.

• Class participation to assess how many genes determine flower color.

• Responses:

  • Some think 2 genes (confidently or uncertainly).

  • Some think 1 gene.

  • Some have no idea.

Understanding Genetic Inheritance

• Focus on understanding the problem through previous knowledge and information provided (data of progeny).

• Data includes a cross between two purple plants resulting in 168 offspring.

• Genotypes of parent plants are unknown, leading to discussions on possible inheritance models.

Expected Phenotype Ratios

• Single gene assumed, yields an expected ratio of 3:1 (dominant to recessive).

• If both parents are heterozygous (Pp), the Punnett square shows:

  • Phenotype ratio: 3:1 (141 purple: 47 non-purple).

Dominance Exceptions

• Exceptions to complete dominance:

  • Incomplete dominance: blended traits.

  • Codominance: both traits expressed.

• Ratios for these scenarios differ, leading to ratios like 1:2:1 (e.g., red, white, pink flowers).

Analyzing Results

• The 3:1 ratio indicates dominant trait but not for single gene inheritance.

• Consideration of two-gene interactions:

  • Possible ratios are associated with different genetic interactions (e.g., epistasis).

• Recessive epistasis results in a modified ratio of 9:3:4 instead of 9:3:3:1.

• Review of flower color gene interactions evidences epistasis, e.g., in squashes and dogs.

Gene Combinations and Phenotypes

• Assigned purple, red, and other phenotypes through knowledge of genetic codes.

• Examining genotypic combinations leads to determining roles of each gene:

  • A ext{-recessive epistasis: little a, little a}.

  • B ext{-hypostatic: little b, little b}.

Recombination Frequency in Genetic Mapping

• Definitions:

  • Recombination frequency (RF): calculated as the number of recombinant offspring/total offspring.

  • Each 1% RF equates to 1 map unit (or centimorgan).

• Example given: If 20% RF, genes are 20 map units apart.

• Genes A/B and A/C mapping examples discussed with calculations:

  • Combining distances for additive distances.

Importance of Mapping Principles

• Discussing chromosome mapping requires understanding breakage points and constructs.

• Forced reevaluation of genetic maps due to double crossover events that alter potential calculations.

Three-Point Mapping Overview

• Gene order identification through progeny.

• Identifying nonrecombinants and double crossovers helps clarify middle gene positioning.

• Understanding looks at parental types and recombinant types crucial for setting gene order.

  • The key in identifying the nonrecombinant and double crossover types.

Identification of Middle Gene

• Aligning and assessing nonrecombinant results to suggest middle gene by examination of recombination.

• Recognizing if one gene appears out of place facilitates logical deduction.

Review of Genetics Concepts

• Understanding gene interactions is pivotal for genetic problem-solving: Mendelian inheritance vs. genetic maps.

• Continuous reinforcement through calculations, practical examples, and holistic gene layout reviews is necessary for exams.

Next Session Preview

• Continuing with genetics mapping, reinforcement of concepts through practice questions.

• Expectation: Develop proficiency in identifying genetic relationships, mapping techniques, and deducing gene orders efficiently.