Lecture Notes: Comprehensive Genetics - Chapter 8

Introduction

Focus on predicting progeny and outcomes of crosses involving three linked genes.
Previous section covered two linked and one unlinked genes.
This section examines all three genes being linked.

Genes: Echinaceas, Light, Hooked on the same chromosome.
- Echinaceas and Light: 8 map units apart.
- Light and Hooked: 12 map units apart.

P1 Cross:
- Fly homozygous for Echinaceas and Hooked crossed with fly homozygous for Light.
F1 Female:
- Important for setting up heterozygous female.
- Chromosome from one parent and whole chromosome from the other.
Test Cross:
- With male expressing all three traits.

F1 Female Produces 8 Gametes: Parental and Crossover Types
- Parental (No Crossover):
  - Chromosome on top and bottom as separate gametes.
- Single Crossovers:
  - Region 1 (Echinaceas to Light):
    - Echinaceas crossover Light Plus.
    - Plus crossover Plus Hooked.
  - Region 2 (Light to Hooked):
    - Ec Plus crossover Plus.
    - Plus Light crossover Hooked.
- Double Crossovers:
  - Echinaceas crosses Light and Hooked.

Coefficient of Coincidence used to determine expected vs. observed frequencies.
Double Crossover Frequency:
- Observed frequency calculated from map distances.
- Total frequency divided into two equal parts.
Single Crossover Frequency:
- Use map distance formula, subtract double crossovers to find single crossover frequency.
Parental Frequency Calculation:
- Total minus sum of crossovers.

Map: Ebony, Hooked, Scarlet genes.
- Ebony and Hooked: 8 map units apart.
- Hooked and Scarlet: 10 map units apart.
Cross Setup:
- Male: Ebony and Hooked but not Scarlet.
- Female: Scarlet but not Ebony or Hooked.

Double Crossovers:
- Coefficient of coincidence given.
- Expected from map distances.
Single Crossovers in Region 1 & 2:
- Formula application as above.
Parental Frequencies:
- Derived from total subtracted by crossovers.