Linkage and Mapping: Introduction and Recombination Frequency
Administrative Announcements
The professor will review their schedule on Friday to potentially add more office hours for next week.
Approximately 50 students are currently not on the schedule, indicating a class size of about 100 students who may need appointments.
In addition to exam review hours, content review hours will also be posted.
An announcement will be made when new appointment slots become available.
Introduction to Chapter 5: Linkage and Mapping
This chapter introduces concepts (linkage and mapping) that are likely entirely new to most students, unless they have a prior background in genetics.
The professor will proceed slowly and encourages students to ask questions for clarity.
Key topics for today's lecture include:
Defining linkage.
Linkage notation.
Calculating recombination frequencies.
The terms "coupling" and "repulsion."
An introduction to two- and three-point crossings (though most detail will be in subsequent lectures).
Review of Independent Assortment (Foundation for Linkage)
P Generation Example: Starts with homozygous parents for two different traits (e.g., purple flower color (dominant) and long pollen shape (dominant), crossed with red flower color (recessive) and brown pollen shape (recessive)).
F1 Generation: All offspring are heterozygous for both traits (e.g., PpLl).
F2 Generation (Interbreeding F1 Heterozygotes):
If genes assort independently (as learned previously), the expected phenotypic ratio in the F2 generation is 9:3:3:1.n * Numerical Example: If 160 F2 progeny were counted, the expected numbers according to the 9:3:3:1 ratio would be 90:30:30:10.
Observed Deviations: Early geneticists, like Gregor Mendel, observed cases where actual F2 progeny ratios (e.g., 84, 21, 21, 55) significantly deviated from the expected 9:3:3:1 ratio.
Running a Chi-square test on such data would likely fail (e.g., with a p-value of 0.05), indicating that the observed deviation is