Linkage and Beyond Study Guide

Here’s a condensed and structured study guide based on your notes for AP Biology:

1. Linkage & Recombination

Thomas Hunt Morgan’s Experiments

• Studied Drosophila melanogaster (fruit flies) to understand inheritance.

• First Experiment: Found that certain traits appeared together more often than expected, suggesting linkage (genes on the same chromosome).

• Parental Phenotypes: More frequent in offspring.

• Recombinant Phenotypes: Less frequent due to crossing over.

• Second Experiment Conclusion:

• Genes must be physically linked on chromosomes.

• However, crossing over can separate linked genes.

Recombination & Gene Mapping

• Alfred Sturtevant (1913):

• Used recombination frequencies to create linkage maps of chromosomes.

• Distance between genes measured in map units (centimorgans, cM).

• The closer two genes are, the less likely they will be separated by crossing over.

Morgan’s Linkage Data Example

• Observed offspring phenotype counts:

• 965 & 944: Parental phenotypes (most frequent).

• 206 & 185: Recombinants (less frequent).

• Recombination frequency = (Recombinants / Total Offspring) × 100 = ~17%.

2. The Role of Mutations

Hermann Muller’s Experiment

• Studied mutations and found that environmental factors (e.g., radiation) cause genetic changes.

• Led to understanding how mutations drive genetic variation.

3. Evolution & Speciation

Theodosius Dobzhansky

• Connected genetics with evolution by studying Drosophila pseudoobscura.

• Defined heterozygous advantage: Some heterozygous genotypes provide survival benefits.

• Reproductive Isolating Mechanisms: How species stay genetically distinct.

Ernst Mayr’s Contributions

• Expanded on Dobzhansky’s work.

• Biological Species Concept (BSC):

• “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.”

• Defined species based on reproductive isolation rather than physical similarities.

Mayr’s Observations on Speciation

• Allopatric Speciation:

• Physical barriers (e.g., islands, mountains) lead to genetic divergence.

• Closely related species are geographically close but reproductively isolated.

4. Timeline of Key Discoveries in Genetics & Evolution

• 350 BCE – Aristotle: Early classification of life.

• 1735 – Linnaeus: Binomial nomenclature.

• 1809 – Lamarck: Early ideas on inheritance of acquired traits.

• 1859 – Darwin: Natural Selection (On the Origin of Species).

• 1863 – Mendel: Laws of inheritance (pea plant experiments).

• 1902 – Sutton: Meiosis theory.

• 1910 – Morgan: Genes are on chromosomes.

• 1913 – Sturtevant: First genetic maps.

• 1920 – Muller: Environmental mutations.

• 1935 – Dobzhansky: Role of isolation in speciation.

• 1942 – Mayr: Biological Species Concept.

Study Tips for AP Bio

1. Understand key experiments (Morgan, Sturtevant, Muller, Dobzhansky, Mayr).

2. Practice Punnett squares with linked genes.

3. Use recombination frequency formulas to calculate map distances.

4. Review vocabulary: linkage, recombination, crossing over, genetic drift, reproductive isolation.

5. Apply concepts to evolution: How genetic variation leads to speciation.