Comprehensive Genetics - Chapter 7: Chromosomal Theory of Inheritance

Rediscovery of Mendel's Work

• Early 1900s: Rediscovery of Mendel's work led to experiments on genetic ratios.
• Scientists like Reginald Punnett, William Bateson, and Edith Rebecca Saunders worked with pea plants to study dihybrid crosses.

Linked vs. Independent Genes

• Experiments with pea plants did not always follow expected 9:3:3:1 ratios.
• Conclusion: Some genes are linked and do not assort independently.

Thomas Hunt Morgan's Work

• Worked with Drosophila (fruit flies) and noted only 4 chromosomes.
• Linked genes would not independently assort, leading to more than 4 genes being linked on chromosomes.

Mendel's Second Law vs. Chromosomal Theory

• Mendel's Second Law: Independent assortment of genes leads to 1:1:1:1 ratio in dihybrid crosses.
• Chromosomal Theory: Genes on the same chromosome should travel together, leading to a linked inheritance pattern.

Morgan's Observations and Crossovers

• Morgan observed unexpected ratios in fly crosses, suggesting linked genes but also detected recombination.
• Proposed the existence of parental types and recombinant types due to crossing over during meiosis.

Evidence of Crossover

• Jansen and Darlington observed chiasmata, indicating physical crossover sites on chromosomes.
• Barbara McClintock and Harriet Creighton provided cytological evidence of crossover with marked corn chromosomes.

Sturtevant's Genetic Mapping

• Alfred Sturtevant used recombination frequencies to construct the first linear chromosome map.
• Recombination frequency used as a measure of genetic distance; frequencies are additive.

Limitations and Use of Genetic Maps

• Map units (centiMorgans) are relative distances based on recombination.
• Maximum observable recombination frequency is 50% due to parental gamete proportions.
• Larger distances underestimate double crossovers due to potential missed exchanges.

Importance of Genetic Maps

• Historical significance: Foundation for the Human Genome Project and sequencing efforts.
• Genetic maps laid groundwork for understanding gene localization and inheritance patterns.

Future Topics

• Upcoming sections will cover more on linkage and mapping with both two and three genes.
• Understanding genetic inheritance across generations.

Overall, the development of the chromosomal theory of inheritance and genetic mapping led to significant advancements in genetics, ultimately paving the way for modern genomics and the Human Genome Project.