1. The Chromosomal Basis of Inheritance
15.1 Mendelian inheritance has its physical basis in the behavior of chromosomes
Melanie Ornelas

2. Vocabulary
Chromosome theory of inheritance- A basic principle in biology stating that genes are located on chromosomes and and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
Wild type- An individual with the phenotype most commonly observed in natural populations; also refers to the phenotype itself.
Mutant phenotypes- Traits alternative to the wild type.

3. Overview Mendel’s “hereditary factors” were genes
Today it is known that genes are located on chromosomes

4. Chromosome theory of inheritance
Began to take form around 1902
States:
Mendelian genes have specific loci on chromosomes
Chromosomes undergo segregation and independent assortment
The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

5. Experimental Evidence
The first solid evidence came from Thomas Hunt Morgan
Morgan’s experiments with fruit flies (Drosophilia melanogaster) provided evidence that chromosomes are the location of Mendel’s heritable factors
Morgan noted wild type (normal phenotypes that were common in the fly populations)
Traits alternative to the wild type are called mutant phenotypes

6. Ideal experimental organism
Several characteristics make fruit flies a convenient organism for genetic studies:
They breed at a high rate
A generation can be bred every two weeks
They have only four pairs of chromosomes

7. Conclusion
In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)
F1 generation- all red eyes
F2 generation- showed 3:1 red:white eye ratio, but only males had white eyes
Morgan determined that the white-eyed mutant allele must be located on the X chromosome
Morgan’s finding supported the chromosome theory of inheritance

9. Video
Start at 3:03

10. Test questions
Q: Where are Mendel’s hereditary factors located in the cell? A: On chromosomes Q: In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? A: Red eyes in a 3:1 ratio