Chapter 15: The Chromosomal Basis of Inheritance

Introduction

This chapter explores how chromosomes carry genetic information and how their behavior during cell division underlies Mendelian inheritance. The study of fruit flies (Drosophila melanogaster) provided key evidence for the chromosome theory of inheritance, linking genes to specific locations on chromosomes.

Chromosome Theory of Inheritance

Definition and Historical Context

• Chromosome theory of inheritance: States that genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns.

• Thomas Hunt Morgan's experiments with Drosophila melanogaster provided convincing evidence for this theory.

Why Fruit Flies Are Used in Genetics

• Produce many offspring in a short time.

• Short generation time (can breed every two weeks).

• Only four pairs of chromosomes, making genetic analysis simpler.

Wild Type vs. Mutant Phenotypes

Definitions

• Wild type: The normal phenotype commonly found in natural populations (e.g., red eyes in fruit flies).

• Mutant phenotype: Traits alternative to the wild type, resulting from genetic mutations (e.g., white eyes in fruit flies).

Relating Behavior of Gene Alleles to Chromosome Pairs

Morgan's Experiment

• Mated male flies with white eyes (mutant) to female flies with red eyes (wild type).

• F1 generation: All had red eyes.

• F2 generation: 3:1 ratio of red to white eyes, but only males had white eyes.

• Conclusion: The white-eyed allele is located on the X chromosome.

• Females: XX; Males: XY.

• This supported the chromosome theory of inheritance.

Relationship Between Genes and Chromosomes

Key Concepts

• Genes are located on chromosomes.

• Chromosomes duplicate before cell division.

• During meiosis I, homologous chromosomes separate; in meiosis II, sister chromatids separate.

• Each chromosome has one version of a gene (allele).

• Offspring inherit one allele from each parent.

• Homologous chromosomes have alleles of a given gene at the same locus.

Key Terms and Definitions

• Gene: A segment of DNA that codes for a specific trait.

• Allele: Different versions of a gene.

• Homologous chromosomes: Chromosome pairs, one from each parent, with genes for the same traits at the same loci.

• Sister chromatids: Identical copies of a chromosome connected by a centromere.

Example: Fruit Fly Eye Color Inheritance

• Red eyes (wild type) are dominant to white eyes (mutant).

• Crosses show sex-linked inheritance, as the gene for eye color is on the X chromosome.

Summary Table: Chromosome Theory of Inheritance

Additional info:

• Further topics in this chapter include sex-linked inheritance, genetic mapping, chromosomal alterations, and exceptions to Mendelian inheritance, which are covered in subsequent slides and notes.