Explain how Bridges’s study of non disjunction in Drosophila helped prove the chromosome theory of inheritance.

Calvin Bridges’ study of nondisjunction in Drosophila melanogaster provided crucial evidence supporting the chromosome theory of inheritance, which posits that genes are located on chromosomes and that these chromosomes are the vehicles for inheritance. Here’s a detailed explanation of how his work helped prove this theory:

1. Background of the Chromosome Theory of Inheritance

• Prior to Bridges’ work, Thomas Hunt Morgan had demonstrated that genes were associated with specific chromosomes through his study of sex-linked traits, such as eye color in Drosophila. He observed that the gene for eye color was linked to the X chromosome.
• However, while Morgan’s work provided strong indications that genes were located on chromosomes, it wasn’t yet irrefutable proof. Bridges aimed to further solidify this theory through his studies on nondisjunction.

2. Nondisjunction Explained

• Nondisjunction refers to the failure of chromosomes to separate properly during meiosis, the process by which gametes (sperm and egg cells) are formed. When nondisjunction occurs, gametes may end up with an abnormal number of chromosomes.
• Bridges studied nondisjunction specifically in female Drosophila and how it affected the inheritance of sex-linked traits, such as eye color.

3. Exceptional Offspring

• Normally, males inherit an X chromosome from their mothers and a Y chromosome from their fathers, while females inherit an X chromosome from each parent. Bridges noticed an anomaly: some male offspring inherited X-linked traits (like eye color) from their fathers, and some females inherited such traits only from their mothers, contrary to expected patterns.
• Bridges termed these flies as “exceptional males” and “exceptional females” because they deviated from the normal inheritance patterns.
• For example, exceptional females had an extra X chromosome (XXY), and exceptional males had only one X chromosome and no Y chromosome (XO).

4. Link Between Chromosomal Abnormalities and Phenotypes

• Bridges demonstrated that these exceptional phenotypes (unusual inheritance patterns) directly correlated with specific chromosomal abnormalities. This meant that the traits (such as white eyes in Drosophila) were directly tied to the number and type of chromosomes the offspring received.
• For instance, the XO males inherited all of their X-linked traits from their fathers, which was only possible because they received a single X chromosome from their father due to nondisjunction in the mother.

5. Microscopic Confirmation

• Bridges used microscopes to confirm that the exceptional flies had abnormal chromosomal compositions, such as XXY females and XO males. These visual confirmations provided direct evidence that phenotypic traits were tied to specific chromosomal arrangements.

6. Proving the Chromosome Theory of Inheritance

• The chromosome theory of inheritance claims that genes are carried on chromosomes and that the behavior of chromosomes during meiosis determines the inheritance of traits. Bridges’ work provided irrefutable proof of this theory.
• His experiments showed that:
  • The presence or absence of specific chromosomes directly affected the inheritance of traits.
  • Chromosomal nondisjunction led to predictable changes in the phenotypes of offspring.
• Therefore, by linking abnormal chromosomal behavior to specific inheritance patterns, Bridges demonstrated that chromosomes are indeed the carriers of genetic information, thus validating the chromosome theory of inheritance.