Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 15.1 Mendelian Inheritance Has Its Physical Basis in the Behavior of Chromosomes
The discovery that genes are located on chromosomes provided strong support for Mendel’s laws of inheritance. Thomas Hunt Morgan’s experiments with fruit flies offered the first solid evidence linking a specific gene to a specific chromosome.
Chromosome Theory of Inheritance
- Proposed around 1902 by Sutton and Boveri
- States that Mendelian genes have specific loci on chromosomes
- Chromosomes undergo segregation and independent assortment during meiosis
- This physical behavior of chromosomes mirrors Mendel’s proposed “heritable factors”
Morgan’s Choice of Experimental Organism
- Thomas Hunt Morgan used Drosophila melanogaster (fruit flies)
- Advantages:
- Easy to breed, short generation time (~2 weeks)
- Hundreds of offspring per mating
- Only four chromosome pairs, easily observed
- Distinct male (XY) and female (XX) sex chromosomes
- Known as the “fly room,” Morgan’s lab became a hub of genetics research
Discovery of a Mutant Phenotype
- After two years of breeding flies, Morgan discovered a white-eyed male, differing from the wild-type red-eyed majority
- Traits common in nature = wild type
- Traits that differ = mutant phenotype, due to mutated alleles
- Morgan developed a gene symbol system (e.g., w = white eyes, w⁺ = red eyes) still used in fly genetics
Linking Genes to Chromosomes
- Morgan crossed a red-eyed female with a white-eyed male
- All F₁ offspring had red eyes → red is dominant
- In F₂, only males showed the white-eyed phenotype
- This pattern suggested the eye color gene is located on the X chromosome
- Males have only one X chromosome, so a single recessive allele is expressed
- Females must inherit two recessive alleles to show the white-eye trait
- No white-eyed females appeared in F₂ because all received a w⁺ allele from their red-eyed fathers
Evidence for Chromosome Theory
- Morgan’s discovery supported the idea that genes reside on chromosomes
- Explained why certain traits are inherited in a sex-linked manner
- Laid the foundation for understanding sex-linked genes and their unique inheritance patterns
Meiosis and Mendel’s Laws
- The law of segregation is explained by the separation of homologous chromosomes in anaphase I
- The law of independent assortment is explained by random alignment of chromosome pairs during metaphase I
- This chromosomal behavior accounts for the genetic patterns observed by Mendel
In a Nutshell
Mendel’s abstract ideas gained physical support through chromosome behavior in meiosis. Morgan’s experiments with fruit flies provided the first clear link between a gene and a chromosome, confirming that genes are inherited through specific loci on chromosomes.