Lecture on Genetic Mapping and Human Genetics

Introduction

• Upcoming lectures: Genetic mapping and human genetics.
• Detailed learning objectives provided.

Background on Chromosomes

• Mendel's laws: Law of segregation and independent assortment.
• Early chromosome theory: Genes might be on chromosomes due to pairing observed in meiosis.
• Discovery: Genes are on chromosomes confirmed by Morgan's work with Drosophila (fruit flies).

Morgan's Experiment

• Discovery of White-Eyed Mutation:

  • Mutant male Drosophila with white eyes.
  • Cross with red-eyed female; F1 progeny all red-eyed (red is dominant).

• Nomenclature for Drosophila Genetics:

  • Dominant allele for red eyes: w+.
  • Recessive allele for white eyes: w.

• F2 Generation Observations:

  • Unexpected results: F2 generation had only white-eyed males.
  • Suggests sex linkage; gene for eye color on X chromosome.

Chromosome and Gene Mapping

• Experiment Analysis:

  • F1 females crossed with F1 males predicted 3:1 red to white ratio.
  • Actual results: Only males had white eyes, suggesting X-linked gene.

• Punnett Square Analysis:

  • Explains unexpected F2 results using X and Y chromosome differences.
  • Hypothesis: Eye color gene is X-linked as Y chromosome lacks this gene.

Sex Chromosomes and Determination

• X and Y Chromosomes:

  • X and Y differ; Y lacks certain genes present on X.

• Sex Determination Systems:

  • Drosophila and humans use XY system: XX female, XY male.
  • Birds use ZW system: ZZ male, ZW female.
  • Other systems include XO in insects, and haploid/diploid in bees.

X-Linked Traits and Disorders

• X-Linked Recessive Conditions:
  • More common in males due to single X chromosome.
  • Examples: Color blindness and hemophilia.

Dosage Compensation and X Inactivation

• Dosage Compensation:

  • Females inactivate one X chromosome (becomes Barr body) to balance gene dosage.

• Calico Cats:

  • Exhibit X-linked mosaicism in fur color (black and orange patches).

Exceptions to Chromosome Theory

• Non-Chromosomal Genes:

  • Mitochondrial and chloroplast genes passed maternally (maternal inheritance).

• Gene Mapping:

  • Mapping genes on chromosomes using recombination frequency.
  • Crossovers in meiosis create recombinant alleles.

Genetic Mapping Experiment

• Recombination and Mapping:

  • Recombinant frequency: Number of recombinants divided by total progeny.
  • Example calculation: 12% recombination frequency between linked traits.

• Maximum Recombination:

  • Maximum measurable recombination frequency is 50% (indicates unlinked genes).

Summary and Conclusion

• Genes are confirmed to be on chromosomes through various experiments.
• Importance of recombination in genetic mapping.
• Understanding linkage and recombination helps in determining gene positions on chromosomes.