Overview
This lecture covers the Chromosomal Theory of Inheritance, genetic linkage, homologous recombination, and how gene positions can be mapped using crossing over frequencies.
Chromosomal Theory of Inheritance
- Chromosomal Theory states that chromosomes carry genes, the units of heredity.
- This theory explains Mendel's laws based on chromosome behavior in meiosis.
- Key observations supporting the theory include independent migration and random sorting of homologous chromosome pairs, and gametes having half the chromosome number of parents.
- Equal genetic contribution from male and female gametes is indicated by their chromosome numbers.
- Fertilization restores the original chromosome number in offspring.
- Physical evidence (Carothers) and experimental support (Morgan) confirmed the theory.
Genetic Linkage
- Genes on the same chromosome are linked and may be inherited together, opposing Mendel's independent assortment.
- Linked genes can become unlinked through crossing over (homologous recombination) during meiosis.
- Parental types have the same allele combinations as parents, while recombinant (nonparental) types result from crossing over.
- The frequency of recombinant offspring is evidence of gene linkage and crossing over.
Homologous Recombination (Crossing Over)
- Homologous recombination occurs when synapsed chromosomes exchange DNA segments at chiasmata during meiosis.
- Crossing over increases genetic diversity by producing new allele combinations.
Genetic Mapping and Recombination Frequency
- Recombination frequency estimates the distance between genes; 1% recombination = 1 centimorgan (cM).
- Genes that are far apart have higher recombination frequencies and are less often inherited together.
- Sturtevant’s genetic maps represented gene order and distances on chromosomes using recombination data.
- Maximum measurable recombination frequency is 50%, indicating unlinked genes.
Mendel’s Traits and Chromosome Mapping
- Mendel’s studied traits were either on different chromosomes or far apart, so he observed independent assortment.
- Mapping genes confirmed that Mendel’s analyzed traits are effectively unlinked.
Key Terms & Definitions
- Chromosomal Theory of Inheritance — Concept that chromosomes carry hereditary units (genes).
- Genetic Linkage — Tendency of genes close together on a chromosome to be inherited together.
- Homologous Recombination/Crossing Over — Exchange of chromosome segments between homologous chromosomes during meiosis.
- Recombination Frequency — Proportion of recombinant offspring, used to estimate gene distance.
- Centimorgan (cM) — Unit of genetic distance corresponding to a 1% recombination frequency.
- Parental Type — Offspring with the same allele combination as the parents.
- Recombinant Type — Offspring with a new combination of alleles due to crossing over.
Action Items / Next Steps
- Review Sturtevant’s genetic mapping process using the linked resource.
- Practice calculating recombination frequencies and constructing simple gene maps.
- Prepare for questions on genetic linkage, crossing over, and chromosome theory.