Overview
This lecture covers the Chromosomal Theory of Inheritance, genetic linkage, homologous recombination (crossing over), and how genetic maps are constructed using recombination frequencies.
Chromosomal Theory of Inheritance
- Chromosomes carry genes, the units of heredity, as proposed by Sutton and Boveri in 1902.
- Key observations supporting the theory include random chromosome sorting, gametes having half the chromosome number, and chromosomes combining at fertilization.
- Carothers and Morgan provided experimental evidence for chromosomes as the basis of Mendelian inheritance.
Genetic Linkage
- Traits on the same chromosome (linked genes) are often inherited together, which can disrupt Mendel’s law of independent assortment.
- Linked genes can become unlinked during meiosis via homologous recombination (crossing over).
Homologous Recombination (Crossing Over)
- During meiosis, homologous chromosomes exchange segments at chiasmata, creating new allele combinations (recombinants).
- Crossing over usually happens more frequently the farther apart two genes are on a chromosome.
- Parental types exhibit the same allele combinations as the parents, while recombinants display new combinations.
Genetic Mapping and Recombination Frequency
- Recombination frequency between genes is used to estimate their distance apart on a chromosome (measured in centimorgans, cM).
- 1% recombination frequency equals 1 cM; higher frequencies indicate greater distance.
- Genes with a recombination frequency of 0.5 assort independently, either being on different chromosomes or very far apart on the same chromosome.
- Alfred Sturtevant constructed the first genetic map using recombination data.
Experimental Evidence and Applications
- Experimental crosses in Drosophila showed that some genes are linked but can be separated via crossing over.
- McClintock, Creighton, and Stern provided physical proof of chromosome segments exchanging during recombination.
Mendel’s Traits and Genetic Maps
- Mendel’s studied traits were either on different chromosomes or far apart, so he only observed independent assortment.
- Modern mapping confirms Mendel’s luck or deliberate exclusion of linked traits.
Key Terms & Definitions
- Chromosomal Theory of Inheritance — the idea that chromosomes are the carriers of genes.
- Genetic Linkage — the tendency of genes close together on a chromosome to be inherited together.
- Homologous Recombination (Crossing Over) — exchange of genetic material between homologous chromosomes during meiosis.
- Recombination Frequency — proportion of recombinant offspring, used to estimate gene distance.
- Centimorgan (cM) — unit of genetic distance; 1 cM = 1% recombination frequency.
- Parental Types — offspring with the same allele combinations as parents.
- Recombinants (Nonparental Types) — offspring with new allele combinations due to crossing over.
Action Items / Next Steps
- Review how to calculate recombination frequencies and genetic map distances.
- Study Sturtevant’s method for creating genetic maps.
- Practice problems with three-point test crosses for gene mapping.