Overview
This lecture uses a creative wedding analogy to explain key concepts in genetics: independent assortment, recombination, gene linkage, recombination frequency, and gene mapping.
Chromosome Behavior in Meiosis
- Homologous chromosomes line up randomly during meiosis, determining which parent’s chromosomes go into gametes.
- The law of independent assortment states alleles on different chromosomes are inherited independently.
- Alleles (gene variants) from one chromosome do not affect alleles inherited from another chromosome.
Recombination and Crossing Over
- Recombination (crossing over) is the exchange of DNA segments between homologous chromosomes during meiosis I.
- Crossing over produces chromosomes with new combinations of maternal and paternal alleles (recombinant chromosomes).
- Recombinant offspring carry allele combinations not found in the parental chromosomes.
Linked Genes and Independent Assortment
- Genes far apart on a chromosome or on different chromosomes assort independently (unlinked genes).
- Linked genes are close together on the same chromosome and tend to be inherited together.
- Linked genes do not follow independent assortment because they are less likely to be separated by recombination.
Recombination Frequency and Genetic Mapping
- Recombination frequency measures how often alleles of two genes are separated by crossing over.
- A recombination frequency of 50% means genes are unlinked and assort independently.
- Linked genes have recombination frequencies less than 50%.
- Gene maps show the order and distance between genes on a chromosome, based on recombination frequencies.
Centimorgans and Gene Mapping
- Distances on gene maps are measured in centimorgans (cM).
- 1 centimorgan = 1% recombination frequency; maximum is 50 cM (unlinked).
- Gene mapping helps locate genes and identify mutations responsible for genetic disorders.
Key Terms & Definitions
- Homologous chromosomes — Chromosomes with the same genes from each parent.
- Allele — Variant form of a gene.
- Independent assortment — Random distribution of chromosome pairs to gametes.
- Recombination (crossing over) — Exchange of DNA between homologous chromosomes.
- Linked genes — Genes located close together on the same chromosome.
- Recombination frequency — Likelihood that alleles on the same chromosome are separated during crossing over.
- Gene map — Diagram showing ordered positions of genes on a chromosome.
- Centimorgan (cM) — Unit of genetic distance equal to 1% recombination frequency.
Action Items / Next Steps
- Review how to calculate recombination frequencies and interpret gene maps.
- Study problems involving linked vs. unlinked genes and predicting offspring genotypes.