Review Question 2: Crossing Over Can someone show me on the SMARTboard what crossing over is and when it occurs? Before crossing over After crossing over Gametes
Gene Linkage and Crossing Over Think about a cross between parents GGYY and ggyy. • What is the genotype of all F1 offspring? • If you cross the F1 offspring, what is the expected phenotypic ratio? GGYY ggyy
Gene Linkage and Crossing Over Sometimes the 9:3:3:1 ratio doesn’t work! • What would happen if G and Y genes are on the same chromosome: g Note that when the genes are on the same chromosome, they are inherited together! G F1 Offspring (GgYy) Y y Gametes (Note: 2 possibilities, not 4 possibilities)
Genetic Disorders & Pattern of Inheritance • LINKED GENES – Genes that are located on the same chromosome and therefore inherited together. • If two heterozygous parents (GgYy) mate, and we know that the genes are linked, the genotypic ratio of the offspring will be: • Normally, the ratio above is observed in crosses. But sometimes a few individuals can end up with the genotypes Ggyy or ggYy. • The ONLY way that this can happen is if crossing over of the chromosomes occurred during gamete formation. • The crossing over of genes on a chromosome is called recombination.
Genetic Disorders & Pattern of Inheritance • A crossover frequency of 5% means the genes are 5 map units away from each other. A crossover frequency of 16% means that the genes are 16 map units away from each other. • Therefore, crossover frequencies can be used to determine gene maps: Ex) Crossover frequency between gene A and B = 12% Crossover frequency between gene B and C = 7% Crossover frequency between gene A and C = 5%
Genetic Disorders & Pattern of Inheritance Select the correct sequence of genes based on the crossover frequencies: B A C D
Gene Recombinations in Nature • Crossing over in meiosis is not the only time recombinations happen in nature: • 1) Transposons: • Segments of DNA that can move along a chromosome (aka – “jumping genes”).
Gene Recombinations in Nature • Crossing over in meiosis is not the only time recombinations happen in nature: • 2) Conjugation: • A form of sexual production in which genetic material is switched between two bacterial cells. This is one way we get antibiotic resistant strains of bacteria!
Gene Therapy • What if we could fix mutated genes to cure genetic diseases such as hemophilia, Parkinson's disease, or sickle-cell anemia? • Gene therapy can be done in three ways: • Gene modification – repair of the mutated gene • Gene insertion – insert a normal copy of the gene into a cell • Gene surgery – remove the mutated gene and insert the normal gene into a cell.