Georgia Performance Standards: Meiosis: Georgia Performance Standards: Using Mendel’s laws, explain the role of meiosis in reproductive variability. Essential Questions: How does meiosis generate variation in offspring?

Forever Linked? Section 11-5 Some genes appear to be inherited together, or “linked.” If two genes are found on the same chromosome, does it mean they are linked forever? Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide. Go to Section:

Linked Genes Warm-up 1. In how many places can crossing over result in genes A and b being on the same chromosome? 2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e? 3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

Linked Genes: It’s easy to see how genes located on different chromosomes assort independently, but what about genes located on the same chromosome? Wouldn’t they generally be inherited together?

Linked Genes First, each chromosome is actually a group of linked genes. Second, Mendel’s principle of independent assortment still holds true.     It is the chromosomes, however, that assort independently, not individual genes.

Genes and Linkage: Crossover events separate and exchange linked genes If two genes are found on the same chromosome, does this mean that they are linked forever? Not at all. Crossing-over separates genes that had been on the same chromosome onto homologous chromosomes. Crossover events separate and exchange linked genes produce new combinations of alleles it helps to generate genetic diversity.

Gene Maps A gene map shows the relative locations of each known gene on a chromosomes Recombination rates measure the frequencies of crossing-over between genes used to construct genetic maps Ex: human genome map

Comparative Scale of a Gene Map Section 11-5 Mapping of Earth’s Features Mapping of Cells, Chromosomes, and Genes Cell Earth Country Chromosome Chromosome fragment State Gene City People Nucleotide base pairs Go to Section:

Figure 11-19 Gene of the Fruit Fly Section 11-5 Exact location on chromosomes Chromosome 2 Go to Section:

Checkpoint Questions: 1. How does the principle of independent assortment apply to chromosomes? 2. What are gene maps, and how are they produced? 3. How does crossing-over make gene mapping possible? 4. If two genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are?