Announcements We will finish linkage today and start bacterial genetics next week ( a day behind according to syllabus) Monday morning is very last chance to turn in a rough draft of “Monk” paper to get feedback by Tuesday morning. They are due in lab, 10/1 or 10/2. The average on Exam 1 was:

Review of Last Lecture Single crossovers Three-point mapping - identifying double cross-over events Determining the order of genes

Outline of Lecture 12 Determining the order of genes, continued Linkage and mapping in haploid organisms Somatic cell hybridization - human chromosome maps

I. The order of genes: 3-point Mapping in Maize bm (brown midrib), v (virescent seedling), and pr (purple aleurone)

Results of Cross

Questions What is the heterozygous arrangement of alleles in the female parent? What is the gene order? What are the map distances between each pair of genes?

II. Linkage and mapping in haploid organisms Chlamydomonas and the Benefits of Sex Reproduction

Life Cycle of Fungi Neurospora and Sordaria Note different meaning of “tetrad”

Tetrad Analysis

Mapping the Centromere Essentially like 2-point mapping problem between one gene locus and the centromere. Identify first-division segregation (may or may not be most common group) from second-division segregation. D = 1/2(second-division segregant asci)/total. For example, if there are 65 first-division asci and 70 second-division asci, then D = 1/2(70/135) = 0.259 or 26 map units.

III. Human Chromosomes have been Mapped by Somatic-cell Hybridization Two cells from mouse and human fused to form heterokaryon (two nuclei in common cytoplasm). Nuclei fuse to form synkaryon and lose human chromosomes over time. Gene products are assayed and correlated with remaining human chromosomes. Genes also mapped by pedigree analysis and recombinant DNA techniques.

Example Gene A: Gene B: Gene C: Gene D:

Human Chromosome Maps

Why didn’t Mendel Observe Linkage? There are 7 chromosomes and 7 genes Did he get one gene per chromosome? Genes are located on four chromosomes, but far enough apart to seem unlinked (frequent crossing over creates independent assortment). He should have seen linkage if he had mated dwarf plants with wrinkled pea, but he apparently didn’t do this experiment.