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Announcements
1. We will finish linkage today and start bacterial
genetics next week ( a day behind according to
syllabus)
2. 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.
3. The average on Exam 1 was:
Review of Last Lecture
I.
Single crossovers
II.
Three-point mapping - identifying double cross-over
events
III.
Determining the order of genes
Outline of Lecture 12
I.
Determining the order of genes, continued
II.
Linkage and mapping in haploid organisms
III. 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
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.