# BIO305: Genetics

We are going to discuss the location of genes on the chromosome, what we mean by crossing over, how this is related to Mendel’s idea of independent assortment, and how we can use this information to map the location of genes on the chromosomes. Thomas Hunt Morgan first constructed genetic maps in his lab in 1910, and then McClintock discovered transposons in 1931.

Chromosome mapping started when scientists studying meiosis and the transfer of genetic material realized that if two genes were next to one another on a chromosome, then the particular alleles carried on those two genes were more likely to be passed on together than two genes far apart on a chromosome. This is because during the recombination stage of meiosis, genes that are closer to one another are more likely to become inherited together and thus passed on. Scientists can apply math to this linking of inheritance in genes and can map the order of genes on a chromosome based on the phenotype of the offspring.

This unit should take you approximately 7.75 hours to complete:

☐    Subunit 4.1: 1 hour

☐    Subunit 4.2: 3 hours

☐    Subunit 4.3: 1.5 hours

☐    Subunit 4.4: 2 hours

☐    Subunit 4.5: 0.25 hours

Unit4 Learning Outcomes
Upon successful completion of this unit, you will be able to: - calculate the distance between genes based on offspring ratios; - calculate the order of the genes based on offspring ratios; and - explain why genes on the same chromosome separate.

4.1 Crossing Over and Genetic Recombination   - Reading: Dr. John W. Kimball’s Biology Pages: “Crossing Over and Genetic Recombination in Meiosis” Link: Dr. John W. Kimball’s Biology Pages: “Crossing Over and Genetic Recombination in Meiosis” (HTML)

Instructions: Read this article for an introduction to the mechanism behind how crossing over happens, according to the most recent scientific data.

4.2 Relating Cross Over Frequency to Distance on Chromosomes   4.2.1 Mapping on the X Chromosome   - Reading: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 6: Recombination and Genetic Maps” Link: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 6: Recombination and Genetic Maps” (PDF)

Instructions: Select the PDF link for “Lecture 6: Recombination and Genetic Maps,” read these lecture notes. Note that genes that are on the same chromosome may separate during meiosis as result of crossing over. This text discusses mapping on the X chromosome in male fruit flies; this can be scored directly, because male fruit flies carry only one copy of the X chromosome. This text is technical, so please plan to read it several times for a full understanding.

Reading these lecture notes should take approximately 1 hour and 30 minutes.

4.2.2 Mapping on the Autosomes   - Reading: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 7: Three-Factor Crosses” Link: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 7: Three-Factor Crosses” (PDF)

Instructions: Select the PDF link for “Lecture 7: Three-Factor Crosses,” and read these lecture notes. These lecture notes discuss mapping the distance of two genes on the autosomes using heterozygote parents. Next, these lecture notes discuss mapping the distance of three genes by using parents that are heterozygotes for all three genes; this is called three-factor cross. This text is technical, so please plan to read it several times for a full understanding.

Reading these lecture notes should take approximately 1 hour and 30 minutes.

4.3 Tetrad Analysis   - Reading: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 8: Tetrad Analysis” Link: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 8: Tetrad Analysis” (PDF)

Instructions: Select the PDF link for “Lecture 8: Tetrad Analysis,” read these lecture notes. These lecture notes explain how to determine from experimental data whether two genes are linked or unlinked. These lecture notes explain the expected phenotype when only Mendel’s laws of segregation and independent assortment determine the phenotypes of the offspring, and these notes also explain the expected offspring phenotype when crossing over plays a role. This text is technical, so please plan to read it several times for a full understanding.

Reading these lecture notes should take approximately 1 hour and 30 minutes.

4.4 Statistical Evaluation of Genetic Linkage   - Reading: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 29: Statistical Evaluation of Linkage I” Link: Massachusetts Institute of Technology: Professor Chris Kaiser’s “Lecture 29: Statistical Evaluation of Linkage I” (PDF)

Instructions: Selec the PDF link for “Lecture 29: Statistical Evaluation of Linkage I,” and read these lecture notes. These lecture notes use family Huntington disease data to draw conclusions about genetic linkage. SSR stands for simple sequence repeats. LOD stands for logarithm of odds. LOD is a statistical test that helps to draw conclusions on genetic linkage; positive scores suggest linkage. This text is technical, so please plan to read it several times for a full understanding.

Reading these lecture notes should take approximately 2 hours.