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UD - BISC 403010 - Class Notes - Week 4

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UD - BISC 403010 - Class Notes - Week 4

School: University of Delaware
Department: Biology
Course: Genetic and Evolutionary Biology
Professor: Erica Selva
Term: Fall 2015
Tags: Biology
Name: Linkage__Recombination__and_the_Mapping_of_Genes_on_Chromosomes.pdf
Description: These notes cover linkage, recombination, and the mapping of genes on chromosomes
Uploaded: 03/19/2017
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background image Instructor: Dr. Olabisi 
Textbook: Genetics: From Genes to Genome by Hartwell 
 
Drosophila melanogaster   -  There are  4  homologous pairs of chromosomes   o  2 pairs of large autosomes 
o  1 pair of very small autosomes 
o  A pair of sex chromosome  
-  Females normally have two X chromosomes  
-  Males -> one X and a tiny Y chromosome  
Human Genome   -  It is made up of  23  chromosome pairs with a total of about 3 billion DNA base pairs  -  There are an estimated 20,000-25,000 human protein-coding genes  Linkage and Recombination  -  Genes linked together on the  same  chromosome usually  assort together   -  Linked genes may become separated by recombination  Genetic Linkage   -  The phenomenon in which genes that are close together on the same chromosome tend  to be transmitted as a unit  -  The proximity of two or more markers on a chromosome; the closer together the  markers are, the lower the probability that they will be separated by chromosomes  Independent assortment   -  When genes are on  separate  chromosomes, or very  far apart  on the same  chromosomes, they  assort independently  due to crossing over (homologous  recombination)  o  Because of the random orientation of homologous chromosome pairs  during meiosis    -  For example,   o  AaBb -> AB, Ab, aB, ab 
o  4 combinations 
Linkage   -  When genes are  close together  on the same chromosome, they are said to be  linked   -  The genes do not independently assort  
-  Two genes on the same chromosome segregate together  
-  For example,  
o  AaBb -> AB, ab 
o  2 combinations  
-  Genes on separate chromosomes are  never  linked  
background image Instructor: Dr. Olabisi 
Textbook: Genetics: From Genes to Genome by Hartwell 
 
Recombination and distance   -  Two very  close-together  genes will have very  few recombination  events and be tightly  linked, while two genes that are slightly further apart will have more recombination 
events and be less tightly linked 
-  The frequency with which genes become separated reflects the physical distance  between them  o  Crossovers during meiosis happen at more or less  random  positions along the  chromosome, so the frequency of crossovers between two genes depends on 
the distance between them 
Properties of Linked vs. Unlinked Genes   -  Linked genes   o  Parentals  >  Recombinants (RF < 50%)  o  Deviation from 1:1:1:1 segregation in F 2  indicates the genes are linked  o  Linked genes must be  syntenic  and sufficiently close together on the same  chromosome so that they do not assort independently     Syntenic genes: genes located on the same chromosome  -  Unlinked genes   o  Parentals  =  Recombinants   o  Occurs either when two genes are on different chromosomes or when they are  sufficiently far apart on the same chromosomes that at least one crossover 
occurs between them in every meiosis  
Recombination frequencies   -  Reflect distances between two genes  
-  The recombination frequency is 
relatively constant  between homologous chromosomes  -  For a given chromosome, N number of breaks will occur, but where they will occur is  random  -  Could be used as a gauge of the  physical distance  separating any two genes on the same  chromosome   -  1 %RF = 1 map unit (m.u.) = 1 centiMorgan (cM)  Two point crosses   -  Can establish the  order  of linked genes and the  distances  between them through  pairwise analysis of recombination frequencies  Limitations of two point crosses  -  Difficult to determine gene order if two genes are  close  together  Actual  distances between genes  do not always add up   -  Pairwise crosses are time and labor consuming 

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School: University of Delaware
Department: Biology
Course: Genetic and Evolutionary Biology
Professor: Erica Selva
Term: Fall 2015
Tags: Biology
Name: Linkage__Recombination__and_the_Mapping_of_Genes_on_Chromosomes.pdf
Description: These notes cover linkage, recombination, and the mapping of genes on chromosomes
Uploaded: 03/19/2017
6 Pages 15 Views 12 Unlocks
  • Better Grades Guarantee
  • 24/7 Homework help
  • Notes, Study Guides, Flashcards + More!
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