Lecture 8

• Who is quoted in the statement “Nothing in biology makes sense except in the light of evolution”?

  Theodosius Dobzhansky
• What are the learning objectives of BIOS3240 – Genetics and Evolution?

  • Understand the principle of genetic linkage
  • Learn how genetic distance is measured
  • Analyze two- and three-point test crosses to measure genetic distances
• What is the maximum recombination fraction (RF) in genetic linkage?

  50%
• How is genetic distance measured?

  By the proportion of offspring that are recombinant
• If there are 1000 offspring and 100 are recombinants, what is the recombination fraction?

  $RF =$$\frac{100}{1000} =$$10\%$
• What is a two-point testcross?

  • A cross between two genes to determine parental types
  • Involves phenotypes and genotypes of offspring
  • Used to calculate recombination frequency
• What is the total number of offspring in the two-point testcross example?

  1000
• What does a linkage map show?

  The position of known genes relative to each other in terms of recombination frequency
• What is the purpose of a three-point testcross?

  • To map genes in groups of three
  • Allows determination of distances and order of genes
  • Involves counting phenotypic classes in progeny
• How many different phenotypic possibilities are there in a three-point testcross?

  8
• What are the four outcomes of a three-point testcross?

  1. No recombination in either interval
  2. Recombination in the first interval only
  3. Recombination in the second interval only
  4. Recombination in both intervals
• How do you determine the most likely outcome in a three-point testcross?

  By identifying the parental classes and comparing recombination events
• What is the total number of offspring in the three-point cross example?

  1000
• What do parental haplotypes represent in a three-point cross?

  • The most common groups of offspring
  • Indicate the original combinations of alleles
  • Help identify the parental classes
• How do you identify double recombinants in a three-point cross?

  By finding the two smallest groups of offspring
• What is the conclusion regarding the gene in the middle of a three-point cross?

  It is the gene that has undergone the most recombination events
• What are the steps to calculate the final genetic map for genes p, j, and r?

  1. Determine the recombination fraction for each interval
  2. Identify parental and double recombinant classes
  3. Draw the genetic map showing distances
• If the recombination fraction between genes p and j is 24%, what is the genetic distance?

  $24cM$
• If the recombination fraction between genes j and r is 12%, what is the genetic distance?

  $12cM$
• What points should be remembered when analyzing a three-point cross?

  • Identify the parental class as the two largest groups
  • Check for matching pairs in double recombinants
  • Determine the gene in the middle by comparing parental and double recombinant classes
• What is the key reading for the course BIOS3240?

  Campbell biology “Linkage and chromosomes”
• In which edition of Campbell biology can you find the relevant chapter for this course?

  12th edition
• What pages of the 12th edition of Campbell biology are particularly relevant for this course?

  Pages 344-357