BIO 201 Chapter 5 PPT

• Linked genes are located close together on the same chromosome and travel together during meiosis, ending up in the same gamete and therefore inherited together
• Chromosomes are called linkage groups, containing a group of genes that are linked together
• The number of linkage groups is the number of types of chromosomes of the species
• In humans, there are 22 autosomal linkage groups, an X chromosome linkage group, and a Y chromosome linkage group
• Genes that are far apart on the same chromosome may independently assort from each other due to crossing over
• Crossing over occurs during prophase I of meiosis at the bivalent (tetrad) stage
• During crossing over, non-sister chromatids of homologous chromosomes exchange DNA segments
• Recombination frequency (%) = (Number of recombinant progeny / Total number of progeny) x 100
• Crossing over during meiosis allows the homologous X chromosomes to exchange pieces of chromosomes, creating new combinations of alleles
• The coupling or repulsion arrangement of linked genes on a chromosome affects the results of a testcross
• Genetic mapping is also known as gene mapping or chromosome mapping
• Genetic maps are calculated using recombination data
• Purpose of genetic mapping is to determine the linear order of linked genes along the same chromosome
• Alfred Sturtevant worked out the first chromosome map in Drosophila using five linked genes on the X chromosome in 1911
• Genetic maps are useful to understand complexity and genetic organization of a species
• Genetic maps help improve understanding of evolutionary relationships between species
• Genetic maps can be used to diagnose and potentially treat inherited human diseases
• Genetic maps aid in predicting the likelihood of producing children with certain inherited diseases
• Genetic maps provide information for improving agriculturally important strains through selective breeding programs
• Percentage of recombinant offspring is correlated with the distance between two genes

  If genes are far apart, many recombinant offspring

  If genes are close, few recombinant offspring
• Map distance = Number of recombinant offspring / Total number of offspring x 100
• One map unit is equivalent to 1% recombination frequency
• Chromosomes are the product of a crossover during meiosis in the heterozygous parent
• Recombinant offspring are fewer in number than nonrecombinant offspring
• The most accurate method of mapping is using the trihybrid (three-point) testcross data directly
• Product rule allows predicting the likelihood of a double crossover from individual probabilities of each single crossover
• Interference is expressed as I = 1 - C, where C is the coefficient of coincidence and C=(observed # of double crossovers)/(expected # of double crossovers)
• Interference of 60% means that 60% of the expected number of crossovers did not occur
• Positive interference occurs when two genes are very close together, decreasing the probability of a second crossover nearby
• Observed number of double crossovers was lower than expected due to positive interference
• Crossing over between linked genes may produce recombinant phenotypes
• If crossing over occurs between 2 normally linked loci, they will sort independently
• Bateson, Saunders, and Punnett discovered 2 traits that did not assort independently


  Sweat pea flower color and pollen shape

  Did not get 9:3:3:1 in F2
• In Morgan's F2 generation he observed a much higher proportion of the combinations of traits found in the parental generation. What was his explanation?


  All 3 genes are located on the X chromosome

  Therefore, they tend to be transmitted together as a unit
• Morgan's 3 hypotheses

  1. Genes for those traits are all located on the same chromosome, so they tend to be inherited together.
  2. Due to crossing over during meiosis, the homologous chromosomes can exchange pieces of chromosomes, creating new combinations of alleles.
  3. The likelihood of crossing over depends on the distance between two genes. Crossing over is more likely to occur between two genes that are far apart.
• Coupling Arrangement

  P1 is heterozygous for both traits and P2 is homozygous recessive for both traits. This results in the nonrecombinant progeny displaying the dominant phenotype or the recessive phenotype for both traits (SAME) and the recombinant progeny showing different phenotypes for each traits (DIFFERENT).
• Repulsion Configuration


  P1 is heterozygous for both traits, but the alleles are on different chromosomes (1 has D and R; other has R and D) and P2 is recessive. The nonrecombinant progeny display the different phenotype for each trait while the recombinant progeny display the same phenotypes (dominant or recessive) for both traits.
• The following testcross produces the progeny shown:

  AaBb X aabb --> 10 AaBb, 40 aaBb, 40 Aabb, and 10 aabb.

  Were the genes in the AaBb parent in coupling or repulsion?


  Genes in AaBb parent are in repulsion.
• Chi Square Analysis is frequently used to determine if the outcome of a dihybrid cross is consistent with linkage or independent assortment
• Do the following data fit a Mendelian ratio?

  Gray body, red eyes: 1,159

  Yellow body, white eyes: 1,017

  Gray body, white eyes: 17

  Yellow body, red eyes: 12

  $x^2=$$2109.8$

  $x^2=$$7.815$