Gene Mapping

• Linked genes are close together on the same chromosome
• Genes on the same chromosome may not show random assortment into gametes
• If genes A and B are very close together, an A B/ab heterozygote may give only A, B and a, b gametes
• If genes D and E are far apart, even on the same chromosome, a DE/de heterozygote may produce 25% DE, 25% De, 25% dE, and 25% de gametes just as would occur if D and E are on different chromosomes
• Crossing over occurs in the 4 strand stage of Meiosis I when already duplicated chromosomes are paired
• Mapping genes on the X chromosome can be done by phenotyping the sons of heterozygous females
• Map distance = percent recombination
• Crossing over occurs in meiosis at fairly random locations along paired chromosomes
• Since synapsis in Meiosis I occurs at the 4 strand stage of paired, duplicated chromosomes, only two of the four chromatids will be recombinant (the other two will still have parental arrangement of alleles), and the exchange created an X-shaped figure called a Chiasma
• Even genes far apart on the same chromosomes can show random assortment
• Map distance is defined as % recombination
• Crossing over involves breakage and rejoining of chromatids and can be affected by:

  • genetic background
  • temperature
  • exposure to radiation or other factors that induce chromosome breaks
  • the degree of base sequence homology
  • the sex of the heterozygous parents
• Recombination is not evenly distributed along the chromosome
• Formation of chiasma may decrease the odds of another forming nearby
• Linked genes are known as "haplotypes" where the alleles on each partner chromosome are shown together
• Coupling configuration occurs when both dominant alleles are on one chromosome and both recessives are on the other
• In repulsion phase, the heterozygous parent has one dominant and one recessive allele on each chromosome
• synteny mapping: locating genes onto specific chromosomes
• synapsis: already duplicated chromosomes have paired
• Most paired chromosomes will undergo at least one crossover
• Crossing over results in a reciprocal exchange of chromatids
• chiasma: the exchange point which is sometimes visible in stained chromosome spreads
• When we know genes are linked, we write the genotype showing those on each chromosome together
• Arrangement of genes in the parent has a dramatic effect on the frequency of each class of progeny
• Genes on the same chromosome belong to the same linkage group