HMB265H1 Lecture Notes - Lecture 8: Thomas Hunt Morgan, Harriet Creighton, Chromosomal Crossover
20 May 2018
School
Department
Course
Professor
Lecture 8: Linkage and Recombination
Thomas Hunt Morgan
• Investigated 2 traits at a time
o Noticed a departure from Mendel’s laws
• Did a dihybrid test cross
• Crossed the F1 generation with a tester strain
o Tester flies are always males that are
recessive for both alleles
o Recombination only occurs in females
• F2 did not show a Mendelian ratio of 1:1
o Majority of the offspring were of parental
genotype
o Rest of the offspring were recombinants
• Morgan predicted that the eye colour gene and the
wing gene were on the same chromosome
o Linked
• Recombination results when crossing-over during
meiosis separates linked genes
• Frans Janssens observed chiasmata, regions in
which nonsister chromatids of homologous
chromosomes cross over each other
• Thomas Hunt Morgan suggested these were sites of chromosome breakage and exchange resulting in genetic
recombination
o Increases diversity
• Physical exchanged among nonsister chromatids; visualized
cytologically as a chiasma
• Typically, several crossing-over events occur within each bivalent or
tetrad in each meiosis (chiasmata physically hold homologous chromosomes together and assure proper
segregation at Anaphase I)
• Resulted in 2 parental types equal in frequency to each other and total
>50%
• Also resulted in 2 recombinant types equal in frequency to each other
and total <50%
• Genetic recombination depends on the reciprocal exchange of parts
between maternal and paternal chromosomes
o Harriet Creighton and Barbara McClintock studied corn
o Curtis Stern studied fruit flies
o Physical markers used to keep track of specific chromosome
parts
▪ Cytological markers
o Genetic markers were points of reference to determine if
particular progeny were the result of recombination
McClintock and Creighton
• Visual confirmation that chromosomes cross over
• Resulted in new allele combinations
• Correlation between genetic crossover and chromosomal crossover
• Found cytological marker – part of chromosome 8 was attached to chromosome 9
• Also found a genetic marker 2 genes in corn on chromosome 9 – C for colour and
wx for Wax
find more resources at oneclass.com
find more resources at oneclass.com
25
HMB265H1 Full Course Notes
Verified Note
25 documents
