Transcript Mendel`s work

Mendel’s work and the genes
Eörs Szathmáry (Alpbach 2005)
Collegium Budapest
Eötvös University Budapest
Concepts of inheritance
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J.B.S. Haldane:
„I inherited my watch from my father”
„I inherited my nose from father”
Geneticists are interested in the latter
„Genetics deals with the question why
organisms that look almost alike are
nevertheless different, in a hereditary
fashion”
The man and the garden
The plant and the work
Traits chosen by Mendel
Prevention of self-fertilization
Mendelian inheritance
• Gametes are
always clean!
• Stoichiometric
paradigm
• Probabilistic
combinations
Segregation of dominant mutation
½ of the offsping
in F2 generation
shows dominant
phenotype
Segregation of recessive mutation
Recessive
phenotype
appears in 1:3
proportion in
generation F2
Two traits together
• A concrete count of
two segregating traits
Two segregating traits
• No linkage
• Punnett’s table
• Independent
combination
Mendel was extremely lucky that his
traits are on different chromosomes
• Some deviations from Mendel’s rules could
not be reconciled in any other way than
assuming that they are linked together as
“beads on a string”
• Morgan has made crosses to analyse linkage
• The concept of recombination was later
linked to the cytological observation of
meiosis (reductive cell division)
The cytology of meiosis
How meiosis is
integrated into
the plant life
cycle
The molecular mechanism of
recombination
Whether recombination
really occurs depends on
the way the Holliday
junction is resolved
Whereas genetic recombination can
generate variation, its evolutionary
role is unclear
• Genetic recombination can generate good
chromosomes out of partially bad ones
• But, unfortunately, the opposite is also true
• Something must generate an asymmetry
because recombination is “costly”
The compementarity principle
The principle of DNA copying
Genes tend to specify the structure
of a protein
• Nucleotides sequence of DNA specifies the
amino acid sequence of proteins
• Proteins can be structural components or
enyzmes
• “Information: the precise determination of
sequence” (Francis Crick)
This shows the genetic code
The genetic code is remarkable
The simplest cells are bacterial
Eukaryotic cells are very complex
Bacterial genes are much simpler
than eukaryotic ones
A vast variety of gene products are
generated by alternative splicing