Mendel’s work and the genes

Collegium Budapest Eötvös University Budapest

Eörs Szathmáry (Alpbach 2005)

Concepts of inheritance

• 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