Lecture 2: Mendelian Genetics
Moravian Sheep Breeders
• Selective breeding practices had produced valuable flocks of merino sheep that made large quantities of soft, fine
• Breeders dilemma
o I possess a ram that would be priceless if its advantages are inherited by its offspring, but if they are not
inherited, then it is no more than the cost of wool, meat and skin.
• Two hypotheses to explain inheritance
o One parent contributes more to an offspring’s inherited traits: Aristotle contended that it was the male and
that a fully formed homunculus was inside the sperm
o Blended inheritance: the traits of the parents are blended in their offspring (like blue and yellow to make
green) – explained single offspring, but not siblings, or the next generation
• Abbott Cyril Napp presides over the Augustinian monastery in Brunn, in the province of Morovia, in Austria
o Proposed that breeders could improve their ability to predict what traits would appear in their offspring if
they addressed three questions?
▪ What is inherited?
▪ How is it inherited?
▪ What is the role of chance in heredity?
• Entered the Augustinian monastery in Brunn in 1843 at age 21
• Napp sent Mendel to University of Vienna to study physics, maths, chemistry, botany, palaeontology and plant
• Returned from Vienna in 1853, began genetics experiments in 1854, published results in 1865 at the age of 43
• Key to Mendel’s success was how he set up experiments
o Model was pea plants
• Need to identify organisms that are more amenable to genetic analysis-model organisms
o Short generation times
o Can be inbred (self-fertilize)
o Simple reproductive biology
o Small size (easy to grow/breed)
o Large numbers of progeny
• Well characterized, cultivated plant
• Could be selfed – pollen from the plant could be used to pollinate its own flowers—allows inbreeded
o Had both male and female parts
• Could obtain and maintain pure-breeding lines- these always bred true producing the same trait generation upon
• Could be readily cross-fertilized to create hybrids between pure-breeding lines—could have carefully controlled
matings and reciprocal crosses to rules out the effect of one parent versus the other
• Could examine clear-cute (qualitative/discrete) traits where there were 2 forms of the trait – “either-or” choices—
unambiguously distinguish forms of the trait
• Could have a large number of plants and progeny, so could subject the data to statistical
analysis- Mendel did quantitative analyses that produced robust results and aided