6.11 Mendel and our understanding of genetics

Gregor Mendel

In the mid-1800s, Gregor Mendel, a monk in what is now the Czech Republic, carried out careful breeding experiments on pea plants. He studied single characteristics, such as tall or dwarf stems, and counted the offspring in large numbers.

What he found

1. Crossing pure-breeding tall plants with dwarf plants gave offspring that were all tall.
2. Crossing these offspring with each other gave 3 tall : 1 dwarf.

He concluded that:

• characteristics are determined by "units" that are passed on unchanged
• each plant has two units for each characteristic, one from each parent
• some units are dominant and can hide others (recessive), which can reappear in a later generation.

Why it took so long to be accepted

Mendel published in 1866, but his work was largely ignored until after his death because:

• chromosomes, genes and DNA had not been discovered, so there was no way to explain his "units"
• he was a monk, not a well-known scientist, and published in a little-read journal
• many scientists did not understand or accept his mathematical approach.

What came next

• Late 1800s: chromosomes were seen during cell division.
• Early 1900s: scientists saw that chromosomes and Mendel's units behaved in the same way, so the units must be on chromosomes. They were called genes.
• 1953: the structure of DNA was worked out, explaining how genes work.