Sunday, October 24, 2010

The genetics of garden peas- with credit to Gregor Mendel

This year we are growing two varieties of pea (Pisum sativum) in our garden:

Early crop Massey (which I now find are also called Melbourne Market)
As I watched the two plants grow, I started to remember snippets of biology lessons: pea flower colour, dwarf or tall, Mendel's Garden... I looked up my favourite biology text, Neil Campbell's Biology (now Campbell and Reece- RIP Neil Campbell) and sure enough, I was observing the same differences between my pea plants as Gregor Mendel did, in an Austrian monastry 150 years ago.

If you don't know Mendel: he was an Augustinian monk, and I suspect he is one of the few monks who can lay legitimate and honourable claim to fatherhood? (Are there others?) Mendel is considered "the father of modern genetics" because of his studies of the inheritance of pea characteristics.

This was a fifty years before anyone had shown that it is DNA that carries traits between generations (thank you Oswald Avery), and a hundred years before anyone could see how DNA could specify different characteristics (thank you James Watson and Francis Crick). Gregor Mendel meticulously studied the characteristics of peas, and controlled pollination (peas normally self-pollinate) to study the effect of interbreeding different characteristics. From this he realised that there was something that was transmitted between generations that enabled the features of a parent plant to be transmitted to its offspring. More importantly, he realised that this something was independently inherited for each feature of the plant- for example, whether a plant had purple or white flowers didn't influence whether it had yellow or green seeds.

Mendel's genius came when he applied mathematical techniques to analyse the offspring of parents with different features, and realised that for each feature in a plant (eg flower colour), a plant inherits information from both parents, yet some versions are "dominant" so mask the effect of the "recessive" ("not dominant" version). So, for example, the offspring of a purple flowering and a white flowering pea plant will have purple flowers (purple is dominant over white). However, these offspring still contain the "white" information, because their offspring (grandchildren of the original white- and purple-flowering plants) can show the white flowering trait.

The characteristics Mendel studied included flower colour (purple/white), flower position (on a side shoot/at the top of the main shoot), seed colour (yellow/green), seed shape (round/wrinkled), pod shape (inflated/constricted), pod colour (yellow/green), and height (tall (2m)/dwarf (0.5m)) (the illustration to the left is taken from Campbell and Reece's Biology.)

This whole thought process about Mendel came about because the two pea varieties I have differ in five of these Mendelian characteristics:

Flower colour: Early Crop Massey- white, Golden Podded- purple
Seed colour: Early Crop Massey- green, Golden Podded- yellow
Pod shape: Early Crop Massey- inflated, Golden Podded- constricted
Pod colour: Early Crop Massey- green, Golden Podded- yellow
Height: Early Crop Massey- 0.5m (dwarf), Golden Podded- 2m (tall)

I have no plans to do the intercrosses to re-prove Mendel's findings, but it's pretty cool to think that peas haven't changed much in 150 years!

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