The secret life of genes
Updated: Jan 17, 2020
It's been known for thousands of years that we inherit things: children look like their parents, plants and dogs are bred for their useful qualities, etc. Exactly how this works, however, was pretty mysterious. That was until the discovery of the gene.
Genes were discovered by a monk with a keen interest in gardening called Gregor Mendel. Simply by breeding pea plants with different characteristics like seed shapes and colours, he identified the main rules of how genes are passed on (inherited) and affect things we can see.
Let's have a look at one of the experiments Gregor did. First, he took a pure-bred pea plant that makes yellow seeds and one that made green seeds. He then crossed them together (i.e. pollinated one with the other to mate them). However many times he crossed these two characteristics (phenotypes); out of the hundreds of baby pea plants (offspring), every single one had yellow seeds.
-> That's odd, whats happened to the green seed genes? Have they disappeared completely?
These are questions Gregor was also wondering (probably), so he decided to cross these yellow-seeded plants from 'Generation 1' with each other (plants don't mind mating with their siblings).
When Gregor crossed the Generation 1 plants together, out of the hundreds of offspring, 75% always had yellow seeds and 25% had green seeds.
-> So the green seed genes were there all along!
One important discovery: you can carry genes in your DNA and pass them onto your offspring without actually showing the characteristics of that gene.
A second important discovery: how come there was always this 3:1 ratio in the second generation? We would have maybe expected a 50:50 split, or maybe some greeny-yellow seeds in there, but this was not the case. It turns out this 3:1 ratio can be explained by the the laws of inheritance that Gregor identified.
Based on this experiment and many more (and a whole lot of thinking about potential explanations), Gregor managed to create a theory to explain all the results he saw. He said that:
1) a gene that controls an aspect of the plant, e.g. seed colour, can come in different flavours (called alleles) e.g. green and yellow
2) every plant contains two copies of every gene. One comes from the father and one from the mother. The choice between which allele is passed onto the child is purely random.
3) which phenotype the plant has e.g. seed colour, depends on which 2 alleles it has, and also what types of alleles they are. Some alleles can be dominant (phenotype seen even if only 1 allele is there), and some recessive (phenotype only seen when 2 of that allele is there).
In green vs yellow seed case, Gregor explained the results seen by saying that the yellow allele is dominant (called Y) and the green allele is recessive (g).
This 'model' (or explanation), neatly explained all of the pea plant data, and turned out to be exactly right. These rules form the basis of classical genetics and has been really useful in experiments to figure out what genes are doing and whether they are dominant or recessive. There are many extra layers of rules on top of these ones, but what was maybe the most interesting thing from this model is that not all genes are inherited in the same way, and genes are also not expressed (i.e. you can't see their effects) in the same way.