Mendel’s Gene Ideas
In the 19th century, a European monk called Gregor Mendel performed experiments in the garden of his abbey that would forever change the course of biology. Until that point, farmers and agriculturalists had been attempting to grow hybrids of plants with mixed success, because they didn’t really know what laws dictated how genetic information is passed on. Though Mendel had never looked through a microscope and had no idea what the actual process of DNA replication and meiosis was, he still was able to determine four fundamental rules of genetics through the power of his well-designed experiments.
Mendel’s experimental subjects were peas (Pisum sativum). Not very exciting, but perfect for what he wanted: they had short generation times, a large number of offspring, can both self-pollinate or pollinate with others, and they were available in lots of varieties. Their physical characteristics like colour and shape were also very obvious, allowing him to visually track the changes through the generations. Remember that the expression of a gene is called a character, like pea colour, and the expression of an allele is called a trait, like green or yellow pea colour.
Mendel selected pure-bred pea plants with particular traits and cross-bred them in order to see what phenotypes they expressed in different generations. Using carefully planned experiments, he figured out patterns of inheritance.
In a typical experiment, Mendel would take two very different pure-bred plants and cross them together. For example, he crossed a pure-bred yellow pea plant with a pure-bred green pea plant. This mating is called hybridisation. Note: we call the first generation the P generation (for parent), the second generation the F1 generation, and the third generation the F2 generation, usually given by self-pollination.
In completing this yellow and green pea cross, Mendel found this relationship:
In the F1 generation, all plants produced yellow peas. Even though the parents were green and yellow, the F1 generation didn’t mix the colours to be greeny-yellow—they were just yellow.
Then, when Mendel crossed the F1 gen with the F1 gen (through self-pollination), he found that only three quarters of the plants had yellow peas, and one quarter mysteriously were green. This 3:1 ratio is recurring—remember it.
This happened every single time Mendel crosses plants in this way. He realised that this regularity must be the key to some underlying mechanism of inheritance.
If crossing two different genes plants could blend the genes, then we’d expect the F1 generation to be a greeny-yellow colour. However, this isn’t the case. So what happened to the green in the F1 generation? Obviously it hasn’t disappeared completely, because it crops up again in F2. Mendel reasoned that it was hidden from view, and termed the yellow colour as a dominant trait and the green colour as a recessive trait. We now know that there are dominant alleles and recessive alleles, and only one is expressed in the phenotype.
In order to come up with the law of segregation, Mendel noted four related concepts.
- Offspring have different physical characteristics because genes have different “versions”, called alleles. These account for variation.
- For each gene, an offspring inherits one allele from each parent.
- If two alleles at a locus are different, then the dominant allele will determine what the organism looks like. The recessive allele will have no observable effect.
- The two alleles for a gene of one parent separate when gametes are formed. They end up in different gametes, so the egg or sperm only get one of the two alleles present in the somatic cells of the organism. For example, if the gene has a dominant and a recessive allele—this separation means that 50% of gametes will end up with the dominant allele, and 50% will end up with the recessive one. This is called the Law of Segregation.