Gregor Mendel was a nineteenth century monk widely known for his work with peas and genetics. It was his work in the abbey gardens that lead to the rules for simple inheritance. The Law of Segregation states that when an organism is producing sex cells (eggs and sperm) each cell gets one and only one copy of the genes. The Law of Independent Assortment states that each pair of genes divides independently of other genes. What copy or allele of gene A you get has no effect on which copy of gene B.
And although his work was lost for many years before being rediscovered in the twentieth century and later fused with Darwin’s ideas of natural selection, Mendel’s work sets him apart as a giant upon whose shoulders we have built our knowledge of biology.
However, the funny thing is, Mendel got pretty lucky.
Firstly, the Law of Segregation assumes that each organism only has two copies of a gene. And for the majority of plants and animals this is correct. However, there are exceptions. The peas he worked with do, in fact, have 2 sets of chromosomes (this is known as being diploid, or a 2N organism). However, other nearby plants were not. Apples can have an odd number of chromosomes (3N). Tobacco is a tetraploid (4N). Domestic oats are hexaploid (6N). Strawberries may have as many as eight copies of their chromosomes! It was quite fortunate that Mendel didn’t turn his attention to these plants since their crosses can get more complicated.
Secondly, we now know that the Law of Independent assortment is not an absolute rule. When Mendel was working he assumed that each gene was free-floating and totally free from any other gene. This is not correct. Genes are tied together through their sequences into large collections of proteins and DNA known as chromosomes. And while chromosomes can switch around their genes with neighboring sister chromosomes during sperm or egg development, genes that are too close can affect each other’s chances.
Mendel worked with seven visible traits when he looked at peas. Each one just happened to be on a different chromosome and free from any possible interference. It would have been terribly easy for him to choose a trait that would not have been so easily understood. One of his laws may have never even gotten off of the ground.
Mendel was, overall, correct in his observations and the creation of his laws. However, it is quite amusing to sit back and consider just how lucky he was.