Luck in the time of pea plants: The story of Gregor Mendel.

 Gregor Mendel. 

 If you did GCSE science, odds are you remember this name. There may also be a chance that you can recall Gregor Mendel being the man with the pea plants who somehow changed the idea of genes and inheritance. 

 

Hopefully, this flashback to the horrors of GCSE biology hasn't filled you with too much dread and you are still reading this article. 

The fact is that Gregor Mendel's discoveries did change genetics. And if genetics is something that interests you, this article is definitely worth a read! There is a lot more going on with Mendel and his pea plants than you were ever taught at GCSE. And if you never experienced the UK education system and never heard of Mendel, the story is fascinating and you should also continue reading.

So, to begin with, a bit about the man himself. Gregor Mendel was born on the 20th of July 1822 in what was at the time, the Austrian Empire (the place he was born is now the Czech Republic). In about 1844, he began training as a Catholic priest and entered St Thomas's Abbey in Brno. From 1850, he worked as a substitute high school teacher and as a teacher of physics, although he failed to become a certified teacher. In 1867, he became abbot of the monastery, where he focused on admin and running the monastery. He died in 1884 in Brno. 

Around this time, it was believed that inheritance was 'blended', with characteristics being an average of the parent's characteristics. For example, if a person with light brown hair had a child with a person with dark brown hair, the child would have medium brown hair. Or if a red flower produced offspring when fertilized with a white flower, all the offspring would be pink. This, as Mendel would soon discover is not the case. 

In 1854, Mendel started experimenting in the monastery's 4.9-acre garden. In this garden, he did something or rather grew something that changed what we knew about inheritance.

In terms of how he did it... well, in very simple terms, he grew some pea plants and recorded what they looked like.

I am not even oversimplifying that much.

Mendel chose to work with pea plants as they were easy to grow and could be both self-pollinating and cross-pollinated. This allowed Mendel to create 'true breeding' lines but also meant he could control pollination by hand. He would transfer pollen from one plant to another and was able to prevent them from self-pollinating when he wanted cross-pollination. I suppose another bonus was that you would have the peas to eat afterwards. Maybe that is why the monks put up with Mendel's pea plants taking over the garden! Between 1856-1863, Mendel grew 28,000 plants. 

Mendel chose seven traits to focus on: seed shape, flower colour, seed coat tint, pod shape, unripe pod colour, flower location, and plant height. He began by true-breeding and then cross-bred the true breeds to different varieties. For example, he fertilized tall plants by short plants or yellow peas to green peas. He noted ratios in the characteristics that would appear in the next generation of plants. For example, seed colour. When a green pea was crossed with a yellow pea, the offspring always had yellow seeds. But in the second generation, the green peas reappeared, with 1 green pea plant for every 3 yellow pea plants. Why? Well, this comes down to what we call 'alleles' - versions of a gene. All the pea plants would have a gene controlling colour - a gene making it green and a gene making it yellow are the alleles. The pea plants would have two alleles for each gene- like humans do.

So, Mendel's yellow plants would have had two yellow alleles each, whilst his green plants had two green alleles each. They were true bred so can't have had any other alleles for colour. Their offspring inherited one yellow allele and one green allele each. The next offspring could have ended up with two yellow alleles, one yellow and one green, or two greens. The two greens resulted in a green plant. 

 Visual representation of pea plant colour. The grid on the right is called a Punnet square, and these little squares are important tools for measuring inheritance. Here we can see the allele each parent pea plant may have had. 

This led Mendel to devise the terms 'dominant, referring to versions of genes that need one copy to show their characteristic, and 'recessive', referring to genes that need two of the same versions. For example, the plants were only green when they had two green alleles, whilst the plants could be yellow when they had one. Interestingly, Mendel never actually used the term 'gene'- he used the term 'factors'. The term gene was introduced in 1909 by Wilhelm Johannsen. However, Mendel was still the first to even deduce the inheritance of individual inheritable units.

These inheritable units would lead to the development of what is now known as Mendel's Law of Inheritance- or Mendelian inheritance. There were 5:

1.Characteristics are 'discrete'. This means that they are individual and can't be combined. For example, green and yellow plants do not result in light green or dark yellow plants.

2.Genetic characteristics have alternative forms; one from each parent. This is what we call alleles.

3.One allele is dominant, and the phenotype (the appearance) reflects this. 

4. Gametes are created by random segregation. Okay, this is a bit of a weird one. But it basically means that the cells needed for reproduction, that contain half of the genetic material, will randomly form with the alleles for each characteristic randomly being assigned to each gamete.

5. Different traits have independent assortment. This means that individual genes do not get inherited together.

Now a bit of context, Mendel first published what he discovered in 1865. But his work was largely ignored. Charles Darwin was also attempting to explain inheritance around this time -but was unaware of Mendel's discovery. It has been suggested that if Darwin had found the paper, modern genetics would have been discovered a lot earlier. 

Mendel's work was not rediscovered until the early 20th century - by Hugo de Vries and Carl Correns. They actually managed to recreate the experiments without first reading the work, but it is thought that de Vries didn't understand his results until finding Mendel's paper. Biologists quickly replicated Mendel's results and combined this with their understanding of Darwin's natural selection. This, in the 1930's and 1940's, led to the modern ideas of evolutionary biology. Although, in the Soviet Union and People's Republic of China, Mendelian genetics was rejected resulting in imprisonment and execution of Mendelian geneticists. 

Dominant and recessive inheritance is quite a major idea in genetics. Many characteristics and diseases are classed as dominant and recessive. Cystic fibrous is a recessive disease meaning that those with the condition need two copies of a mutated allele, whilst Marfan syndrome is dominant. Recessive disease also has 'carriers' where a person has only one copy of the mutated allele. This means that they do not suffer themselves - but they are able to pass on this allele. For example, in cystic fibrous, if a carrier of the cystic fibrous allele has a child with another carrier, their child has a 25 (1 in 4) chance of inheriting both alleles and having the disease.

This understanding of inheritance has had a massive impact in treating genetic disorders as well as research into prevention. This is why Mendel is known as the 'Father of Modern Genetics'. 

But here is the thing -Mendel was rather lucky! Now, luck isn't a bad thing in science- major discoveries were made by luck and good timing. We also can't deny that Mendel put a lot of hard work in his experiments, and it was his correct deductions that led to the discovery in the end. However, not every form of inheritance follows Mendelian laws- and Mendel was fortunate that none of these other forms of inheritance made an appearance in his experiments.

One major form of inheritance that does not follow Mendelian law is sex-linked inheritance. This can happen in plants- as some plants are able to produce both male and female plants. This is when the expression of the allele is dependent on the gender of the individual (or the plant) as it is located on a sex chromosome. So, humans in particular have two sex chromosomes. Females are always XX, whilst males are XY. Haemophilia is an example of a sex-linked disease. The gene is located on the X chromosome - and as males only have one X chromosome, if they inherit the mutated gene, they will have the disease. Females will need to inherit two copies- as they would have a 'back up' gene. 

 

 

Another example of a non-mendelian inheritance is codominance, with blood groups being a key example of this. In humans, there are four blood groups - A, B, AB, and O. This is ignoring the + or - which is denoting the rhesus group and a different system. The letter system refers to an antigen on the surface of a red blood cell. An A allele results in the A antigen being produced, whilst a B allele results in the B antigen. Antibodies are also produced to recognise 'foreign' antigens, which means a person with only the A allele produces antibodies that attach the B antigen and vice versa. However, if a person inherits both an A allele and a B allele, the red blood cells produce both antigens and no antibodies. Hence, AB blood group. Meanwhile, the O blood group produces no antibodies or no antigens. So, if a person inherits an O allele or an A allele, they will only produce A antigens - and anti B antibodies giving them blood group A. If a person inherits an O allele and a B allele, they will be blood group B. 

 

Autosomal linkage is also a form of inheritance that Mendel did not encounter. It actually proves that his fifth law is not always true. In this form of inheritance, genes located together on the same autosome (non sex chromosome) are likely to be inherited together- i.e. they are linked. This happens quite a bit in fruit flies (which are actually really important in genetics). The genes for body colour and wing length are located together. If one fly has a grey body with long wings and has offspring with a fly that has a black body and short wings, the offspring are far more likely to inherit a black body with short wings or a grey body with long wings, than a grey body with short wings or a black body with short wings. This is more or less the opposite of Independent Assortment. 

Finally, epistasis is also something Mendel is lucky he did not encounter, as this would have completely messed up his experiments. This phenomenon is when one gene locus affects another gene locus. An example is fur colour in mice. Two genes control fur colour - A and B, which B controlling the expression of gene A. Think of it like a checkpoint of a gate. For expression of A to be allowed, a mice must be BB or Bb. If the mouse is BB or Bb, then has AA or Aa, the mouse will have black bands in its fur. If the mouse is BB or Bb, then has aa, it will have solid black fur. BUT if the mouse is bb, the gate is closed, and the fur colour will only ever be white regardless of the A alleles. This is what we call recessive epistasis - as two copies of bb are needed to prevent the expression. The opposite can occur. Dominant epistasis: in this case Bb or BB would close the gate.

So! This gives four forms of inheritance that Mendel was lucky not to have encountered. This is not to disparage Mendel's work at all. His discoveries ultimately changed genetics for the better, and whilst his laws do not work all the time, they do work more than they do not. But if Mendel's experiments had been affected by these other methods of inheritance, it is certainly possible that the rules of dominance and recessive may not have been discovered at all. For this, Mendel was lucky, and by extension so are all those who benefited from his discoveries and work.