Mendel part 1: Peas and the laws of inheritance

A story of eye colours, seed colours, and the least creatively named scientific paper of all time

My sister and I have bright blue-green eyes. They’re quite striking, have won us many compliments over the years, and have done very little to encourage a sense of modesty in either of us through our lifetimes. To me, the strangest thing about our eyes is how much of a fuss some people make over them: they’re just eyes, and I’ve had the same ones my whole life. For some people, the strangest thing is the colour changing in the light: in some lights deep blue, in others light green. For some people, the strangest thing about our eyes is actually the eyes of our parents: two pairs of standard issue dark brown eyes. How, some people muse, could two people with brown eyes produce not one, but two children with startling blue-green eyes.

That’s because, instinctively, we see inheritance as the inheriting of noticeable, tangible features from one generation to the next: my partner has her father’s kinked pinkie finger, but her mother’s hair. The reality of how these traits are passed down is, as is obvious to an observant person, more complicated than this, and we first learnt a bit about how exactly this works through the work of one Gregor Mendel in the mid-19th century.

If you’ve ever even started to learn about genetics, as people generally do at some point in high school, you’re probably familiar with Mendel. If not familiar by name, then familiar with his work. Or it’ll at least ring a bell once I start talking about it. Mendel was born in Moravia in Austria (though the region is in modern-day Czechoslovakia) in 1822 and was an Augustinian friar in an abbey in the city of Brno. In those times, such religious positions were positions of great learning, not only spiritual but also scientific and philosophic, so it makes at least a little sense that his name has lived on as the father of modern genetics rather than a great spiritual leader. I don’t want to go any further into his life history, partially because I’m here to talk about the science and legacy of his experiments, but mainly because so many other sources go into his life so much better than I ever could and I don’t want to be in their shadow.

So what was Mendel’s work all about? The answer is simple: peas. Mendel’s legacy was contained more or less in one paper published in 1866 based on experiments which had begun in 1856 and were conducted on 28,000 pea plants. It had the characteristically exciting name ‘Experiments on plant hybrids’ and was cited only three times in the following 35 years. But the work inside it would end up being the birth of modern genetics.

Mendel’s ‘Experiments on Plant Hybrids’

Mendel’s work began with breeding ‘true-breeding’ varieties of peas for various traits. A true-breeding specimen for a trait, say, the trait of having yellow seeds, means the offspring of that specimen and another true-breeding specimen for that trait will always have yellow seeds. If either or both of the specimens merely has yellow seeds but is not true-breeding for the yellow peas, there is a chance the offspring of the pair will not have yellow seeds, and instead have some other colour of seeds, such as green*. True-breeding was commonly done for hundreds of years by plant and animal breeders before anything of the underlying molecular mechanisms was understood, allowing Mendel to complete this step of the experiment with relative ease.

Mendel’s next step was to take two specimens which were true-breeding for two opposing traits (say, one specimen which was true-breeding for green seeds and another which was true-breeding for yellow seeds) and breed them together. This was Mendel’s first ‘cross’, as he was crossing two lineages of pea plant. In the case of the seed colour cross, every single one of the offspring of this cross had yellow seeds. These offspring were known as F1, denoting the first ‘Filial’ generation. Mendel’s final step was to self-fertilise** the F1 generation to create more offspring (known as the F2 generation. Most of the F2 generation had yellow seeds, but about a quarter had green seeds.

The experiment was repeated for six other types of traits: the shape of the seeds, the shape of the pea pods, the colour of the pods, the length of the stems, the colour of the flowers, and the position of the flowers on the stems. As with the seed colour example, in each experiment, the F2 had the same ratio of traits: ¾ had one trait, and ¼ had the other. His results, based on thousands of individual pea plant specimens, couldn’t be coincidental. So how could this be explained?

The outcome: Mendel’s laws

Mendel’s results are often explained with reference to what are known as Mendel’s ‘laws’ – principles he derived from his experiments with peas. The number of these laws varies from two to three to four – sometimes two are called laws, the other two are called postulates, sometimes three are known as principles and one isn’t acknowledged, but for the sake of completeness I’ll talk about all four here (and call them all laws for ease). Not all of these laws have stood the test of time, which I will be addressing in my next article on the topic. It’s important to understand the laws and what they mean before understanding why they don’t all stand up.

Law 1: The laws of paired factors

Each organism has two ‘factors’ for a single trait (now more is known, these factors are known as genes). An organism can be ‘homozygous’ (the two genes are identical) or ‘heterozygous’ (the two genes are different) for these factors. True-breeding specimens are always homozygous for the gene influencing the trait they are true-breeding for.

Law 2: The law of dominance

For any two different genes, one will be ‘dominant’, and one will be ‘recessive’. The dominant gene will be the one influencing the trait. For instance, yellow seeds carry a gene dominant to the gene for green, so any heterozygous individual carrying a yellow seed gene and a green seed gene will have yellow seeds.

Law 3: The law of segregation

Each parent contributes one of their genes for each trait to their offspring. So, when the true-breeding parents crossed, each offspring had one yellow seed gene and one green seed gene. As the yellow seed gene was dominant, all their offspring (the F1 peas) had yellow seeds.

Then, in the F2 peas, each parent cell had a 50/50 chance of giving a green seed gene or a yellow seed gene to the offspring. The result was three different kinds of offspring, offspring with two yellow seed genes (which had yellow seeds), offspring with one of each gene (which also had yellow seeds due to the law of dominance), and offspring with two green seed genes (which had green seeds) in a ratio of 1:2:1. That’s a little confusing to read and understand, so I’ve included a favourite tool of genetics teachers below: the Punnett square.

In the image, the upper case Y represents the gene for yellow seeds, and the lower case y represents the gene for green seeds.

We can see from the table that the 1:2:1 ratio in the pairs of genes the offspring will have corresponds to a 3:1 ratio in yellow:green seeds. So even though both parents have yellow seeds, ¼ of their offspring will have green seeds. To somehow make all of this about me again, this is why my sister and I have blue eyes even though neither of our parents do. My parents carry one blue eyes gene and one brown eyes gene each, but the brown eyes gene is dominant over the blue eyes gene, so they have brown eyes. However, they both contributed their blue eyes genes to both me and my sister, so we both have blue eyes.

Law 4: The law of independent assortment

Each gene assorts independently when being transferred to the next generation. That is, if a pea plant has a yellow seeds gene and a tall stems gene from one parent, and a green seeds gene and a short stems gene from the other parent, when that pea plant contributes genes to its offspring the gene it gives for stem height will be chosen independently of the gene it gives for seed colour. It’s equally likely to give the yellow seeds gene and tall stems gene as it is to give the yellow seeds gene and short stems gene (and the same likelihood to give the other two possibilities).

In the follow up to this article, I'll be talking about the shortcomings of Mendel's laws, one of his lesser known studies, and how closely Mendel's model of inheritance resembles ours.

*As green seeds is a recessive trait in peas, both parental specimens would have to be not true-breeding for the offspring to have a chance of having yellow seeds

**Self-fertilising is possible in many plant species including peas. It simply means the same organism provides both male and female sex cells to create its offspring, without the need for another organism.