Mendel’s Three Laws of Genetics & How to Grow and Save Seeds

Gardeners Understandably Want to Grow and Save Homegrown Seeds. But Not All Seeds Are Worth Saving As Explained in Mendel’s Three Laws of Genetics.

Having grown a successful crop many gardeners want to repeat that success. Saving seed from a successful crop makes sense to them as they imagine the will get the same results when they subsequently sow their saved homegrown seed. Sadly that isn’t always the case. Often the resulting crop is vastly inferior with low yields of poor quality crops. It may surprise them to then discover that the results they experience are totally predictable to the extent they can be stated as immutable Laws of Genetics, with clearly defines mathematical biological ratios, as discovered by Gregor Mendel in the 1860s.

Long before genes, genomes, hybridisation and Genetically Modified Organisms (GMOs) were known, Austrian monk Gregor Johann Mendel conducted experiments in his monastery’s garden. He planted 5000 peas (Pisum sativum) between 1856 and 1863, and later published his results entitled, Versuche über Pflanzen-Hybriden (Experiments on Plant Hybridization), in 1865.

Mendel came up with what is now known as Mendel’s Three Laws of Genetics. And these three laws are the basis of Mendelian Genetics which dictate precisely what works, and what doesn’t when we save and sow our homegrown seeds today.

What Has Mendel’s Three Laws of Genetics To Do With How to Grow and Save Seeds Today?

I’ll start by writing a simple explanation and then, for those interested, I’ll dig into the science of genetics that Mendel discovered. The only thing that has changed since Mendel’s day is some of the terminology used today. Mendel used the word trait to describe things such as the height of a plant, the colour of the seed etc. Today we use words like gene, DNA and RNA that were coined after Mendel’s initial work.

It’s hard to write anything about genetics without using some technical words. So I’ve added a section at the end of this post to explain any technical terms I have to use. Plus I’ve added them to the gardening dictionary on this site.

It is however important to firstly understand that though Mendel’s experiments brought order and clarity what we now know as genetics, it was ignored for some time after he made his discoveries. Secondly, the whole idea of systematically breeding animals and plants was not new.

Hippocrates, (c 460-c375 BCE) believed that characteristics could be inherited through a process he called pangenesis. He thought all of our organs gave off invisible “seeds” that were transmitted during mating and used as building blocks to form new life.

Prior to Hippocrates, there is a 6000 year old Babylonian tablet that describes the pedigree of horses and talks about inherited characteristics. Ancient mankind applied basic genetics to the breeding of cultivated crops and domesticated animals. Clearly, they didn’t understand the science behind the mechanism of heredity but they did apply the basic principles.

So let’s start with some really important concepts that are essential when we are considering How to Grow and Save Seeds Today

The Basics of How to Grow and Save Seeds

When we buy seeds or plants there are two types of seeds. Heirloom seeds and F1 seeds.

Heirloom Varieties

The first type is from the established seed varieties that have been around for years. They are sometimes called heirloom seed. This type of seed is genetically diverse so the results are slightly variable. But for both plant and gardener that’s good. It means that not every plant will mature on the same day and we can harvest over a long period. So rather than all your crop being ready at the same time, it will be ready to harvest over several weeks. And if we sow new seeds every 2-3 weeks we can have crops ready throughout a long season.

Because the seeds are a bit variable the plants can gradually evolve to suit local conditions. That’s good for the plant as it means it can be selectively bred. For example, if some of the seeds are slightly more resistant to a pest or disease attack, they will survive when others don’t. That means that if I collect carrot seed from the best of my crop then I’ve actually selected the seed from the plants that did best in my growing conditions. I’m selectively breeding from plants that like my weather, soil type and tolerate pests and diseases a bit better than others. The same plants grown on a different soil type a few miles away can also have seed harvested from their best plants. But it will be from the plants that do best on their soil type. They will be slightly different.

So though we might have started with the same variety of seed, what we save is very slightly different for each location. And if we did this for generation after generation we’d see more and more diversity and end up with local cultivars of the same species or variety.

Rare Breeds .. Farm Animals 

This used to be quite common in farm animals. Where I live in Devon we have both South and North Devon cattle. They are totally different.

In the Channel Islands, local selection of cattle produced Guernsey and Jersey cattle. There was even an Alderney cow, but the pure breed is now extinct. They are all cows but each was slightly different due to selective breeding on their individual islands. Because they were om islands they didn’t cross breed with one another they gradually evolved to be slightly different and ideally suited to their own very specific environment and the wishes of the farmers.

A similar thing has happened with dogs. Over many generations, dogs were selectively bred for different traits. So we have dachshunds that were bred in Germany for hunting. The name translates as Badger dogs, and they have been selected for short legs so they can hunt in the confines of badger setts. The longer-legged greyhound was bred in ancient Egypt as a coursing dog. A fast-running dog that could flush and pursue game. The collie was selected for its ability to herd animals and needed both longer legs than the dachshund and less speed than the greyhound.

Each dog breed is now a breed in their own right, We refer to them as purebred lines and record their pedigree in the same way that cattle, sheep and pigs are bred with pure pedigree genetic lines.

When we do this with plants we get purebred lines of heirloom plants. For example, there are lots of peas that we can grow, but they have different names, such as Alderman, Oregon Sugar Pod and the rare Glory of Devon (awarded the RHS Award of Merit in 1899). These are pure lines. All of them are open-pollinated varieties that if we breed them true, ie only with others of the same named variety, will remain true to type for many generations. This is because they are inbred within that variety. And whilst it’s true that if we keep selecting our own seed in our own garden, year after year, it will eventually deviate enough from the original named variety as to become a new variety, what we find in practice is that seed is shared and mixed and so remains reasonably true to type.

If however our location is isolated, such as the individual Channels Islands are, then we eventually end up with new versions of what was standard. Darwin saw this in the Galapagos Islands where the tortoises and sparrows on each island evolved to local conditions and became distinctly different.

F1 Seeds

The second type of seed is called F1. These are produced when you take two distinct pure lines and cross them to get an offspring. Often these offspring are better than the sum of the parents due to Hybrid Vigour. It’s a matter where 2+2 can actually equal 5. Or maybe even 6, 7 or more.

F1s are very uniform. Every plant is likely to be ready for harvest on the same day and it’s why farmers and processors love them. They can clear harvest a field or greenhouse of even-sized crops in one day and be virtually guaranteed that every single plant will be the same!

F1 plants have revolutionised farming and growing and have helped feed a hungry world.

But as gardeners, we don’t want all our crops to be ready the same day. We usually want to harvest over a period of time.

There is a place for F1 plants in some cases. The yields are often a bit better, so if yield is really important to you then they can be worth considering. But where F1s have been selected for yield and other factors such as the ability to be able to be transported for miles, often from overseas, and still have good shelf life, these traits aren’t important to gardeners. And the shelf life and yield traits are sometimes at the cost of flavour!

Why F1 Seeds Are Bad News For Seed Savers

F1 seed has many attributes and many gardeners grow them despite the drawbacks. But there’s one drawback that can’t be ignored. F1 seed doesn’t breed true.

What Mendel discovered was that the seed produced by F1 plants, Ie the F2 seed, was predictable in that it produced inconsistency when planted. It was predictably bad!

When you take the seed of an F1 you get an F2 and it is predictably inconsistent. In fact, Mendel’s work in the 1800s determined the degree of predictability.

For example, when he investigated peas and looked at the tall and short trait he discovered something quite strange.

He started with a group of “parent” peas which he crossed to form the F1 generation (First Filial Generation). When planted, all these seeds grew tall peas. So far, so good. In many cases, this type of cross also gives Hybrid Vigour as noted previously. So they are good seeds, very uniform and yield well.

Mendel then took some of the F1s and crossed them to get the F2 (Second Filial Generation) seeds. That’s like a gardener growing an F1 crop and saving seed from it. The seed they then have is F2 seed.

What Mendel found this time was that he didn’t get 100% tall plants this time. They grew 75% tall plants and 25% short plants.

We call the above cross a monohybrid cross. We are only interested in one trait, hence monohybrid cross. And when traits are visible like this geneticists call them a phenotype.

But Mendel didn’t study just one trait. He studied seven in total.

For example, he looked at height and flower colour. We call this a dihybrid cross (di meaning two in the same way as it does when we use the word dicotyledon when differentiating from monocotyledons).

And if we check out two traits at once, we get a combination of the two. Some offspring will be tall with white flowers, some will be tall with blue flowers, some will be short with white flowers and some short with blue flowers. And the ratio will be 9:3:3:1. This is the genotype ratio.

The F2 Generation Problem

The problem with F2 plants is they are inconsistent, (though in a predictable way) and this is explained in Mendel’s Three Laws of Genetics.

If what you want are tall plants with white flowers then only some of them will be like this. Some will either be short, have blue flowers or be both short and blue!

And the more traits you are interested in the more mixed and inconsistent it becomes. And what’s worse is that Mendel discovered that the traits he tested were not linked in any way. He didn’t, for example, find that all tall plants had white flowers and all short ones had blue flowers. As previously shown they are independent of one another. This fact is so important that it became one of his three laws of genetics.

Obviously both tall and white are highly visible (they are phenotypes). But the same principle applies to those traits you can’t see, such as flavour. Unfortunately, height and flavour aren’t linked. In fact, flavour is even more complex as it is determined by a range of traits such as sugar content, acidity, texture etc. And all these are independently inherited. Flavour is polygenic and I explain this in more detail later.

So if we save seed from an FI, and like the F1 because it was high yielding, great flavour, etc., then we will be disappointed with the F2, as much of what we value will be absent.

A simple way of thinking about this is to consider what happens when we cross two pedigree dogs. We get a mongrel (and yes, they can be lovable). Mongrels have a mix of the characteristics of both but not always the best of both! That’s because some genes are dominant and some recessive.

Dominant & Recessive Genes & Mendel’s Three Laws of Genetics

Every pea, in fact, every living thing, has dominant and recessive genes. Traits are determined by genes. Part of the genetic code (an allele) is passed on from parents to their offspring. In other words, genes are something we inherit from our parents. And of course, we inherit genes from both parents.

What happens is that each parent gives one piece of code for each trait, say height. And these combine to be expressed in the potential height of the offspring. I say potential as obviously an individual with “tall genes” will not be tall if undernourished or impacted by anything else that will mask the genetic potential.

But if both parents have the gene for tall then we can expect the offspring to be tall when two heterozygous individuals cross. It doesn’t matter if this applies to Mendel’s peas or the height of our children, the same principle applies.

But as you’ll recognise life isn’t that simple. There aren’t just tall people and short people. People come in all sizes. And that is often due to a whole range of circumstances outside of genetic control.

But if all other things are equal what Mendel discovered is that some traits were dominant and some not. Let me explain

We each have two copies of the gene for height. And we pass one copy onto our offspring. So we might have two tall genes, let’s call them TT or we might have one tall and one short. We can refer to them as Tt (they could be tT but we normally write the capital letter first) Depending on which of these pieces of code (geneticists call them alleles) we give then they will give different results when they combine with our partner’s genes. We could end with offspring that have TT, Tt, tT or tt.

Dominant and Recessive Genes

That actually means that the ratio of tall to short offspring is 3T and 1t. And do you remember the piece I wrote and highlighted earlier? What Mendel found this time was that he didn’t get 100% tall plants this time. The seeds grew 75% tall plants and 25% short plants.

That’s it a 3:1 ratio as explained in Mendel’s Three Laws of Genetics. You can see how it works in the four square grid on this page.

And the reason for this is something Mendel discovered. He discovered that one gene is dominant and one recessive. In this case, T is dominant and the three with a T in their mix are tall. That’s because T is dominant in any combination. TT, tT or Tt. It’s only when we get tt that the recessive genes are expressed and we get short peas .. or people or whatever.

Mendel’s work focused on traits where just one allele influenced the genetic expression. Traits aren’t always that simple. They are often influenced by a number of heritable traits. For example, eye colour in humans is not simply brown or blue, dominant or recessive. We have brown, blue, hazel, green, violet etc as eye colour descriptors and many attempts have been made to classify human eye colour.

When more than one gene influences a given trait it is said to be polygenic.

More on Mendel’s Three Laws of Genetics

When we consider the inheritance of more than one trait in the F2 generation and want to have seed exhibiting multiple positive traits, it gets even more complex. If we are lucky we might get plenty of tall plants, with small sharp fruits that don’t keep well. That’s not a lot of good if we wanted tall plants with large fruit, a sweet flavour and good keeping quality. One out of four isn’t good. Neither is three out of seven or any other combination of numbers if what we really want is ten out of ten!

What Traits Did Mendel Experiment With When Determining Mendel’s Three Laws of Genetics?

Gregor Mendel wasn’t happy testing a few traits. He tested no fewer than seven.

They were: –

Plant size (tall or dwarf)

Flower colour (purple or white)

Pea shape (round or wrinkled)

Position of flowers (axial or terminal)

Pea colour (green or yellow)

Pod shape (constricted or inflated)

Pod colour (green or yellow)

What Are Mendel’s Three Laws of Genetics?

Mendel’s Three Laws are the :

Law of Independent Assortment 

Law of Dominance

Law of Segregation

Law of Independent Assortment 

Mendel’s Law of Independent Assortment is based on the fact that Mendel discovered that genes act independently of each other. So, for example, tall genes aren’t linked to pea colour or pea shape. In reproduction, they sort themselves independently. The genes passed down for a given trait in the sperm or egg are independent of one another.

Law of Dominance

Mendel’s Law of Dominance is about how the dominant trait for a characteristic will conceal the recessive trait for the same characteristic. For example, he found that the gene for Tall was the dominant trait for height in his 1860s pea experiments. Tall (T) is dominant over short (t) which is recessive.

Law of Segregation

Gregor Mendel discovered that despite the fact that two alleles exist for each trait, they segregate during reproduction so that only one is passed on during crossing. These then combine with the single allele from the other parent so that the offspring has two alleles for each trait. 

Punnett Squares – Monohybrid & Dihybrid Crosses

A Punnet Square is a grid diagram that shows the genotype and phenotype of crosses according to Mendelian inheritance. It is named after English geneticist, Reginald C Punnett and was first used in 1942.

With monohybrid crosses, there are four squares in the grid and the genotype can be seen to be 1:2:2 and the phenotype is 3:1

With dihybrid crosses, there are 16 squares in the Punnett square. It shows the phenotype is 9:3:3:1.

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