Darwin’s On the Origin of Species was a huge success when it was published, but Gregor Mendel’s work went unnoticed for over 3 decades. Once it was discovered, people realized how this humble researcher had managed to outline the basic principles of genetics. How could this have gone undiscovered for so long?

Why this is important…This work is the starting point for understanding all genetics.

Foundations

Mendel’s work reveals some of the underlying motivation for studying nature – predictability. If we understand how something works, then we can control it, or at least predict what will happen next. For example, early astronomers tried to understand the movement of the sun, moon and stars in order to predict seasonal changes while growing crops. Although scientific experiments do not need to have a practical application like this, it’s often why they are done in the first place.

Mendel knew the pattern of the plant life cycle. Like animals, plants go through repeating cycles of the following processes. All of these will be discussed in more depth later.

Adult organisms produce specific sex cells (sperm, egg, ovule, pollen) with half of their genetic material (this is the result of meiosis)
These sex cells combine (fertilization) to produce a genetically unique individual
This new individual grows into an adult (though mitosis and cell division)

Mendel’s work won’t reveal all the details of genetics, but it is a starting point for many future genetic concepts we will study.

Key terms

Characteristic – a physical or functional aspect of an organism that is genetically controlled; for example, eye color, flower color, or height

Trait – specific value for a characteristic; for example, yellow, white, red or pink flower colors

Genotype – genetic makeup based on the type of genes an organism inherited;
this is usually expressed as a combination of two alleles (EX: AA, Aa, or aa)

Phenotype – the observable trait of an organism resulting from its genotype;
this is the more technical term for ‘trait’

Gene – a segment of DNA that codes for a specific trait

Allele – a variation of a gene; for example, a gene for flower color might have purple or yellow versions

Dominant (adj.) – type of allele (or trait) that hides others

Recessive (adj.) – type of allele (or trait) that is hidden by dominant alleles

Zygote – the cell formed as a result of fertilization

Heterozygous (adj.) – genotype of two different alleles for the same gene
(Greek: hetero = other, different and zygote = yolk, referring to the offspring)

Homozygous (adj.) – genotype of two of the same alleles for the same gene
(Greek: homo = same and zygote = yolk, referring to the offspring)

Details

Mendel’s big new idea was that parents passed on unchanging factors. We now call these factors genes. Before this, most people would not have guessed that they pass from one generation to the next unchanged. When scientists started to see chromosomes under a microscope in the 1880s, this idea made more sense.

Mendel’s work on pea plants (scientific name: Pisium sativum) in his abbey garden was a big undertaking, even though his methods were simple. For 7 years he recorded the traits of 28,000 plants. He looked at 7 different characteristics, each of which had just two possible traits (Figure 1-8). You can read an English translation of his work on the Mendelweb website.

Seven characteristics of pea plants shown from seed shape and color to flower color to pod shape and color.

Figure 1-9. Pea characteristics. Mendel looked at 7 characteristics of pea plants to reveal the patterns of inheritance. ("Mendel's pea characteristics" by Mariana Ruiz Villarreal, LadyofHats is in the Public Domain, CC0)

After analyzing his data, he noticed some significant patterns. I imagine that this is exactly what he was hoping for – patterns that would allow him and others to get predictable results from plant breeding. He summarized these patterns in three observations that we now call his Laws of Inheritance.

Law of Dominance – Dominant traits hide (or mask) other traits. The traits that get hidden are recessive. The recessive trait will only reveal itself if no dominant trait is present.
Law of Segregation – Each parent has two genes for each characteristic and passes on one gene to the next generation.
Law of Independent Assortment – Each characteristic is inherited separately. For example, flower color genes are on a different chromosome than plant height genes, so they are inherited separately.

These three laws are an important starting point. If you didn’t fully understand them, it may be helpful to look over the key terms again then reread the laws. Terminology is hard to escape in genetics, but with some repetition, the ideas should become clear to you.

Video 1f: Mendel’s Experiments and Laws

After watching this video, you should be able to:

describe how his experiments led him to each of the 3 separate laws
describe each of the laws of inheritance

Inheritance is part of the basic life cycle. When organisms reproduce, each parent contributes one copy of each gene to their offspring. When the parents have sex and the sex cells combine (fertilization), a new individual is formed called the zygote (Figure 1-9, in the bottom in grey). The zygote is just one cell, but it has all the genetic information it needs to grow up into an adult. Depending on what mix of the alleles it got from parents, the genetics of the zygote can be described as either:

Heterozygous if it got two different alleles
Homozygous if it got two of the same alleles.

Flowering plant life cycle. Flower shown with expanded detail of male and female reproductive structure and what happens with fertilization.

Figure 1-10. Plant life cycle. ("Angiosperm life cycle diagram" by Mariana Ruiz Villarreal, LadyofHats is in the Public Domain, CC0)

The mix of alleles that the zygote has is its genotype. For simplicity, we usually just look at one gene at a time. (Humans have 20,000 genes, so you could see how that would be overwhelming.) One way to communicate the genotype is by calling it homozygous or heterozygous. A shorter way to communicate the genotype is to use letters to symbolize each allele. I find it easiest to use the letter ‘A’ in most cases. Since there are usually two alleles, we can use the following convention:

Capital letters (such as ‘A’) symbolize dominant alleles
Lowercase letters (such as ‘a’) symbolize recessive alleles

Genotypes can be written as two letters together.

Homozygous dominant = AA
Heterozygous = Aa
Homozygous recessive = aa

Applying the Law of Dominance, we can determine the individual’s phenotype. The phenotype is more than just the appearance. All the characteristics Mendel observed were based on physical appearance, but phenotypes also include hidden traits like personality, ability to taste or smell certain chemicals, or the ability to make certain digestive enzymes. Any individual with a dominant allele (either AA or Aa) will have the dominant phenotype. Only individuals with all recessive alleles (aa) will display the recessive phenotype.

Stop & Think #8: If freckles are a dominant trait, what genotypes could someone with freckles have?

Applying the Law of Segregation, we can determine the offspring from two parents. As long as we know the genotypes of both parents, we can determine the genotype of all potential offspring. And if we know the genotype of offspring, we can figure out their phenotype. Of course, this is just a prediction. Each time fertilization happens, chance determines which combination they will get.

Punnett squares are a useful way to predict potential offspring. Video 1e guides you through this process. In brief, the parents’ alleles are written on the edge of a 2x2 table (Punnett square). The parent alleles represent sex cells (sperm or egg). One allele from each parent fills in the squares, simulating fertilization. The letters in each square are the predicted genotypes of offspring.

Video 1g: Applying the Law of Segregation to inheritance problems

After watching this video, you should be able to:

Describe key pieces of information you need to solve an inheritance problem
Construct a Punnett square
Interpret phenotypes from genotypes
Give the probability for offspring of different genotypes or phenotypes

Each potential offspring is just a probability. The offspring “created” in a Punnett square don’t represent real children, only probabilities. There are different ways to represent this mathematically. Let’s explore these in a simple scenario.

Sample genetic problem with flower color

First, we need to track what alleles we are working with by making a simple legend:
A = purple flowers
a = white flowers
Next, we need to know the genotypes of the parents. In this case both parents are heterozygous, so
Aa x Aa
To predict possible offspring, we can use a Punnett square. The single letters on the top and left side represent the single allele carried by the sex cells. The letter pairs inside the squares represent genotypes of possible offspring.

	A	a
A	AA	Aa
a	Aa	aa

Finally, we can use the information from the Punnett square to answer questions about potential offspring. There are many ways to express probabilities, but the most straightforward is usually a percentage of the offspring, represented by the genotypes in the Punnett square above.
1. If we are asked about genotypes for the situation above, we could report this as:
  25% AA
  50% Aa
  25% aa
2. If we are asked about phenotypes, we need to interpret the genotypes in the Punnett square above based on the legend we made (in step 1) and the Law of Dominance. Based on this, the offspring would likely be:
  75% purple flowered
  25% white flowered

It is worth repeating that these are just mathematical calculations, not actual results. Results could end up as 80% purple flowered and 20% white flowered, or 60/40, or even 90/10. The “results” are really just the likely predictions based on probabilities.

Stop & Think #9: Purple flowers are dominant to white flowers in peas.
a. If you breed two plants, AA and Aa, what percentage of the offspring will have purple flowers?
b. If you breed two plants, Aa and aa, what percentage of the offspring will have purple flowers?
c. If you bred a purple and white flowered plant and half of the offspring had white flowers, what are the genotypes of the parents?

Applying the Law of Independent Assortment is more complicated. We rarely look at more than 2 genes at once, since it gets overwhelming quickly. The key thing to remember when working with more than one gene is that each child should get 2 copies of every gene. If the parents’ genotypes are AABb and aaBB, then offspring should have similar genotypes – they should have four alleles as well. The sex cells from parents are going to carry one allele for each gene. A parent with a genotype AaBb could produce any of the following sex cells: AB, Ab, aB, or ab. Watch Video 1f for instructions on how to solve inheritance problems that involve more than one gene.

Video 1h: Applying the Law of Independent Assortment to inheritance problems

After watching this video, you should be able to:

determine possible allele combinations in sex cells
construct a Punnett square using two traits (there are two methods for this)
determine the potential offspring combinations of genotypes or phenotypes

Stop & Think #10: Purple flowers (A) are dominant to white flowers (a) in peas. Tall plants (B) are dominant to dwarf plants (b).

Breed parents with the following genotypes: AaBb and aabb. What percentage of offspring will be tall and purple-flowered? What percentage will be dwarf with white flowers?
Breed parents with the following genotypes: AABb and aabb. Give the ratio of all phenotypes.

These inheritance principles are an important starting point to learn more about genetics. The idea that we have two sets of genes and only pass on one, will remain constant. We should recognize that the traits Mendel studied are unusual in a few ways.

Most characteristics have more than two traits. All the traits he studied happened to have only two.
Many human traits are controlled by more than one gene. This results in more variation, as it does in hair color and eye color. Traits Mendel looked at were controlled by one gene.
Most human alleles do not follow this simple law of dominance. Many genes have alleles that are partially dominant, resulting in a blending effect.

Mendel’s work provides a general outline, but there is much more to learn about genetics in future chapters.

Appreciating good science

Mendel did some outstanding science. For such a simple experiment, the results were enormously important. Most experiments are very small and don’t change the world. But even small experiments are worth doing well. We should take a moment to appreciate his practice of science and borrow from the things he did well.

Focused on the natural world. All science should limit itself to the physical world governed by natural laws. He didn’t test the effects of spiritual matters by blessing certain plants or testing how prayer affected them.
Ideas were testable. By breeding certain plants together, he was able to test the patterns that he proposed. Untestable ideas often have to do with supernatural causes or value judgements. If he said that plants would grow purple flowers because he was a good person, that is not testable in an objective way because goodness is very subjective.
Recorded empirical observations. He observed traits that he could record objectively and consistently. He didn’t use vague or inconsistent traits that would require him to make a judgement, like plant health or how good the peas tasted.
Repeated his experiment. Not only did he use more than just a couple plants, he repeated the experiment for many generations and over 7 years with thousands of plants. This gave him confidence that there was a true pattern behind his observations, not just coincidence.
Based conclusions on evidence. The data he gathered were used to support his ideas about how inheritance worked in peas. He didn’t guess or ignore data that didn’t agree with his ideas.
Opened his work for others to review. He described his experiment and results to other scientists. He presented this at a scientific meeting and then published a paper.

Despite Mendel’s careful recordkeeping and excellent experimental methods, his work didn’t get much attention. Most people didn’t even know about Mendel’s 1866 publication until four scientists independently re-discovered it around 1900. Part of the reason for this was that Mendel did not announce it as a grand new theory that explained inheritance for all plants and animals. He published it with the humble title, “Experiments on Plant Hybridization” and didn’t seem to know how broadly it applied. He also worked in a monastery in the Austrian Empire, isolated from most of the rest of the scientific community.

It may also be that the scientists working on genetics were not quite ready to accept his ideas, much less apply it to most living beings. Even Charles Darwin, who’s theory of natural selection discussed inheritance, thought that organisms could pass down traits they acquired over their lifetime. Most scientists accepted the idea that traits from parents were essentially averaged or blended in offspring. This seems logical, since children look like a mix of each parent. But it doesn’t explain why siblings look different or why people don’t all start to look the same over time. Mendel’s work solved some long-standing mysteries about inheritance, but people were not ready to embrace it.

Another good source for information about solid scientific practices is Understanding Science 101 developed by the University of California Museum of Paleontology.

Learning objectives

Determine gametes, perform crosses, and calculate probabilities for outcomes of one- and two-character crosses using the principles of Mendelian Inheritance
Identify characteristics of good science

Stop & Think Questions Explained

8. Possible genotypes for someone with freckles: AA or Aa.

9. For flower colors:

- 100% of offspring would have purple flowers (all would have at least one dominant allele)
- 50% would be purple-flowered (half would be Aa, half would be aa)
- Parents were Aa and aa. Explanation: we know the white-flowered plant must have only recessive alleles (aa). The purple-flowered parent could be AA or Aa, but since half the offspring were white, they must have gotten a recessive allele from this parent also – which means it must have been Aa.

10. For flower color and height:

1. - 25% will be tall and purple-flowered (50% will be purple-flowered and 50% will be tall, but the combination is 0.5 x 0.5). 25% will be dwarf with white flowers (the reasoning is identical).
  - 1 purple-flowered tall : 1 purple-flowered dwarf
    If ratios don’t make much sense to you, this could also be expressed as half (or 50%) purple-flowered tall plants and half (or 50%) purple-flowered dwarf plants.