- Introduction to heredity
- Fertilization terminology: gametes, zygotes, haploid, diploid
- Alleles and genes
- Worked example: Punnett squares
- Mendel and his peas
- The law of segregation
- The law of independent assortment
- Probabilities in genetics
- Mendelian genetics
Mendel and his peas
How Austrian monk Gregor Mendel laid the foundations of genetics. Mendel's life, experiments, and pea plants.
How can we study inheritance?
When spending time with your own family, friends, and neighbors, you may have noticed that many traits run in families. For instance, members of a family may share similar facial features, hair color (like the brother and sister below), or a predisposition to health problems such as diabetes. Characteristics that run in families often have a genetic basis, meaning that they depend on genetic information a person inherits from his or her parents.
Image of a brother and sister who both have distinctive reddish hair.
What if you wanted to figure out how genetic information is transmitted between generations? For instance, you might be curious how traits can "skip" a generation, or why one child in a family may suffer from a genetic disease while another does not. How could you go about asking these kinds of questions scientifically?
An obvious first idea would be to study human inheritance patterns directly, but that turns out to be a tricky proposition (see the pop-up below for details). In this article, we'll see how a nineteenth-century monk named Gregor Mendel instead uncovered the key principles of inheritance using a simple, familiar system: the pea plant.
The monk in the garden: Gregor Mendel
Johann Gregor Mendel (1822–1884), often called the “father of genetics,” was a teacher, lifelong learner, scientist, and man of faith. It would be fair to say that Mendel had a lot of grit: he persevered through difficult circumstances to make some of the most important discoveries in biology.
Portrait of Gregor Mendel.
As a young man, Mendel had difficulty paying for his education due to his family's limited means, and he also suffered bouts of physical illness and depression; still, he persevered to graduate from high school and, later, university. After finishing university, he joined the Augustinian Abbey of St. Thomas in Brno, in what is now the Czech Republic. At the time, the monastery was the cultural and intellectual hub of the region, and Mendel was immediately exposed to new teachings and ideas.
His decision to join the order (against the wishes of his father, who expected him to carry on the family farm) appears to have been motivated in part by a desire to continue his education and pursue his scientific interests. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels.
Research on heredity
In 1856, Mendel began a decade-long research project to investigate patterns of inheritance. Although he began his research using mice, he later switched to honeybees and plants, ultimately settling on garden peas as his primary model system. A model system is an organism that makes it easy for a researcher to investigate a particular scientific question, such as how traits are inherited. By studying a model system, researchers can learn general principles that apply to other, harder-to-study organisms or biological systems, such as humans.
Mendel studied the inheritance of seven different features in peas, including height, flower color, seed color, and seed shape. To do so, he first established pea lines with two different forms of a feature, such as tall vs. short height. He grew these lines for generations until they were pure-breeding (always produced offspring identical to the parent), then bred them to each other and observed how the traits were inherited.
In addition to recording how the plants in each generation looked, Mendel counted the exact number of plants that showed each trait. Strikingly, he found very similar patterns of inheritance for all seven features he studied:
- One form of a feature, such as tall, always concealed the other form, such as short, in the first generation after the cross. Mendel called the visible form the dominant trait and the hidden form the recessive trait.
- In the second generation, after plants were allowed to self-fertilize (pollinate themselves), the hidden form of the trait reappeared in a minority of the plants. Specifically, there were always about plants that showed the dominant trait (e.g., tall) for every plant that showed the recessive trait (e.g., short), making a ratio.
- Mendel also found that the features were inherited independently: one feature, such as plant height, did not influence inheritance of other features, such as flower color or seed shape.
Representation of results from one of Mendel's experiments. When a tall and short plant are crossed, all of the offspring are tall. If the offspring self-fertilize, they produce tall and short plants in a ratio of 3:1 in the next generation. Mendel's actual counts were 787 tall:277 short plants in this generation (2.84:1 ratio).
In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. Based on the patterns he observed, the counting data he collected, and a mathematical analysis of his results, Mendel proposed a model of inheritance in which:
- Characteristics such as flower color, plant height, and seed shape were controlled by pairs of heritable factors that came in different versions.
- One version of a factor (the dominant form) could mask the presence of another version (the recessive form).
- The two paired factors separated during gamete production, such that each gamete (sperm or egg) randomly received just one factor.
- The factors controlling different characteristics were inherited independently of one another.
We'll take a closer look at how Mendel reached these conclusions in the articles on the law of segregation and the law of independent assortment. In 1866, Mendel published his observations and his model of inheritance, under the title Experiments in Plant Hybridization, in the Proceedings of the Natural History Society of Brünn.
Mendel's work went largely unnoticed by the scientific community during his lifetime. How could this have been the case?
In part, Mendel's contemporaries failed to recognize the importance of his work because his findings went against prevailing (popular) ideas about inheritance. In addition, although we now see Mendel's mathematical approach to biology as innovative and pioneering, it was new, unfamiliar, and perhaps confusing or unintuitive to other biologists of the time.
In the mid-1800s, when Mendel was doing his experiments, most biologists subscribed to the idea of blending inheritance. Blending inheritance wasn't a formal, scientific hypothesis, but rather, a general model in which inheritance involved the permanent blending of parents' characteristics in their offspring (producing offspring with an intermediate form of a characteristic). The blending model fit well with some observations of human inheritance: for instance, children often look a bit like both of their parents.
But the blending model could not explain why Mendel crossed a tall and a short pea plant and got only tall plants, or why self-fertilization of one of those tall plants would produce a ratio of tall to short plants in the next generation. Instead, if the blending model were correct, a tall plant crossed with a short plant should produce a medium plant, which would go on to produce more medium plants (see below).
Image comparing the predictions of the blending model with Mendel's actual results for a cross between a tall pea plant and a short pea plant.
The blending model predicts that all the offspring from the cross should be of medium height, and that if those offspring self-fertilize, all the plants in the next generation will also be of medium height.
Mendel instead observed that all the offspring of the cross were tall, and that when they self-fertilized, they produced tall and short plants in a ratio of 3:1.
As it turns out, both pea plant height and human height (along with many other characteristics in a wide range of organisms) are controlled by pairs of heritable factors that come in distinctive versions, just as Mendel proposed. In humans, however, there are many different factors (genes) that contribute fractionally to height and vary among individuals. This makes it difficult to see the contribution of any one factor and produces inheritance patterns that can resemble blending. In Mendel's experiments, in contrast, there was just one factor that differed between the tall and short pea plants, allowing Mendel to clearly see the underlying pattern of inheritance.
In 1868, Mendel became abbot of his monastery and largely set aside his scientific pursuits in favor of his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until around 1900 that his work was rediscovered, reproduced, and revitalized. Its rediscoverers were biologists on the brink of discovering the chromosomal basis of heredity – that is, about to realize that Mendel's “heritable factors” were carried on chromosomes.
Mendel’s model system: The pea plant
Mendel carried out his key experiments using the garden pea, Pisum sativum, as a model system. Pea plants make a convenient system for studies of inheritance, and they are still studied by some geneticists today.
Useful features of peas include their rapid life cycle and the production of lots and lots of seeds. Pea plants also typically self-fertilize, meaning that the same plant makes both the sperm and the egg that come together in fertilization. Mendel took advantage of this property to produce true-breeding pea lines: he self-fertilized and selected peas for many generations until he got lines that consistently made offspring identical to the parent (e.g., always short).
Pea plants are also easy to cross, or mate in a controlled way. This is done by transferring pollen from the anthers (male parts) of a pea plant of one variety to the carpel (female part) of a mature pea plant of a different variety. To prevent the receiving plant from self-fertilizing, Mendel painstakingly removed all of the immature anthers from the plant’s flowers before the cross.
Diagram of pea flowers, showing how a cross is performed. First, a flower on the female parent is emasculated, meaning that the male parts (anthers) are removed with forceps or scissors. Then, pollen is collected from a flower on the male parent plant using a paintbrush. The pollen is dabbed onto the female part (carpel) of the female parent flower that was previously emasculated.
Because peas were so easy to work with and prolific in seed production, Mendel could perform many crosses and examine many individual plants, making sure that his results were consistent (not just a fluke) and accurate (based on many data points).
Mendel’s experimental setup
Once Mendel had established true-breeding pea lines with different traits for one or more features of interest (such as tall vs. short height), he began to investigate how the traits were inherited by carrying out a series of crosses.
First, he crossed one true-breeding parent to another. The plants used in this initial cross are called the generation, or parental generation.
Mendel collected the seeds from the generation cross and grew them up. These offspring were called the generation, short for first filial generation. (Filius means “son” in Latin, so this name is slightly less weird than it seems!)
Once Mendel examined the plants and recorded their traits, he let them self-fertilize naturally, producing lots of seeds. He then collected and grew the seeds from the plants to produce an generation, or second filial generation. Again, he carefully examined the plants and recorded their traits.
Diagram of a cross between a tall plant and a short plant, labeling the P, F1, and F2 generations.
Mendel's experiments extended beyond the generation to , , and later generations, but his model of inheritance was based mostly on the first three generations (, , and ).
Mendel didn’t just record what his plants looked like in each generation (e.g., tall vs. short). Instead, he counted exactly how many plants with each trait were present. This may sound tedious, but by recording numbers and thinking mathematically, Mendel made discoveries that eluded famous scientists of his time (such as Charles Darwin, who carried out similar experiments but didn’t grasp the significance of his results).
You can use the links below to learn more about Mendel's laws of inheritance:
- The law of segregation, describing how individual traits are inherited.
- The law of independent assortment, describing how two or more traits are inherited relative to one another.
Want to join the conversation?
- what is difference between geneotype and phenotype(20 votes)
- genotype = genetic code
phenotype = physical appearance/properties/expression of the gene(70 votes)
- what does it mean when it says that it "self fertilizes"?(15 votes)
- Self fertilizing is when the pollen from one plant is put on the stigma of the same flower or another flower that is growing on the same plant.(27 votes)
- What does F1 generation mean? It doesn`t make sense.(1 vote)
- The F stands for filial, which comes from the Latin word filius, meaning son. F1 means the first generation of children (or baby pea plants).(50 votes)
- why the ratio is always roughly 3:1 not exactly?(13 votes)
- It is correct that it won't be exactly half ,because as Alexander said, there would be slight imperfections (nothing is absolute ,I mean).
But , for this pea plant experiment ,I think it would be that tall and short are compound traits ,like they are made up from a combination of small traits.(7 votes)
- What determines what gamete is chosen to move on? Or is it just random, and each gamete has an equal chance at being passed on to the offspring?(13 votes)
- Does garden pea, Pisum sativum has a discontinue character on height, colour...? mostly those characters are continuous, they are controlled by many genes.(7 votes)
- How can we identify that after breeding multiple times we have got a homozygotic offspring and does multiple breeding results in a homozygote offspring ?(9 votes)
- When 2 parents breed, they each have a genotype, let's say one is AA, the other is aa. Both are purebred and homozygous. In its offspring, they create a new genotype, one letter from each of the parent's genotype. This creates many outcomes, including the heterozygous, Aa.(5 votes)
- why did he use pea plants, why not any other flower?(5 votes)
- because pea was an annual plant and its was a self fertilizing plant and its easy to manually fertilize it(15 votes)
- If a tall plant and a short plant bred together, how would all of the plants end up being tall? Wouldn't 75% become tall and 25% become short based on the 3:1 ratio?(4 votes)
- All plants would end up being tall if the parents are homozygous for the traits controlling each character. 75% would be tall if and only if the p generation is heterozygous for the character while 25% would be short, which conforms to the 3:1 ratio.(10 votes)
- How it can be explained if Gregor Mendal studied eight character?(5 votes)
- Who knows, really depends on what 8 character was. What really important is that those traits are not linked (on the same chromosome) and that those traits are comparable (for example tall and short) and easily distinguishable.
All 3 Mendelian Laws would probably stay the same.(2 votes)
- what is difference between geneotype and phenotype(4 votes)
- Genotype is the specific gene alleles where as the phenotype is the physical expression of the alleles.(4 votes)