pea plant experiment inheritance

Gregor Mendel's Pea Plant Experiment

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Gregor Mendel is considered the father of modern genetics. He was an Austrian monk who worked with pea plants to explain how children inherit features from their parents. His work became the foundation of how scientists understand heredity, and he is widely considered a pioneer in the field of genetics.

Pea Plants and Mendelian Genetics

In Mendel's famous pea plant experiments, he deliberately cross-pollinated pea plants with obviously different features to discover some important things about how offspring inherit traits from their parents.

The Experiments

Mendel measured seven specific characteristics of pea plants:

• Smooth or wrinkled ripe seed
• Yellow or green seed albumen
• Purple or white flower
• Inflated or constricted ripe pod
• Green or yellow unripe pods
• Axial or terminal position of flowers
• Tall or dwarf stem length

What He Discovered

Between 1856 and 1863 Mendel experimented on the Pisum sativum , or pea plant, species. His experiments led him to make three generalizations:

• Offspring acquire one hereditary factor from each parent. This is known as the law of segregation .
• Different traits have an equal opportunity of occurring together. This is known as the law of independent assortment , and today's scientists understand this to be largely inaccurate. Some genes are in fact, linked together and appear more often together.
• Offspring will inherit the dominant trait, and can only inherit the recessive trait if he inherits both recessive factors. This is known as the law of dominance .

Most scientists of his day rejected Mendel's work. It wasn't widely accepted until after he died. During his lifetime, most scientists believed offspring inherited traits by blending, that is offspring inherited an 'average' of the traits of the parents.

Demonstrating Mendelian Genetics

Mendel is said to have tested over 28,000 plants to come to his conclusion. While the scope of his project is probably not realistic for you to recreate, you can study genetics using plants.

Who's the Father?

Who's the Father is an experiment in which students will experiment on plants to predict observable traits. You can recreate the experiment using Wisconsin Fast Plants® ( Brassica rapa ) - which are designed specifically so students can use them to study genetics. They also grow faster - a complete life cycle takes 28-30 days. This experiment will take approximately six weeks of daily observations to complete. It's best suited for older students in middle school or high school who are studying genetics.

• Wisconsin Fast Plants ® Seed, Non-Purple Stem, Hairless (pack of 200)
• Wisconsin Fast Plants ® Seed, Yellow-Green Leaf (pack of 200)
• Wisconsin Fast Plants ® Seed, Non-Purple Stem, Yellow-Green Leaf (pack of 200)
• Potting mix
• Slow-release fertilizer pellets
• Homemade fluorescent lighting system or purchased lighting system
• Homemade growing system (alternatively, you can purchase a watering system )
• Labels for plants
• Stakes and ties
• Q-tips, or bee sticks (you just need a few)

Instructions

• Construct your lighting and watering systems first. Wisconsin Fast Plants® need continuous fluorescent light, and a continuous supply of fertilizer and water. You can either build homemade versions of these, or you can purchase pre-made kits through Carolina Biological. Both options are linked above in the materials list.
• Plant the seeds (you do not need to use all of them) according to the growing instructions . You'll want to start by planting the non-purple, yellow-green leaf seeds (this will be referred to as the first-generation offspring, or O1.) Also plant the non-purple stem, hairless seeds. (These seeds are the mother seeds, referred to as P1). Make sure you label which is which!
• In approximately four to seven days, your plants should grow. Observe the stem and leaf colors of both sets of plants and record your observations in your lab notebook. The best way to quantify your observations is to count the phenotypes (count the number of plants that have non-purple stems, the number of plants that have yellow-green leaves, etc.)
• Discard the mother plants, but maintain the offspring plants.
• Write a hypothesis as to how the offspring plants inherited their observable genetic traits. For example, if you observe that most of your offspring plants have non-purple stems but yellow leaves, you may assign these as dominant traits. If you observe that some of your offspring plants have purple stems and green leaves, you might assume these are recessive traits. Based on your observations, create a testable hypothesis. You'll want to try to guess the father plant's stem and leaf colors based on your hypothesis.
• Intermate the plants using a bee stick or Q-tip. To do this, gently swap the bee stick on one plant, ensuring that the plant has pollen, and then sharing it with another plant. Do this several times to make sure that each plant receives pollen from several other plants, both with like and unlike observable traits. Do this once a day for three days.
• Once the three days are over, cut off any flower buds that were not pollinated.
• Stop watering the plants and let them dry out.
• Harvest the seeds and replant them, essentially starting the process again. These seeds are the second generation of offspring, or O2.
• Make observations about the stem and leaf color of the next generation of plants. Do you think your hypothesis was correct?
• Plant the yellow-green leaf seeds. These will be known as the 'father' or P2.
• After a few days, observe the stem and leaf colors of the P2 plants. Do your observations support your hypothesis?

Video Directions

This video shows how to do genetics labs and will help you tackle the procedure for studying the genetics of your plants.

Online Labs

It's worth noting that if growing peas and making homemade apparatuses are a bit more than you were bargaining for, there are a few great interactive labs online.

Mendel's Peas

This online lab is a replica of Mendel's pea experiments. The lab has a handy menu so you can actually explore the lab before doing anything. The lab takes you through various steps including planting the peas, observing their traits, and then cross pollinating the first plants you grew. This is exactly what Mendel did so students can get a feel for the tedious process he went through to come up with his observations.

While graphically not as exciting, Pea Soup is another online option that helps students observe two traits in pea plants. To get started, you click on the 'begin experiment' button. Then you are brought to a page where you can choose to 'mate' two different peas. Their genotypes are written for you. Then the page will show you all the options available for the 'parents' you selected. The page moves quickly, and you can miss it if you're not writing everything down.

MIT's STAR Genetics

MIT's STAR Genetics lab is a downloadable 'game' of sorts where students can mix and match genotypes of a variety of species including pea plants, fruit flies, and even cows. The program is best suited towards high school students who have a strong understanding of Biology.

Genetics Is Fun

Whether you study pea plants or fruit flies, or simply go home and observe your parents' traits and try to figure out how you got your own, studying genetics can be a lot of fun. While modern genetics identifies a few things that Mendel got wrong, his theories still apply where traits are not linked or influenced by other factors.

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Mendel's Experiments: The Study of Pea Plants & Inheritance

Gregor Mendel was a 19th-century pioneer of genetics who today is remembered almost entirely for two things: being a monk and relentlessly studying different traits of pea plants. Born in 1822 in Austria, Mendel was raised on a farm and attended the University of Vienna in Austria's capital city.

There, he studied science and math, a pairing that would prove invaluable to his future endeavors, which he conducted over an eight-year period entirely at the monastery where he lived.

In addition to formally studying the natural sciences in college, Mendel worked as a gardener in his youth and published research papers on the subject of crop damage by insects before taking up his now-famous work with Pisum sativum, the common pea plant. He maintained the monastery greenhouses and was familiar with the artificial fertilization techniques required to create limitless numbers of hybrid offspring.

An interesting historical footnote: While Mendel's experiments and those of the visionary biologist Charles Darwin both overlapped to a great extent, the latter never learned of Mendel's experiments.

Darwin formulated his ideas about inheritance without knowledge of Mendel's thoroughly detailed propositions about the mechanisms involved. Those propositions continue to inform the field of biological inheritance in the 21st century.

Understanding of Inheritance in the Mid-1800s

From the standpoint of basic qualifications, Mendel was perfectly positioned to make a major breakthrough in the then-all-but-nonexistent field of genetics, and he was blessed with both the environment and the patience to get done what he needed to do. Mendel would end up growing and studying nearly 29,000 pea plants between 1856 and 1863.

When Mendel first began his work with pea plants, the scientific concept of heredity was rooted in the concept of blended inheritance, which held that parental traits were somehow mixed into offspring in the manner of different-colored paints, producing a result that was not quite the mother and not quite the father every time, but that clearly resembled both.

Mendel was intuitively aware from his informal observation of plants that if there was any merit to this idea, it certainly didn't apply to the botanical world.

Mendel was not interested in the appearance of his pea plants per se. He examined them in order to understand which characteristics could be passed on to future generations and exactly how this occurred at a functional level, even if he didn't have the literal tools to see what was occurring at the molecular level.

Pea Plant Characteristics Studied

Mendel focused on the different traits, or characters, that he noticed pea plants exhibiting in a binary manner. That is, an individual plant could show either version A of a given trait or version B of that trait, but nothing in between. For example, some plants had "inflated" pea pods, whereas others looked "pinched," with no ambiguity as to which category a given plant's pods belonged in.

The seven traits Mendel identified as being useful to his aims and their different manifestations were:

• Flower color:  Purple or white.
• Flower position:  Axial (along the side of the stem) or terminal (at the end of the stem).
• Stem length:  Long or short.
• Pod shape:  Inflated or pinched.
• Pod color:  Green or yellow.
• Seed shape:  Round or wrinkled.
• Seed color:  Green or yellow.

Pea Plant Pollination

Pea plants can self-pollinate with no help from people. As useful as this is to plants, it introduced a complication into Mendel's work. He needed to prevent this from happening and allow only cross-pollination (pollination between different plants), since self-pollination in a plant that does not vary for a given trait does not provide helpful information.

In other words, he needed to control what characteristics could show up in the plants he bred, even if he didn't know in advance precisely which ones would manifest themselves and in what proportions.

Mendel's First Experiment

When Mendel began to formulate specific ideas about what he hoped to test and identify, he asked himself a number of basic questions. For example, what would happen when plants that were true-breeding for different versions of the same trait were cross-pollinated?

"True-breeding" means capable of producing one and only one type of offspring, such as when all daughter plants are round-seeded or axial-flowered. A true line shows no variation for the trait in question throughout a theoretically infinite number of generations, and also when any two selected plants in the scheme are bred with each other.

• To be certain his plant lines were true, Mendel spent two years creating them.

If the idea of blended inheritance were valid, blending a line of, say, tall-stemmed plants with a line of short-stemmed plants should result in some tall plants, some short plants and plants along the height spectrum in between, rather like humans. Mendel learned, however, that this did not happen at all. This was both confounding and exciting.

Mendel's Generational Assessment: P, F1, F2

Once Mendel had two sets of plants that differed only at a single trait, he performed a multigenerational assessment in an effort to try to follow the transmission of traits through multiple generations. First, some terminology:

• The parent generation was the P generation , and it included a P1 plant whose members all displayed one version of a trait and a P2 plant whose members all displayed the other version.
• The hybrid offspring of the P generation was the F1 (filial) generation .
• The offspring of the F1 generation was the  F2 generation  (the "grandchildren" of the P generation).

This is called a monohybrid cross : "mono" because only one trait varied, and "hybrid" because offspring represented a mixture, or hybridization, of plants, as one parent has one version of the trait while one had the other version.

For the present example, this trait will be seed shape (round vs. wrinkled). One could also use flower color (white vs. purpl) or seed color (green or yellow).

Mendel's Results (First Experiment)

Mendel assessed genetic crosses from the three generations to assess the heritability of characteristics across generations. When he looked at each generation, he discovered that for all seven of his chosen traits, a predictable pattern emerged.

For example, when he bred true-breeding round-seeded plants (P1) with true-breeding wrinkled-seeded plants (P2):

• All of the plants in the F1 generation had round seeds . This seemed to suggest that the wrinkled trait had been obliterated by the round trait. 
• However, he also found that, while about three-fourths of the plants in the F2 generation has round seeds, about one-fourth of these plants had wrinkled seeds . Clearly, the wrinkled trait had somehow "hidden" in the F1 generation and re-emerged in the F2 generation.

This led to the concept of dominant traits (here, round seeds) and recessive traits (in this case, wrinkled seeds).

This implied that the plants' phenotype (what the plants actually looked like) was not a strict reflection of their genotype (the information that was actually somehow coded into the plants and passed along to subsequent generations).

Mendel then produced some formal ideas to explain this phenomenon, both the mechanism of heritability and the mathematical ratio of a dominant trait to a recessive trait in any circumstance where the composition of allele pairs is known.

Mendel's Theory of Heredity

Mendel crafted a theory of heredity that consisted of four hypotheses:

• Genes  (a gene being the chemical code for a given trait) can come in different types.
• For each characteristic, an organism inherits one  allele  (version of a gene) from each parent.
• When two different alleles are inherited, one may be expressed while the other is not.
• When gametes (sex cells, which in humans are sperm cells and egg cells) are formed, the two alleles of each gene are separated.

The last of these represents the law of segregation , stipulating that the alleles for each trait separate randomly into the gametes.

Today, scientists recognize that the P plants that Mendel had "bred true" were homozygous for the trait he was studying: They had two copies of the same allele at the gene in question.

Since round was clearly dominant over wrinkled, this can be represented by RR and rr, as capital letters signify dominance and lowercase letters indicate recessive traits. When both alleles are present, the trait of the dominant allele was manifested in its phenotype.

The Monohybrid Cross Results Explained

Based on the foregoing, a plant with a genotype RR at the seed-shape gene can only have round seeds, and the same is true of the Rr genotype, as the "r" allele is masked. Only plants with an rr genotype can have wrinkled seeds.

And sure enough, the four possible combinations of genotypes (RR, rR, Rr and rr) yield a 3:1 phenotypic ratio, with about three plants with round seeds for every one plant with wrinkled seeds.

Because all of the P plants were homozygous, RR for the round-seed plants and rr for the wrinkled-seed plants, all of the F1 plants could only have the genotype Rr. This meant that while all of them had round seeds, they were all carriers of the recessive allele, which could therefore appear in subsequent generations thanks to the law of segregation.

This is precisely what happened. Given F1 plants that all had an Rr genotype, their offspring (the F2 plants) could have any of the four genotypes listed above. The ratios were not exactly 3:1 owing to the randomness of the gamete pairings in fertilization, but the more offspring that were produced, the closer the ratio came to being exactly 3:1.

Mendel's Second Experiment

Next, Mendel created dihybrid crosses , wherein he looked at two traits at once rather than just one. The parents were still true-breeding for both traits, for example, round seeds with green pods and wrinkled seeds with yellow pods, with green dominant over yellow. The corresponding genotypes were therefore RRGG and rrgg.

As before, the F1 plants all looked like the parent with both dominant traits. The ratios of the four possible phenotypes in the F2 generation (round-green, round-yellow, wrinkled-green, wrinkled-yellow) turned out to be 9:3:3:1

This bore out Mendel's suspicion that different traits were inherited independently of one another, leading him to posit the law of independent assortment . This principle explains why you might have the same eye color as one of your siblings, but a different hair color; each trait is fed into the system in a manner that is blind to all of the others.

Linked Genes on Chromosomes

Today, we know the real picture is a little more complicated, because in fact, genes that happen to be physically close to each other on chromosomes can be inherited together thanks to chromosome exchange during gamete formation.

In the real world, if you looked at limited geographical areas of the U.S., you would expect to find more New York Yankees and Boston Red Sox fans in close proximity than either Yankees-Los Angeles Dodgers fans or Red Sox-Dodgers fans in the same area, because Boston and New York are close together and both are close to 3,000 miles from Los Angeles.

Mendelian Inheritance

As it happens, not all traits obey this pattern of inheritance. But those that do are called Mendelian traits . Returning to the dihybrid cross mentioned above, there are sixteen possible genotypes:

RRGG, RRgG, RRGg, RRgg, RrGG, RrgG, RrGg, Rrgg, rRGG, rRgG, rRGg, rRgg, rrGG, rrGg, rrgG, rrgg

When you work out the phenotypes, you see that the probability ratio of

round green, round yellow, wrinkled green, wrinkled yellow

turns out to be 9:3:3:1. Mendel's painstaking counting of his different plant types revealed that the ratios were close enough to this prediction for him to conclude that his hypotheses were correct.

• Note: A genotype of rR is functionally equivalent to Rr. The only difference is which parent contributes which allele to the mix.

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• Scitable by Nature Education: Gregor Mendel and the Principles of Inheritance
• Biology LibreTexts: Mendel's Pea Plants
• OpenText BC: Concepts of Biology: Laws of Inheritance
• Forbes Magazine: How Mendel Channeled Darwin

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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• Mendel Laws Of Inheritance

Mendel's Laws of Inheritance

Inheritance can be defined as the process of how a child receives genetic information from the parent. The whole process of heredity is dependent upon inheritance and it is the reason that the offsprings are similar to the parents. This simply means that due to inheritance, the members of the same family possess similar characteristics.

It was only during the mid 19th century that people started to understand inheritance in a proper way. This understanding of inheritance was made possible by a scientist named Gregor Mendel, who formulated certain laws to understand inheritance known as Mendel’s laws of inheritance.

Table of Contents

Mendel’s Laws of Inheritance

Why was pea plant selected for mendel’s experiments, mendel’s experiments, conclusions from mendel’s experiments, mendel’s laws, key points on mendel’s laws.

Between 1856-1863, Mendel conducted the hybridization experiments on the garden peas. During that period, he chose some distinct characteristics of the peas and conducted some cross-pollination/ artificial pollination on the pea lines that showed stable trait inheritance and underwent continuous self-pollination. Such pea lines are called true-breeding pea lines.

Also Refer:   Mendel’s Laws of Inheritance: Mendel’s Contribution

He selected a pea plant for his experiments for the following reasons:

• The pea plant can be easily grown and maintained.
• They are naturally self-pollinating but can also be cross-pollinated.
• It is an annual plant, therefore, many generations can be studied within a short period of time.
• It has several contrasting characters.

Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were:

Monohybrid Cross

Dihybrid cross.

While experimenting, Mendel found that certain factors were always being transferred down to the offspring in a stable way. Those factors are now called genes i.e. genes can be called the units of inheritance.

Mendel experimented on a pea plant and considered 7 main contrasting traits in the plants. Then, he conducted both experiments to determine the inheritance laws. A brief explanation of the two experiments is given below.

In this experiment, Mendel took two pea plants of opposite traits (one short and one tall) and crossed them. He found the first generation offspring were tall and called it F1 progeny. Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1. To know more about this experiment, visit Monohybrid Cross – Inheritance Of One Gene .

Mendel even conducted this experiment with other contrasting traits like green peas vs yellow peas, round vs wrinkled, etc. In all the cases, he found that the results were similar. From this, he formulated the laws of Segregation And Dominance .

In a dihybrid cross experiment, Mendel considered two traits, each having two alleles. He crossed wrinkled-green seed and round-yellow seeds and observed that all the first generation progeny (F1 progeny) were round-yellow. This meant that dominant traits were the round shape and yellow colour.

He then self-pollinated the F1 progeny and obtained 4 different traits: round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds in the ratio 9:3:3:1.

Check Dihybrid Cross and Inheritance of Two Genes to know more about this cross.

After conducting research for other traits, the results were found to be similar. From this experiment, Mendel formulated his second law of inheritance i.e. law of Independent Assortment.

• The genetic makeup of the plant is known as the genotype. On the contrary, the physical appearance of the plant is known as phenotype.
• The genes are transferred from parents to the offspring in pairs known as alleles.
• During gametogenesis when the chromosomes are halved, there is a 50% chance of one of the two alleles to fuse with the allele of the gamete of the other parent.
• When the alleles are the same, they are known as homozygous alleles and when the alleles are different they are known as heterozygous alleles.

Also Refer:   Mendelian Genetics

The two experiments lead to the formulation of Mendel’s laws known as laws of inheritance which are:

• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

This is also called Mendel’s first law of inheritance. According to the law of dominance, hybrid offspring will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called the recessive traits while the alleles that determine the trait are known as the dominant traits.

The law of segregation states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. In other words, allele (alternative form of the gene) pairs segregate during the formation of gamete and re-unite randomly during fertilization. This is also known as Mendel’s third law of inheritance.

Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of traits segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunity to occur together.

• The law of inheritance was proposed by Gregor Mendel after conducting experiments on pea plants for seven years.
• Mendel’s laws of inheritance include law of dominance, law of segregation and law of independent assortment.
• The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring.
• The law of independent assortment states that the inheritance of one pair of genes is independent of inheritance of another pair.

Also Read:   Non-Mendelian Inheritance

Stay tuned with BYJU’S to learn more about Mendel’s Laws of Inheritance. You can also download the BYJU’S app for further reference on Mendel’s laws.

Frequently Asked Questions

What are the three laws of inheritance proposed by mendel.

The three laws of inheritance proposed by Mendel include:

Which is the universally accepted law of inheritance?

Law of segregation is the universally accepted law of inheritance. It is the only law without any exceptions. It states that each trait consists of two alleles which segregate during the formation of gametes and one allele from each parent combines during fertilization.

Why is the law of segregation known as the law of purity of gametes?

The law of segregation is known as the law of purity of gametes because a gamete carries only a recessive or a dominant allele but not both the alleles.

Why was the pea plant used in Mendel’s experiments?

Mendel picked pea plants in his experiments because the pea plant has different observable traits. It can be grown easily in large numbers and its reproduction can be manipulated. Also, pea has both male and female reproductive organs, so they can self-pollinate as well as cross-pollinate.

What was the main aim of Mendel’s experiments?

The main aim of Mendel’s experiments was:

• To determine whether the traits would always be recessive.
• Whether traits affect each other as they are inherited.
• Whether traits could be transformed by DNA.

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very nice. it is the best to study about genetics

Genetic inheritance is so interesting

It helped me a lot Thanks

It is so amazing thanks a lot

Superb, it’s interesting.

It is very useful becoz all details explain in simple manner with examples

AWESOME, the above notes are fabulous

well that helped me a lot

Thanks, It helped me a lot!! Impeccable notes !!😍

Nice resource 👍

It helped me alot

If Mendel gave three law the what is the law of unit of characters and who proposed this law . Please clear my doubt a little bit faster , it is little important for me.

The Law of unit characters was proposed by Mendel. He explained that the inheritance of a trait is controlled by unit characters or factors, which are passed from parents to offspring through the gametes. These factors are now known as genes. Each factor exists in pairs, which are known as alleles.

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21 Mendel’s Experiments

By the end of this section, you will be able to:

• Explain the scientific reasons for the success of Mendel’s experimental work
• Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles

Johann Gregor Mendel (1822–1884) (Figure 1) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics that is used to study a specific biological phenomenon to gain understanding to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural history society. He demonstrated that traits are transmitted faithfully from parents to offspring in specific patterns. In 1866, he published his work, Experiments in Plant Hybridization, 1 in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community, which incorrectly believed that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring. This hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation is the range of small differences we see among individuals in a characteristic like human height. It does appear that offspring are a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. Mendel worked instead with traits that show discontinuous variation . Discontinuous variation is the variation seen among individuals when each individual shows one of two—or a very few—easily distinguishable traits, such as violet or white flowers. Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring as would have been expected at the time, but that they were inherited as distinct traits. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime; in fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Crosses

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, meaning that pollen encounters ova within the same flower. The flower petals remain sealed tightly until pollination is completed to prevent the pollination of other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true-breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendel performed hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.

Plants used in first-generation crosses were called P , or parental generation, plants (Figure 2). Mendel collected the seeds produced by the P plants that resulted from each cross and grew them the following season. These offspring were called the F 1 , or the first filial (filial = daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 generation, F 4 generation, and so on, but it was the ratio of characteristics in the P, F 1 , and F 2 generations that were the most intriguing and became the basis of Mendel’s postulates.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea-pod size, pea-pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants and reported results from thousands of F 2 plants.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he was using plants that bred true for white or violet flower color. Irrespective of the number of generations that Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical. This was an important check to make sure that the two varieties of pea plants only differed with respect to one trait, flower color.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait had completely disappeared in the F 1 generation.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that 705 plants in the F 2 generation had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which parent—male or female—contributed which trait. This is called a reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics that Mendel examined, the F 1 and F 2 generations behaved in the same way that they behaved for flower color. One of the two traits would disappear completely from the F 1 generation, only to reappear in the F 2 generation at a ratio of roughly 3:1 (Figure 3).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these dominant and recessive traits, respectively. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-colored flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (and were not blended) in the plants of the F 1 generation. Mendel proposed that this was because the plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of their two copies to their offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic, or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

CONCEPTS IN ACTION

For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of inheritance, visit the Mendel’s Peas web lab .

Also, check out the following video as review

• Johann Gregor Mendel, “Versuche über Pflanzenhybriden.” Verhandlungen des naturforschenden Vereines in Brünn , Bd. IV für das Jahr, 1865 Abhandlungen (1866):3–47. [for English translation, see http://www.mendelweb.org/Mendel.plain.html]

Introductory Biology: Evolutionary and Ecological Perspectives Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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