Gregor Mendel’s Peas (Part 1)

 Gregor Mendel posing as an abbotThis is the first of two articles looking at the life and work of Gregor Mendel- the 19th century monk who founded the science of genetics.Gregor Mendel discovered how genes are passed from parent to offspring- including the offspring of all plants, animals and humans.

You may wish to refer to Explaining Genetic Inheritance before reading this article.

Gregor Mendel-who was he?

Gregor Mendel was born in 1822 in Silesia in what is now the Czech Republic. From a very young age he was interested in gardening, an interest he learnt from his father who had been a farmer.

In 1843 at the age of 21 he became a monk at the Abbey of St.Thomas in the city of Brno in the Czech Republic.

Between 1851 and 1853 he was sponsored by the Abbey to study physics at the University of Vienna in Austria. While studying in Vienna he learned valuable lessons about how to prepare, carry out and analyze results of scientific experiments.Brno higlighted on map of Czech Republic

After completing his studies in Vienna he returned to the Abbey to lead the life of a monk; he also developed an interest in researching heredity.

He started off by studying the heredity of mice before switching his attention to the common garden pea. (Pisum sativum)

Reasons why Mendel choose the garden pea for his research

1) The common garden pea is easy to cross pollinate

The common garden pea is fast growing and easy to ‘cross pollinate’ artificially with other pea plants.’Cross pollination’ is achieved if you stroke the hairs of a small paint brush….

Paint brush with pollen sticking on hairs

….onto the anthers of a mature flower.

Cross section of a pea plant demonstrating process of cross fertilisation

With pollen sticking to its hairs you then stroke the brush onto the stigma of a second mature flower. This starts the process of fertilisation of the ovules and the production of seeds. However, you must make sure that the anthers of the second flower have previously been removed so that self pollination cannot take place.

brushing stigma of second pea plant with pollen, making sure that anthers are removed

The fertilized ovules now develop into seed ’embryos’ before turning into peas. The ovaries, which contain the seeds, become the pea ‘pods’.

cross section of pea pods showing ovary walls and fertilised ovules

2) The common garden pea readily self pollinates

Not only is the common garden pea easy to cross pollinate, but it can also self pollinate. Self pollination takes place when pollen is tranferred from the anthers onto the stigma of the same flower, beginning the process of self fertilisation. The pea plant is one of the few plants in the plant kingdom that self pollinates.

self pollination of pea plant

3) Large numbers of pea plants can be grown on a small plot of land

This is a picture of Mendel’s pea garden at St Thomas’s Abbey. Notice the size of the garden and consider how many pea plants can be grown in such a small plot.

Mendel's garden at St Thomas's Monastery in Brno

4) There are many sub species of common garden pea 

In total Mendel chose 22 sub species of pure bred common garden pea plants to study. With its many varieties, the common garden pea was ideal for Mendel to use in his experiments.

Within these 22 sub species Mendel isolated seven key traits whose heredity he would research; research that would be meticulous, take eight years to complete and involve the breeding of 29 000 pea plants!

His big idea was to understand which variations of a single ‘trait’ were passed onto any offspring.

  • What is a ‘trait’?

In the example below the color of the flower is the ‘trait’. The different colors of flower are the variations of the ‘trait’.

white purple crossbreed meme

5) Different varieties of pea seeds were readily available in Brno in the 1850’s

A regular supply of seeds from many different varieties of pea plant was readily available in the 1850’s in Brno from local seed merchants.

19th century corner shop

Mendel’s seven traits

The following is a summary of the seven ‘traits’ whose heredity Mendel studied in his comprehensive eight year programme of cross breeding and self pollination:

Trait 1 Mendel crossbred or allowed plants with white and purple flowers to self pollinate. Would offspring have white or purple flowers?

flower colour white or purple

Trait 2  He crossbred plants with flowers located near the central stem of the plant (‘axial’ position) or near the top of the stem. (‘terminal’ position) He also allowed these plants to self pollinate. Where would the flowers of any offspring be located? In the ‘axial’ or ‘terminal’ position?

The ‘trait’ is the position of the flowers in relation to the stem. This trait has two possible variations- either the flowers are located in the  ‘axial’ or ‘terminal’ position.

mendelian inheritance-axial and terminal leaf position

Trait 3 He crossbred or allowed pea plants with long or short stems to self pollinate. Would any offspring have short or long stems?

The trait is the length of the stem. This trait has two possible variations- any offspring would develop either short or long stems.

mendelian inheritance-Long or short stems

Trait 4 He crossbred or allowed plants whose seeds were round or wrinkled in shape  to self pollinate. Would offspring produce round or wrinkled seeds?

The ‘trait’ is the seed shape; this trait has two variations-either round or wrinkled seeds.

mendelian inheritance-peas smooth and wrinkled

Trait 5 Mendel crossbred or allowed pea plants whose seeds were colored yellow or green to self pollinate. Would any offspring produce yellow or green seeds?

The ‘trait’ is the color of the seeds; this trait has two variations- either yellow or green seeds.

mendelian inheritance-yellow and green peas

Trait 6 He crossbred or allowed plants whose ripe pod shape was ‘inflated’ or ‘constricted’ to self pollinate. Would any offspring have inflated or constricted pods?

The ‘trait’ is the pod shape; this trait has two variations-either ‘inflated’ or ‘constricted’ pods.

mendelian inheritance-pods smooth and bumpy

Trait 7 He crossbred or allowed pea plants whose unripe pod color was yellow or green to self pollinate. Would offspring have green or yellow pod colors?

The ‘trait’ is the unripe pod color; this trait has two variations-the unripe pods are either colored green or yellow.

 

mendelian inheritance-pod colour yellow or green

Mendel’s conclusions about his experiments

By 1863 Mendel had finally completed his experiments.We discuss the results of those experiments and Mendel’s discoveries about the laws of inheritance in Mendel’s Pea Plants (Part 2). 

St Thomas monastery Brno in the early 1800s from a drawing by A. Winkler (1793-1832)

St Thomas’s Abbey in Brno in the 1800’s

He had reached conclusions about the heredity of the common pea plant and presented those conclusions to the Brno Natural History Society in 1865 in a paper called Versuche über Pflanzenhybriden. (Experiments on Plant Hybridisation)

Reaction to Mendel’s findings

Scientists who listened to Mendel explaining his conclusions at the meeting of the Brno Natural History Society did not fully appreciate the significance of Mendel’s ground breaking conclusions about the laws of inheritance.

Eminent scientists of the 1860’s who read Mendel’s research paper dismissed his findings, believing that Mendel had merely provided answers about the laws of cross breeding pea plants rather than  fundamental laws of inheritance.

 Photograph of Gregor Mendel as an Abbot

Gregor Mendel

Mendel rediscovered

Mendel died in obscurity in 1884 with his research explaining the laws of heredity largely forgotten.

His findings was not rediscovered until the 1900’s when he posthumously received full recognition for his considerable achievements.

Even the great Charles Darwin, a contemporary of Mendel’s, never heard about Mendel’s research during his own lifetime.

Charles Darwin may have solved the mysteries of evolution, but unlike Mendel he was never able to explain the laws of heredity.

Even if eminent 19th century scientists were not able to understand Mendel, we understand what he was saying, even if we cannot speak any German!

Versuche über Pflanzenhybriden-gregor mendel original publication

Click to enlarge

Mendel is explaining ‘genotypes’ and possible combinations of ‘dominant’ and ‘recessive’ alleles, ‘BB’,’Bb’,’bb’ etc

This Punnett square below shows two cross bred pea plants-one has green pods and the other has yellow pods.

  • What information is the Punnett square conveying about the phenotype and genotype of yellow and green pea pods? (answer at bottom of page)

Punnett square peas-yellow or green unripened pea pod‘Blending Inheritance’- before Gregor Mendel

So if 19th century scientists did not understand or appreciate the significance of Mendel’s research, what did they believe about the nature of inheritance?

blended inheritance-red and white paint mixed produces pink

Before Gregor Mendel’s research was rediscovered in the 1900’s, many scientists believed in the hypothesis of ‘blending inheritance’.

‘Blending inheritance’ advocated that offspring inherit a blend or mixture of visible characteristics from both parents.

So if the hypothesis of ‘blending inheritance’ were applied to flowers, red flowers crossed with white flowers would, after a few generations, produce pink flowers.

However we know this does always not happen. For example the offspring of red and white cyclamen flowers is not always pink.

Red white pink cyclamen flowers

This false hypothesis of ‘blending’ the inheritance red and white flowers over three generations can be seen in this diagram:

Blending Inheritance explained in 3 generations

‘Blending Inheritance’, if applied to humans, would mean that the offspring of a tall and short parent would be a child of medium height- that is a mixture of the heights of both parents…

Bao Xi shu worlds tallest man walking next to much shorter wife

Or if you cross a zebra and and horse the resulting offspring would ‘blend’ to look like a ‘zorse’….

zorse- white marking and zebra markings combined

A lion and a tiger blended together would produce a liger. This phenotype looks half lion and half tiger.

Liger, half lion and half tiger, sitting with his trainer

In the absence of any understanding of ‘genotypes’ it is understandable how many 19th century scientists became fixated on providing explanations about the nature of inheritance based on the ‘phenotypes’ they saw with their own eyes.

It was Mendel who looked beyond the ‘phenotypes’ and explained how it is the ‘genotype’, that mixture of dominant and recessive alleles, that provides the answer to understanding inheritance.

Explanation of Punnett Square

The yellow color variation of the ‘pea pod color gene’ is the dominant allele. The green color variation of the ‘pea pod color gene’ is the recessive allele.

unripened green pea pod with two recessive alleles 'yy'The dominant variation of the gene (or in this case the dominant allele ‘Y’) is always the ‘phenotype’, even if an organism only has one copy of the dominant allele.

The recessive variation of the gene (or recessive allele ‘y’) is never the ‘phenotype’ when an organism has one recessive allele and one dominant allele.

One pea plant that is cross bred has green pods ; every green pod has two recessive ‘yy’ alleles.

The other pea plant that is cross bred has yellow pods; every yellow pod has one dominant allele (Y) and one recessive allele (y).

After crossbreeding there is a half chance that any offspring’s pea pods will be yellow and a half chance they will be green.unripened yellow pea pod with one dominant and one recessive gene

There is also a half chance of any offspring having a ‘Yy’ genotype and a half chance of having a ‘yy’ genotype

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