“Come to the garden and see my children,” said the monk to the

bishop.

The Gregor Mendel Story

Gregor Mendel

Born in the 1822 in Moravia (now in the Czech Republic).

Son of a tenant farmer; joined the monastery to get an education.

Deeply interested in science, particularly in heredity.

Gregor Mendel

At the monastery in Brno, Moravia, Mendel received the support of Abbot Napp.

From 1851 - 1855, studied at the University of Vienna, but did not finish any degree.

What was known...

At the time, it was thought that heredity was more or less random and unpredictable.

Most scientists believed heredity was controlled by a liquid factor that blended in the offspring. For example, a child with one light- skinned and one dark-skinned parent often had medium-toned skin.

Mendel wondered if there were predictable patterns to heredity.

Gregor Mendel

With the encouragement of Abbot Napp, Mendel conducted his own studies of heredity in peas.

Collected data from 1856 through 1865.

Mendel chose to study single, simple traits individually instead of all traits in all plants at once.

Gregor MendelMendel presented his work to the Association of Natural Research of Brno in 1865.

However, his quantitative approach to research went over everyone’s heads! Further, the “blending” hypothesis was so widely accepted, no one saw any reason to doubt it. Mendel’s work was seen as a mere curiosity.

Gregor Mendel

In 1868, Gregor was made Abbot of the Brno monastery.

His religious work left little time for research, and he eventually gave up research entirely, though still convinced he was right.

Gregor Mendel

In 1900, Mendel’s work was rediscovered by Hugo de Vries in the Netherlands and Erich von Tschermak-Seysnegg in Austria.

Though sometimes criticized in its detail, the main body of Mendel’s work still stands.

Mendel’s Law of Dominance

Traits are controlled by pairs of “genes.”

Each “gene” can be one of two “alleles”: dominant or recessive.

If at least one dominant allele is inherited, the individual will show the dominant trait. Recessive traits are seen only when two recessive alleles are inherited.

Now we know this only holds true for single gene traits.

Law of Segregation

In the cell are pairs of “genes,” one pair for each trait.

During gamete formation, these pairs are separated from one another. Each member of the pair ends up in a separate gamete.

This law still fits what we know today about gamete formation, except that most traits are controlled by multiple genes.

Law of Independent Assortment

At the end of gamete formation, the “genes” for each trait have been sorted into separate gametes.

Each pair of traits sorts independently of other pairs of genes; that is, a dominant allele for one trait doesn’t necessarily follow the dominant allele for another trait.

We now know that while we have millions of genes, we have only 23 pairs of chromosomes. Obviously most genes must be linked with others on the same chromosome, though crossing over during meiosis does mix genes further.

Monohybrid Cross

A monohybrid cross illustrates the Law of Dominance and the Law of Segregation. Pure-breeding purple and pure-breeding white plants produce all purple plants in the F1 generation.

Monohybrid crossWhen F1 plants are bred together, 3/4 of the offspring are purple, 1/4 are white. The recessive trait did not disappear, but it only appears if an individual inherits two recessive alleles. The parents were all carriers of the recessive allele.

Monohybrid Cross

How it works: Each plant has two chromosomes with the flower color gene (one from each parent). All of the F1 plants were homozygous,

having either two dominant alleles or two recessive alleles. Each individual can put ONE of each chromosome in their gametes. In this

case, the P generation plant with the dominant traits only had the dominant allele.

(Law of Segregation is at work here.)

Monohybrid Cross

(Law of Dominance is at work here.)

A purple P generation parent was always crossed with a white P generation parent, so that each offspring received one dominant (P) allele and one recessive (p) allele. All offspring were heterozygous.

Monohybrid Cross

All of F1 plants had purple flowers because of the dominant allele they received from the purple P generation parent. However, the recessive

allele did not disappear. Each plant F1 could donate either the dominant allele OR the recessive allele to the next generation. Once again, this

shows the Law of Segregation.

Monohybrid Cross

The appearance of the F2 offspring depends on which combination of alleles it received from the F1 parents. Each F1 parent had a 50:50 chance of donating either the dominant (P) or the recessive (p) allele. There are three ways of getting a purple F2 offspring and one way of getting a white offspring. Notice how the Law of Dominance works here.

Punnett SquareA Punnett square is one way to determine the possible outcomes and the probability of each outcome in a monohybrid cross. It can also be figured mathematically. Notice that what goes across the top and down the side of the square are the possible gametes

that each parent can produce.

Exceptions to the rules...

The traits that Mendel studied in peas showed complete dominance.

Not all single-gene traits show complete dominance, however. Some traits appear to “blend,” which is why the blending hypothesis was so strong for such a long time.

Incomplete DominanceNotice the appearance of

“blending” in the heterozygous snapdragons. Even in

complete dominance, both alleles in a heterozygote are

expressed, but only the dominant one is apparent in

the phenotype. In incomplete dominance, both are

expressed, and both show in the phenotype in such a way

that they appear to blend together.

Incomplete Dominance

An example of incomplete dominance in animals.

Genetically, co-dominance works just like incomplete dominance: both alleles are expressed in the phenotype.The only difference between the two is that the expressions of the two alleles do not blend, but are both seen in the phenotype.

A classic example is the roan horse, offspring of a white and a chestnut horse, which has both white and red hairs, making it look pink.

Co-dominance

Co-dominance

Red Roan

Blue Roan

Summary

Mendel’s work showed that heredity is controlled not by “blending” of liquid factors, but by some kind of particle that we now call “genes.”

His discovery showed that heredity follows the laws of probability, and the probability of certain outcomes of crosses can be predicted.