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Mendelian Genetics. About Gregor Mendel. Gregor Mendel was an Austrian monk who studied science and math in the mid-19 th century (1850’s) -- Mendel was also a priest and a high school teacher at the monastery

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About Gregor Mendel

Gregor Mendel was an Austrian monk who studied science and math in the mid-19th century (1850’s)

-- Mendel was also a priest and a high school teacher at the monastery

One of the main jobs of Gregor Mendel was to tend the garden at the monastery, and one of Mendel’s favorite plants to grow was the garden pea

-- Mendel cultivated several varieties of garden pea

Through his work over many years with pea plants, Mendel stumbled upon the secret of inheritence

-- thus, Mendel is considered the father of modern genetics


About Pea Plants

Pea plants are special because they can self-pollinate

-- in other words, the pollen (male sex cells formed in plant meiosis) and the ovary (female sex cells formed in plant meiosis) from the same plant join together to form plant offspring

-- therefore, all of the offspring of the plant have the same DNA and therefore the same genes and traits as the parent

-- pea plants that self-pollinate are called true breeding

Pea plants can also pollinate ovaries of other pea plants through a process called cross pollination

-- pea plants that cross-pollinate are called cross breeding hybrids (or hybrids)


What Mendel Did With Peas

Mendel looked at 7 specific traits of his pea plants. Each trait came in two possible varieties:

-- Seed Shape (either round or wrinkled)

-- Seed Color (either yellow or green)

-- Seed Coat Color (either gray or white)

-- Pod Shape (either smooth or constricted)

-- Pod Color (either yellow or green)

-- Flower Position (either axial or terminal)

-- Plant height (either tall or short)


What Mendel Did (cont’d)

Mendel took his true-breeding pea plants and did what is called selective cross-breeding

-- in other words, he intentionally pollinated the flowers of a plant with one variety of a trait with the pollen of a plant with the other variety for a trait

-- to ensure that the pea plants didn’t self-pollinate, he removed the stamens (pollen producers) from the plant receiving the pollen

What was Mendel’s hypothesis?

Mendel believe that the offspring would show traits that were a combination of the traits of the parents.


Mendel’s Results

The results of Mendel’s experiment were quite surprising!

Each of the offspring (F1 generation) had only one variety of the trait shown in the parent generation (P generation)

For example, when looking at seed shape, in a cross between a round seed parent plant and a smooth seed parent plant, all of the F1 plants had round seeds!

Mendel called the variety of the trait that appeared in the F1 generation dominant


The Results of Mendel’s Experiment

Parent Generation

F1 Generation

Gray Seed Coat Only

All axial flowers

Seed Coat Color White Gray

All Yellow Seeds

All Round Seeds

All Smooth Seeds

All Green Pods

All Tall Plants


Mendel’s Second Cross

If the results of Mendel’s first experiment were surprising, the results of his second experiment were even more surprising. . .

For his 2nd experiment, Mendel let the plants of the F1 generation self-pollinate to create an F2 generation

Mendel’s Hypothesis:

Mendel assumed that all of the plants of the F2 generation would have the exact same traits as the F1 parent

The Result:

Most of the F2 plants had the same variety of the trait as the parent, however, about ¼ of the F2 plants showed the other variety of the trait, that was missing in the F1 generation, but present in the parent generation


Tall vs. Short Plants – An Example of Mendel’s Crosses

P (parent) generation

Tall x Short

F1 (1st filial) generation:

All tall plants

F2 (2nd filial) generation: ¾ tall; ¼ short


In Terms of Modern Genetics. . .What Really Happened?

In modern genetics, we call all the varieties of a trait alleles

Mendel happened to test plants with two different alleles

-- for example, for the trait of plant height, there were two alleles, T, for tall plants, and t, for short plants

***we give both alleles for the same trait the same letter. The dominant allele is capitalized and the recessive allele is lower case***

What you need to remember is that cells that express traits are diploid in chromosome number. . .and that each chromosome in a pair contains genes for the same trait.


Why is Diploid Important?

Even though both homologous chromosomes contain genes for the same trait, each chromosome may have a different allele for the trait

Let’s look at the cross of tall and short plants. . .

In the parent generation, each plant was true breeding, meaning both copies of the chromosome coding for plant height had the same allele for the trait of plant height:

-- the tall plant had two tall alleles, T and T

-- the short plant had two short alleles, t and t

When the plants were cross pollinated, the offspring got one chromosome, and therefore one allele from each parent


The F1 Generation of Pea Plants

Since all of the F1 generation got one allele from each parent, each plant in the F1 generation got one tall allele, T, and one short allele, t

-- all the plants were Tt, however, since T (tall) is dominant to t (short), all the plants appeared tall

When the F1 plants were allowed to self-pollinate, however, the cross could have gone in several different ways:

-- the pollen either gave a T allele or a t allele

-- the ovary either gave a T allele or a t allele

*** Remember that through meiosis, the sex cell contains only one of the two chromosomes, which one it contains is completely random***


The F2 Generation

The F2 generation could have several possible combinations of alleles from the Tt x Tt cross. . .

If the pollen gaveIf the ovary gaveThe F2 plant would be

T T TT (tall)

T t Tt (tall)

t T tT (tall)

t t tt (short)

Therefore, ¾ of the offspring should be tall and ¼ of the offspring should be short. Exactly what Mendel figured out!


Genetics Vocabulary

Trait – a characteristic expressed by an organism

-- in other words, your height is a trait

Allele – the variety of gene that you have that codes for a trait

-- pea plants can either be tall or short, tall and short are alleles for the trait of height

Phenotype – the actual trait that is expressed, what we see

-- we either see the pea plant to be tall or short. Tall or short is the phenotype of the pea plant

Genotype – the combination of alleles that an organism has for a trait

-- for example, pea plants can either be TT, Tt, or tt


More Genetics Vocabulary

Dominant – the allele that is expressed when both alleles are present

-- in peas, tall is dominant to short

Recessive – the allele that is not expressed when both alleles are present

-- in peas, short is recessive

Homozygous – when an organism has two of the same alleles for a trait

-- if a pea plant is TT, it is homozygous dominant

-- if a pea plant is tt, it is homozygous recessive

Heterozygous – when an organism has two different alleles for the same trait

-- all Tt pea plants are heterozygous


Predicting the Offspring

Now that we know how alleles are passed onto the next generation, we can predict what future generations will look like

For example, when a pea plant with yellow seeds is crossed with a pea plant with green seeds, what will the offspring look like?

-- as long as we know the genotypes of the parents, and which trait is dominant and which is recessive, we can figure out the genotypes and phenotypes of offspring. . .

-- we use a tool called a Punnett Square to figure out the traits of the offspring


The Punnett Square

A Punnett Square is a box made of four squares (at least simple ones have four squares).

The alleles of one parent are written across the top and the alleles of the other parent are written across the side, so that one allele sits across each row and above each column.

The combination of alleles that result are the possible results for the offspring of that cross

Female Parent’s alleles

Male Parent’s alleles


Punnett Square Practice Problems

In pea plants, the allele for tall plants is dominant to the allele for short plants. If a homozygous dominant parent is crossed with a heterozygous pea plant, what would be the genotypic and phenotypic ratios of the offspring?

Genotypic Ratio:

Phenotypic Ratio:


Punnett Square Practice Problems

In pea plants, the allele for yellow seeds is dominant to the allele for green seeds. If a heterozygous yellow pea plant is crossed with a green pea plant, what are the genotypic and phenotypic ratios of the offspring?

Genotypic Ratio:

Phenotypic Ratio: