2.  Gregor Mendel was a priest from the mid 19 th century who conducted experiments with pea plants in his garden.

3.  While working in his garden, Mendel wondered why different pea plants grew tall, while others were short and why some had green seeds, and others yellow.  He called all these characteristics traits.

4.  Mendel experimented with thousands of pea plants to understand the process of heredity – the passing down of traits from parents to offspring.  Genetics is the scientific study of heredity.

5.  If you look at a flower, you can see that the petals surround two structures.  The first is the pistil. It produces eggs, the female sex cell of plants.  The second is the stamen. Stamens produce pollen, the male sex cell. It contains sperm.

7.  When the egg and pollen (sperm) cells join to produce a new organism, this is called fertilization.  Pea plants are usually self-pollinating – meaning they can pollinate themselves, even when they are alone.  Mendel developed a method to cross- pollinate – meaning he moved the pollen from one plant to the pistol of another. This is called “cross pollination” or “crossed” for short.

8.  Mendel decided to cross plants that looked completely different to see what would happen.  He started with purebred plants. Plants that he saw that generation after generation, produced plants that looked identical to the parent plant.  He called the Parent generation the “P generation” for short.

9.  In one experiment, Mendel crossed purebred tall plants with purebred short plants.  All of offspring were Tall! The short trait had disappeared!

10.  The offspring generation are called “F generations.”  The “F” stands for filial – the Latin word for “daughter” and “son.”  The first generation of offspring are noted by a little number 1 after the letter “F” (F 1 ).  The second generation (from mating the F 1 ’s together) form the F 2 generation (and so forth).

11.  When the plants in the F 1 generation were full grown, Mendel allowed them to self- pollinate.  Surprisingly, the plants in the F 2 generation were a mix of tall and short plants!  The shortness trait had reappeared!

12.  Mendel counted the tall and short plants.  About 3/4 th of the plants were tall, while only 1/4 th of the plants were short.

13.  Mendel also crossed pea plants with other contrasting traits.  In all of Mendel’s crosses, only one form of the trait appeared in the F 1 generation.  However, in the F 2 generation, the “lost” form of the trait always reappeared in about 1/4 th of the plants!

14.  Mendel reached several conclusions on the basis of his experimental results.  He reasoned that individual factors, or sets of genetic “information,” must control the inheritance of traits for peas!  The factors must come in PAIRS!  One from the male parent and one from the female parent!

15.  The last conclusion Mendel made was that one factor in the pair can mask or hide the other!  In Mendel's experiments, tallness in pea plants masked the short pea characteristic.

16.  Today, we use the word gene instead of “factor.”  Alleles are the different forms of the same gene.  Plant Height = Gene › Tall plants = allele › Short plants = allele

17.  Each pea plant inherits two alleles from its parents – one from the egg and one from the sperm.  A pea plant may inherit: › two alleles for tall stems, or › two alleles for short stems, or › one allele for tall and another for short.

18.  An organism’s traits are controlled by the alleles it inherits from its parents.  Some alleles are dominant, while other alleles are recessive.  A dominant allele is one whose traits ALWAYS shows up in the organism when that allele is present.  A recessive allele, on the other hand, is hidden by a dominant trait. It only shows up when it is ALONE.

19.  In Mendel’s cross for stem height, the purebred tall plants in the P generation had two alleles for tall stems.  The purebred short plants had two alleles for short stems.  The F1 plants each inherited an allele for tall stems from the tall parent and an allele for short stems from the short parent.

20.  When you have two genes that are different (not purebred) then you are called a hybrid Think About It: Name another thing you think about when you hear the word “hybrid.” What does it mean in your example?

21.  Geneticists use letters to represent alleles.  A dominant allele is ALWAYS represented with a capital letter › Tall plants = T  A recessive allele is ALWAYS represented by a lowercase version of THE SAME LETTER OF THE DOMINANT ALLELE › Short plants = t  This way, you always know that the “T’s” go together!

22.  When a plant inherits two dominant alleles for tall stems, its alleles are written as TT.  When a plant inherits two recessive alleles for short stems, its alleles are written as tt.  When a plant inherits one allele for tall stems and one allele for short stems, its alleles are written as Tt.

23.  Unfortunately, the importance of Mendel’s discovery was not recognized during his lifetime.  Then, in 1900, three different scientists rediscovered Mendel’s work.  These scientists quickly recognized the importance of Mendel’s ideas.  Because of his work, Mendel is often called the Father of Genetics.