Mendels Experiments 4 1 1 Notes Gregor Mendel

Mendel’s Experiments 4. 1. 1 Notes

Gregor Mendel was a priest from the mid 19 th century who conducted experiments in his garden. Mendel is considered the “Father of Genetics!”

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

Gregor Mendel experimented with thousands of pea plants to understand the process of heredity – the passing down of traits through generations. Genetics is the scientific study of heredity.

Mendel’s Experiments 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.

Mendel’s Experiments Egg + Sperm/Pollen = fertilization. Pea plants are usually self-pollinating – meaning they can pollinate themselves, even when they are alone. Mendel developed a method to crosspollinate – meaning he moved the pollen from one plant to the pistol of another. This is called “cross pollination” or “crossed” for short.

Crossing Pea Plants 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.

The F 1 Offspring In one experiment, Mendel crossed purebred tall plants with purebred short plants. Scientists today call these parent plants the parental generation (P generation). All of offspring were Tall!

The F 1 Offspring 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).

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

The F 2 Offspring Mendel counted the tall and short plants. About 3/4 ths of the plants were tall, while only 1/4 th of the plants were short.

Experiments with Other Traits 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!

4. 1. 2 Notes: Dominant vs. Recessive

Dominant and Recessive Alleles 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 from peas! The factors must come in PAIRS! One from the male parent and one from the female parent!

Dominant and Recessive Alleles 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.

Genes and Alleles 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

Genes and Alleles 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, two alleles for short stems, or one of each.

Genes and Alleles 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 it is present! A recessive allele, on the other hand, is hidden by a dominant trait. It only shows up when it is ALONE!

Alleles in Mendel’s Crosses 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 F 1 plants each inherited an allele for tall stems from the tall parent and an allele for short stems from the short parent.

Alleles in Mendel’s Crosses When you have two genes that are different (not purebred) then you are called a Hybrid! › Name another thing you think about when you hear the word “Hybrid. ” What does it mean in your example?

Symbols for Alleles 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!

Symbols for Alleles 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.

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.