Download Mendel`s Experiments

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Dr. Kamal Omer,
Associate Prof. of biochemistry
and molecular biology.
Mendel’s Experiments
Gregor Mendel was a priest from the mid 19th century who conducted experiments in his garden. Mendel is
considered the “Father of Genetics!” Genetics is the scientific study of heredity.
 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.
 Mendel experimented with thousands of pea plants to understand the process of heredity: the
passing down of traits through generations.
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.
 When the egg and pollen (sperm) cells join, this is called fertilization.
 Pea plants are usually self-pollinating – meaning they can pollinate themselves, even when they are
alone.
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 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”.
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”.
The F1 Offspring:
 In one experiment, Mendel crossed pure-bred tall plants with purebred short plants.
 Scientists today call these parent plants the parental generation (P generation).
 All of offspring were Tall!
The F1 Offspring:
 The offspring generation are called “F generations.”
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 The “F” stands for filial – the Latin word for “daughter” and “son.”
 The first generations of offspring are noted by a little number 1 after the letter “F” (F1).
 The second generations (from mating the F1’s together) form the F2 generation (and so forth).
The F2 Offspring:
 When the plants in the F1 generation were full grown, Mendel allowed them to self-pollinate.
 Surprisingly, the plants in the F2 generation were a mix of tall and short plants!
 The shortness trait had reappeared!.
 Mendel counted the tall and short plants.
 About 3/4ths of the plants were tall, while only 1/4th of the plants were short.
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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 F1 generation.
 However, in the F2 generation, the “lost” form of the trait always reappeared in about 1/4th of the
plants!.
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!
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The factors must come in PAIRS!
One from the male parent and one from the female parent!
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
 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.
 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 F1 plants each inherited an allele for tall stems from the tall parent and an allele for short stems
from the short parent.
 When you have two genes that are different (not purebred) then you are called a Hybrid!
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!
 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.
 The importance of Mendel’s discovery was not recognized during his lifetime.
 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.
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