Mendel and Heredity
Section 6.3
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Objectives
• SWBAT describe the patterns of inheritance
that Mendel’s data revealed.
• SWBAT summarize Mendel’s law of
segregation.
• SWBAT explain genetic variance.
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Vocabulary
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Trait
Genetics
Purebred
Cross
Law of Segregation
Allele
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Key Concept
• Gregor Mendel showed that traits are inherited
as discrete units.
– Mendel demonstrated through his research the
mechanism for inheritance of which Darwin and
Wallace were ignorant.
– Too bad Mendel was an Austrian monk whose
research was published in obscure journals (not
attracting attention).
– Darwin’s natural selection along with Mendel’s
discovery of heredity, explained how evolution occurs.
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Mendel
• Mendel and Darwin were contemporaries.
– Mendel had read Darwin’s books and accepted
the Theory of Evolution by Natural Selection.
– Darwin was unaware of Mendel.
– Mendel died in 1884 (two years after Darwin).
– His work was not rediscovered until 1900 by a trio
of botanists.
– Building on Mendel’s work, the Chromosome
Theory of Heredity was proposed in 1902.
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Evolution – Basic Forces
• Self-replication
• Heritability - a system by which
characteristics can be passed from one
generation to the next (here on Earth, DNA is
that system).
• Genetic variance
• Genetic mutation – mistakes in copying DNA
and environmental mutagens.
• Natural and artificial selection
Mendel
– In 1909 the word gene is coined.
– In 1911, scientists began working with fruit flies to
test the Chromosome Theory of Heredity.
– Beginning in 1936, biologists melded the Theory
of Evolution by Natural Selection with Mendelian
genetics to create the Neo-Darwinian Synthesis.
• The Neo-Darwinian Synthesis (also called the Modern
Synthesis or simply Darwinism) is the current
evolutionary theory (though it has undergone changes
as we have improved our understanding of genetics).
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The Groundwork for Genetics
• Mendel laid the
groundwork for Genetics.
• Identified Traits as the
distinguishing characteristics that
are inherited.
– Traits are things such as eye color, leaf
shape, tail length, running speed,
perfect pitch, etc.
• Gregor Mendel showed that traits
are inherited as discrete units.
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The Groundwork for Genetics
• In Mendel’s day the common wisdom among
scientists was that traits were blended (like
mixing “red and white paint to get pink).
– Both Darwin and Wallace tended to share this view,
though they both expressed doubts.
• Mendel’s work dispelled this common wisdom –
eventually giving birth to the science of genetics.
• Genetics is the study of biological inheritance
patterns and variation.
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Genetics
• Mendel’s great insight was the discovery that
traits are inherited as discrete units from the
parental generation.
– He showed that traits are like different colored
marbles mixed together – each marble can still be
picked out separately.
– He recognized that each organism inherits two
copies of each “discrete unit” – now called genes
– one from each parent.
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Mendel’s Experimental Design
• Mendel’s three key experimental decisions:
– He used only purebred pea plants;
– He exercised control over the breeding (making
sure the desired crosses took place);
– And his observations were of “either-or” traits
that appeared in only two alternate forms.
• He did not look at traits that might have been codominant or polygenic.
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Mendel’s Experimental Design
• Mendel chose pea plants because they reproduce
quickly and he could easily control how they
mate.
– Pea plants, if uncontrolled, typically self-pollinate.
– After allowing for self-pollination for many generation,
the lineage becomes “uniform or purebred.”
• The offspring of purebred parents inherit all of the parent
organisms’ characteristics.
• Therefore, using purebred lines, Mendel knew exactly the
specific traits of each pea plant.
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Mendel’s Experimental Design
By starting with purebreds, Mendel knew that any variations in
offspring resulted from his experiments.
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Mendel’s Experimental Design
• Mendel chose to follow seven different either-or
traits (no intermediate features):
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Flower color
Pea shape
Pea color
Pod shape
Pod color
Flower position
Plant height
• Each trait assorted independently.
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Mendel’s Results
The mating of two organisms is called a cross. P means parent
generation. F1 means first filial generation. F2 means second
filial generation.
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• Mendel did not cross only two plants – he
crossed many plants and thus observed
patterns.
• He noticed each cross yielded similar ratios in
the F2 generation.
– About three-fourths of the plants had the purple
flowers and about one-fourth had the white
flowers.
– This ratio kept coming up time-after-time!
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Mendel saw a 3:1 ratio occur time-and-timeagain.
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Conclusions
• Mendel drew three important conclusions:
– Traits are inherited as discrete units.
– Organisms inherit two copies of each gene,
one from each parent.
– The two copies segregate during gamete
formation.
• The last two conclusions are called the
law of segregation.
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Conclusions
This should remind you
of meiosis!
Allele – is any of the alternative
forms of a gene that may occur
at a specific locus.
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