Section 6.3: Mendel and Heredity

Mendel and Heredity
Section 6.3
• SWBAT describe the patterns of inheritance
that Mendel’s data revealed.
• SWBAT summarize Mendel’s law of
• SWBAT explain genetic variance.
Law of Segregation
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.
• 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.
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
– 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).
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.
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.
• 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.
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.
Mendel’s Experimental Design
• Mendel chose pea plants because they reproduce
quickly and he could easily control how they
– 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.
Mendel’s Experimental Design
By starting with purebreds, Mendel knew that any variations in
offspring resulted from his experiments.
Mendel’s Experimental Design
• Mendel chose to follow seven different either-or
traits (no intermediate features):
Flower color
Pea shape
Pea color
Pod shape
Pod color
Flower position
Plant height
• Each trait assorted independently.
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.
• Mendel did not cross only two plants – he
crossed many plants and thus observed
• 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
– This ratio kept coming up time-after-time!
Mendel saw a 3:1 ratio occur time-and-timeagain.
• 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
• The last two conclusions are called the
law of segregation.
This should remind you
of meiosis!
Allele – is any of the alternative
forms of a gene that may occur
at a specific locus.