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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

• 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.

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

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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 co-

dominant 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):– 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-time-again.

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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.