understanding heredity part 1 1 1. the work of gregor mendel 2

UNDERSTANDING HEREDITY

Part 1

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THE WORK OF GREGOR MENDEL

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The Work of Gregor Mendel

•ALL living things have a set of characteristics that are contained in genes.

•These genes come from our parents and are found in every cell in our body.

•Genetics – the scientific study of heredity

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The Work of Gregor Mendel, cont.

•Gregor Mendel – an Austrian Monk born in 1822

•He laid the foundation for much of our understanding of inheritance patterns

•Credited as the “Father of Genetics”

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•He was a mathematician/botanist and was in charge of the monastery garden

•He noticed that individual plants of the same species were not identical


• He wanted to know why they were not identical, so he experimented on pea plants to help answer the question.

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•He selected seven traits found in pea plants to study:

1. Seed shape2. Seed color3. Pod shape4. Pod color5. Plant height6. Flower color7. Flower position

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Why pea plants?1. Pea plants are pure breeding –

they produce identical offspring when they self-pollinate

2. They grow fast3. They have traits in distinct

alternate forms (either/or)

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•These characteristics allowed Mendel to control the outcome when he cross-pollinated plants with contrasting traits

•The resulting offspring are called a monohybrid cross

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How did he make these monohybrid crosses?1. Mendel prevented self-

pollination in the plants by removing the stamen

2. He dusted the pollen from one stamen onto another plant’s pistil (cross polination)

3. The result: cross-breed plants

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What happened next?•He called the original plants the Parent (P) generation

•The offspring produced by the P generation were the F1 generation; also called hybrids

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•All of the hybrids showed the traits of only one of their parents…

•The traits from the other parent had disappeared!

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Where did those traits go?•To answer that question, Mendel let the F1 plants self-pollinate

•This produced the F2 generation:

~ ¾ of the plants showed the traits of their parents (the F1 generation)

~ ¼ of the plants showed the traits of their grandparents (the P generation)

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• This lead Mendel to make two conclusions about what he called biological inheritance (we call it genes):

1. Traits are passed from one generation to the next

2. Each trait is found in at least 2 contrasting forms

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The Work of Gregor Mendel, cont.• He further concluded that:a. Traits are inherited as distinct units from

the parentb. Organisms inherit 2 copies of each unit

(one per parent)c. Organisms donate one of those copies

when they make gametesd. The 2 copies separate (segregate) during

gamete formation• These conclusions became know as the Law of Segregation

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MODERN GENETICS

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Modern Genetics

• Some traits are dominant over other traits

• The unit that seems to disappear is recessive – it can only be expressed when 2 recessive traits combine

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Modern Genetics, cont.

• We refer to traits as genes• Genes are sections of chromosomes• Each form of the gene is called an allele

• An organism can be:Homozygous – having 2 identical alleles ORHeterozygous – having 2 different alleles

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Modern Genetics, cont.

• Phenotype – the physical characteristics of the organism (what it looks like)

• Genotype – the genetic makeup of the organism (what is actually there)

The phenotype of an organism is the result of:1. The Genotype2. Environmental pressures

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PROBABILITY AND PUNNETT SQUARES

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Probabilities• Mendel realized that the Principle of Probability (the likeliness that a particular event will occur) could be used to predict and explain the results of genetic crosses.

• If there are 2 possible outcomes, then there is a 1 in 2 or 50% chance of each outcome occurring.

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Probabilities, cont.

• Example: If you flip a coin 3 times in a row, what are the chances it will be heads up every time?

½ x ½ x ½ = 1/8

• Probabilities can predict the average outcome of a large number of events – not the outcome of an individual event.

• For that, we need Punnett Squares…

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Punnett Squares

• The gene combination that might result from a genetic cross can be predicted and compared with a Punnett Square

• The dominant allele is represented by a capital letter (like T for tall)

• The recessive allele is represented by a lower case letter for the same trait (like t for short)

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Punnett Squares, cont.

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Punnett Squares, cont.•Monohybrid cross - cross involving a single trait

ex. flower color

•Dihybrid cross - cross involving two traits

ex. flower color & plant height

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NON-MENDELIAN GENETICS

Beyond Dominant & Recessive Alleles

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Beyond Dominant & Recessive Alleles

• Principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes

• This accounts for the genetic variations among organisms of the same species!

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Beyond Dominant & Recessive Alleles, cont.

• The majority of genes have more than two alleles

• Many traits are controlled by more than one gene

• Most of the genes that affect the physical appearance of an organism are found on the autosomes

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1. Incomplete dominance – case where one allele is not completely dominant over another; produces an intermediate type

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2. Codominance – both alleles contribute to the phenotype; it is a blend of the two alleles

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3. Multiple Alleles – one individual can only have two alleles but more than two alleles can exist in a population.

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4. Polygenic traits – many traits are produced by the interaction of several genes

Examples: hair, eye and skin color

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5. The characteristics of an organism are also determined by the environment it lives in

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understanding heredity part 1 1 1. the work of gregor mendel 2

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