Understanding heredity
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Understanding Heredity. Part 1. 1. The work of gregor mendel. 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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Understanding Heredity

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Understanding Heredity

Part 1

1


The work of gregormendel


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


The Work of GregorMendel, 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”


The Work of Gregor Mendel, cont.

  • 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


The Work of Gregor Mendel, cont.

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


The Work of Gregor Mendel, cont.

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

  • Seed shape

  • Seed color

  • Pod shape

  • Pod color

  • Plant height

  • Flower color

  • Flower position


The Work of Gregor Mendel, cont.

Why pea plants?

  • Pea plants are pure breeding – they produce identical offspring when they self-pollinate

  • They grow fast

  • They have traits in distinct alternate forms (either/or)


The Work of Gregor Mendel, cont.

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

  • The resulting offspring are called a monohybrid cross


The Work of Gregor Mendel, cont.

How did he make these monohybrid crosses?

  • Mendel prevented self-pollination in the plants by removing the stamen

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

  • The result: cross-breed plants


The Work of Gregor Mendel, cont.

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


The Work of Gregor Mendel, cont.

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

  • The traits from the other parent had disappeared!


The Work of Gregor Mendel, cont.

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)


The Work of Gregor Mendel, cont.

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

  • Traits are passed from one generation to the next

  • Each trait is found in at least 2 contrasting forms


The Work of Gregor Mendel, cont.

  • He further concluded that:

  • Traits are inherited as distinct units from the parent

  • Organisms inherit 2 copies of each unit (one per parent)

  • Organisms donate one of those copies when they make gametes

  • The 2 copies separate (segregate) during gamete formation

  • These conclusions became know as the Law of Segregation


Modern genetics


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


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

    OR

    Heterozygous – having 2 different alleles


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:

  • The Genotype

  • Environmental pressures


Probability and Punnett Squares


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.


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…


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)


PunnettSquares, cont.


Punnett Squares, cont.

  • Monohybrid cross - cross involving a single trait

    ex. flower color

  • Dihybrid cross - cross involving two traits

    ex. flower color & plant height


Non-mendelian genetics

Beyond Dominant & Recessive Alleles


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!


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


Beyond Dominant & Recessive Alleles, cont.

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


Beyond Dominant & Recessive Alleles, cont.

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


Beyond Dominant & Recessive Alleles, cont.

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


Beyond Dominant & Recessive Alleles, cont.

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

Examples: hair, eye and skin color


Beyond Dominant & Recessive Alleles, cont.

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


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