Mendel

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Mendels Laws of Heredity

• Why we look the way we look...

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What is heredity?

• The passing on of characteristics (traits) from
  parents to offspring
• Genetics is the study of heredity

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Inheritance Theory Prior to Mendel

• 1. Traits blended
• Trait characteristics to be passed from parent
  to offspring
• bloodlines thought traits passed through the
  blood
• 2. Problem with blending cannot account for
  unexpected traits

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Mendel- Father of Genetics

• Personal history
• 1. Austrian monk
• 2. Teacher of high school natural science- love
  of evolution, nature, meteorology
• 3. for the fun of it crossed peas and mice-
  saw inheritance patterns
• 4. pea plants- a formal test

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Mendel used peas because

• They reproduce sexually
• They have two distinct, male and female, sex
  cells called gametes
• Their traits are distinctive and easy to isolate
• Reproduce quickly

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Mendel crossed them

• Fertilization - the uniting of male and female
  gametes
• Cross - combining gametes from parents with
  different traits

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Steps of Mendel's Experiment
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What Did Mendel Find?

• He discovered different laws and rules that
  explain factors affecting heredity.

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 Mendel studied the inheritance of one trait (for
example plant's height, color of  flowers or
color and shape of seeds). A cross in which only
one trait is studied is called a monohybrid
cross.
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Mendel first cross pollinated tall pea plants
,identified asTT (height of plants in this
variety were about six feet tall ), with each
other.
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Mendel noticed, that only tall plants were
produced. He came to a conclusion, that the
tall variety of a pea plant, must contain some
factor for tallness.
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X
He then crossed short with short and the result
was all short.
Once again he concluded that pea plant must
contain some factor for height
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The next step of Mendel's experimentwas to cross
tall pea plants (TT) withshort pea plants (tt).
The resulting plants were labeled Tt And only
tall plant were produced
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Mendels next experiment involved allowing the
tall plants from the tall short cross to self
pollinate. The result was a mixture of tall and
short plants
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Mendel named each generation Starting
generation P (parent) generation. The
following offspring generation was called F1
- first filial generation F2 - second filial
generation, and so on.
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P
F1
F2
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Mendel experimented with many traits
The results of the crosses were always the same.
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• When pure bred plants were crossed
• the offspring always had the trait of
• the parents.
• When two different alleles for a
• trait were crossed the offspring
• always showed the dominant trait.
• When the offspring of the previous
• were allowed to self pollinate the
• recessive trait showed up again.

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Rule of Unit Factors

• Each organism has two alleles for each trait one
  from each parent
• -Alleles - different forms
• of the same gene
• -Genes - located on chromosomes, they control
  how an organism develops

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Rule of Dominance
Some genes (alleles) are dominant and others are
recessive. The phenotype (trait) of a dominant
gene will be seen when it is paired with a
recessive gene.
TT Tt both result in a TALL plant, because T is
dominant over t. t is recessive. tt will result
in a short plant.
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Law of Segregation

• Each gene (allele) separates from the other so
  that the offspring get only one gene from each
  parent for a given trait.

TT x tt
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Law of Independent Assortment

• The genes for different traits are inherited
  independent of each other.

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Phenotype Genotype

• Phenotype - the way an organism looks
• red hair or brown hair
• genotype - the gene combination of an organism
• AA or Aa or aa

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Phenotype

• Phenotype
• Physical characteristics

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Genotype

• Phenotype
• Physical characteristics
• Genotype
• Genes we inherit from our parents

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Heterozygous Homozygous

• Heterozygous - if the two alleles for a trait are
  different (Aa)
• Homozygous - if the two alleles for a trait are
  the same (AA or aa)

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Questions...

• How many alleles does an organism have for each
  trait?
• What is an allele?
• How many alleles does a parent pass on to each
  offspring for each trait

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Questions...

• What do we call the trait that is observed?
• What case (upper or lower) is it written in?
• What about the one that disappears?
• What case is it written in?

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Punnett Square
Many years later a method of showing the crosses
was developed. It is referred to as the Punnett
Square and shows the probability of the offspring
having a certain trait.
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Punnett Squares

t Tt
Tt Tt
The genes from one parent go here.
The genes from the other parent go here.
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Punnett Squares
T T
t T
t T T
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Punnett Squares
T T
t t Tt
t Tt Tt
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Punnett Squares
T T
t t Tt
t Tt Tt
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Punnett Squares
T T
t Tt Tt
t Tt Tt
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Punnett Squares
T T
t Tt Tt
t Tt Tt
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Punnett Squares
T T
t Tt Tt
t Tt Tt
T
T
T
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Punnett Squares
T T
t Tt Tt
t Tt Tt
T
T
T
Offspring
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Punnett Squares
T T
t Tt Tt
t Tt Tt
F1 generation
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Interpreting the Results
The genotype for all the offspring is Tt. The
genotype ratio is Tt - 4/4 The phenotype for
all the offspring is tall. The phenotype ratio
is tall - 4/4
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Punnett Squares
T t
T ?? ??
t ?? ??
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Punnett Squares
T t
T TT Tt
t Tt tt
F2 generation
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Next, give the genotype and ratios
of the offspring (F2 generation).
T t
T TT Tt
t Tt tt
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Punnett Squares
T t
T TT Tt
t Tt tt
Genotype ratio TT 1/4
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Punnett Squares
T t
T TT Tt
t Tt tt
Genotype ratio TT - 1, Tt 2/4
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Punnett Squares
T t
T TT Tt
t Tt tt
Genotype ratio TT 1/4, Tt 2/4, tt 1/4
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Next, give the phenotype ratios of the
offspring (F2 generation).
T t
T TT Tt
t Tt tt
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Punnett Squares
T t
T TT Tt
t Tt tt
Genotype ratio TT 1/4, Tt 2/4, tt 1/4
Phenotype ratio Tall 3/4
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Punnett Squares
T t
T TT Tt
t Tt tt
Genotype ratio TT 1/4, Tt 2/4, tt 1/4
Phenotype ratio Tall 3/4, short 1/4
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This is a monohybrid cross. We worked with only
one trait. The height of the plant.
T t
T TT Tt
t Tt tt
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We crossed two pea plants which contained both
tall and short information.
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Questions...

• What is the phenotype?
• What is the genotype?
• What is homozygous?
• What is heterozygous?
• What is monohybrid crossing?

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Patterns of Inheritance

• Dominant /Recessive
• Codominance
• Incomplete Dominance
• Multiple Alleles
• Polygenic
• X-Linked
• Maternal

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Dominant/Recessive

• Trait is controlled by two alleles one of the
  alleles is dominant , the other recessive.
• Example the height of pea plants

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Codominance

• The alleles are neither dominant nor
  
• recessive both alleles are expressed
• in the offspring
• A hybrid will have a mixture of the alleles,
• not just one or the other.
• Symbols for codominant alleles are special
• Example of chicken feather color
• (FB black feathers)(FW white feathers)

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Incomplete Dominance

• The alleles for a trait blend .
• An example would be a red flower is
  crossed with a white flower and the resulting
  plant produces pink flowers.

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Multiple Alleles

• The trait is controlled by genes that have more
  than two alleles
• The organism inherits only two of the alleles
• Example Human Blood Types A, B, O
• Alleles IA, IB, and i

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Polygenic Inheritance

• More than one pair of genes determine the
  phenotype.
• Many phenotypes are possible
• Example Height, skin color

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Sex-Liked Inheritance

• Genes controlling sex of an organism (X,Y) are
  not identical in length.
• The X chromosome is longer than the Y and
  therefore contains more genes.
• Males receive only one set of those genes from
  their mother.
• If a recessive allele is received there is no
  dominant to block it.

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Maternal Inheritance

• Mitochondrial DNA is inherited from mothers
  because the egg cell has the mitochondria in it

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Genetic Disorders

• Can be caused by mutations in the genes
• Genetic disorder can result in minor or major
  health problems
• Examples Cystic Fibrosis, Huntingtons Disease,
  Sickle-cell Disease, Hemophilia, and Down Syndrome