Classical (Mendelian) Genetics by Michael Stanley

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Classical (Mendelian) Geneticsby Michael Stanley
Gregor Mendel
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Vocabulary

• Genetics The scientific study of heredity
• Allele Alternate forms of a gene/factor.
• Genotype combination of alleles an organism has.
• Phenotype How an organism appears.
• Dominant An allele which is expressed (masks the
  other).
• Recessive An allele which is present but remains
  unexpressed (masked)
• Homozygous Both alleles for a trait are the
  same.
• Heterozygous The organism's alleles for a trait
  are different.

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History

• Principles of genetics were developed in the mid
  19th century by Gregor Mendel an Austrian Monk
• Developed these principles without ANY scientific
  equipment - only his mind.
• Experimented with pea plants, by crossing various
  strains and observing the characteristics of
  their offspring.
• Studied the following characteristics
• Pea color (Green, yellow)
• Pea shape (round, wrinkled)
• Flower color (purple, white)
• Plant height (tall, short)
• Made the following observations (example given is
  pea shape)
• When he crossed a round pea and wrinkled pea, the
  offspring (F1 gen.) always had round peas.
• When he crossed these F1 plants, however, he
  would get offspring which produced round and
  wrinkled peas in a 31 ratio.

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Laws of Inheretance

• Law of Segregation When gametes (sperm egg etc)
  are formed each gamete will receive one allele or
  the other.
• Law of independent assortment Two or more
  alleles will separate independently of each other
  when gametes are formed

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Punnett Squares

• Genetic problems can be easily solved using a
  tool called a punnett square.
• Tool for calculating genetic probabilities

A punnett square
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Monohybrid cross (cross with only 1 trait)

• Problem
• Using this is a several step process, look at the
  following example
• Tallness (T) is dominant over shortness (t) in
  pea plants. A Homozygous tall plant (TT) is
  crossed with a short plant (tt). What is the
  genotypic makeup of the offspring? The phenotypic
  makeup ?

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Punnet process

1. Determine alleles of each parent, these are given
   as TT, and tt respectively.
2. Take each possible allele of each parent,
   separate them, and place each allele either along
   the top, or along the side of the punnett square.

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Punnett process continued

• Lastly, write the letter for each allele across
  each column or down each row. The resultant mix
  is the genotype for the offspring. In this case,
  each offspring has a Tt (heterozygous tall)
  genotype, and simply a "Tall" phenotype.

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Punnett process continued

• Lets take this a step further and cross these F1
  offspring (Tt) to see what genotypes and
  phenotypes we get.
• Since each parent can contribute a T and a t to
  the offspring, the punnett square should look
  like this.

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Punnett process continued

• Here we have some more interesting results
  First we now have 3 genotypes (TT, Tt, tt) in a
  121 genotypic ratio. We now have 2 different
  phenotypes (Tall short) in a 31 Phenotypic
  ratio. This is the common outcome from such
  crosses.

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Dihybrid crosses

• Dihybrid crosses are made when phenotypes and
  genotypes composed of 2 independent alleles are
  analyzed.
• Process is very similar to monohybrid crosses.
• Example
• 2 traits are being analyzed
• Plant height (Tt) with tall being dominant to
  short,
• Flower color (Ww) with Purple flowers being
  dominant to white.

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Dihybrid cross example

• The cross with a pure-breeding (homozygous)
  Tall,Purple plant with a pure-breeding Short,
  white plant should look like this.

F1 generation
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Dihybrid cross example continued

• Take the offspring and cross them since they are
  donating alleles for 2 traits, each parent in the
  f1 generation can give 4 possible combination of
  alleles. TW, Tw, tW, or tw. The cross should
  look like this. (The mathematical foil method
  can often be used here)

F2 Generation
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Dihybrid cross example continued

• Note that there is a 9331 phenotypic ratio.
  9/16 showing both dominant traits, 3/16 3/16
  showing one of the recessive traits, and 1/16
  showing both recessive traits.
• Also note that this also indicates that these
  alleles are separating independently of each
  other. This is evidence of Mendel's Law of
  independent assortment

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Chromosomes and Classical Genetics

• Walter Sutton in 1902 proposed that chromosomes
  were the physical carriers of Mendel's alleles
• Problems arose however regarding the following
  question
• Why are the number of alleles which undergo
  independent assortment greater than the number of
  chromosomes of an organism?
• This was explained understanding of 2 additional
  factors Sex Linkage and crossing over

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Sex Linkage

• All chromosomes are homologous except on sex
  chromosomes.
• Sex chromosomes are either X or Y.
• If an organism is XX, it is a female, if XY it is
  male.
• If a recessive allele exists on the X chromosome.
  It will not have a corresponding allele on the Y
  chromosome, and will therefore always be
  expressed

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Sex linkage example

• Recessive gene for white eye color located on the
  Xw chromosome of Drosophila.
• All Males which receive this gene during
  fertilization (50) will express this.
• If a female receives the Xw chromosome. It will
  usually not be expressed since she carries an X
  chromosome with the normal gene

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Human Sex Linkage

• Hemophilia
• Disorder of the blood where clotting does not
  occur properly due to a faulty protein.
• Occurs on the X chromosome, and is recessive.
• Thus a vast majority of those affected are males.
• First known person known to carry the disorder
  was Queen Victoria of England. Thus all those
  affected are related to European royalty.

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Hemophilia and Royalty
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Other Factors Multiple Alleles

• Phenotypes are controlled by more than 1 allele.
  Eg. Blood types are regulated by 3 separate
  genes.
• ABO Blood typing
• Humans have multiple types of surface antigens on
  RBC's
• The nature of these surface proteins determines a
  person's Blood Type.
• There are 3 alleles which determine blood type
  IA, IB, or IO. This is referred to as having
  multiple alleles
• Human blood types are designated as A, B or O.
• Type A denotes having the A surface antigen, and
  is denoted by IA
• Type B denotes having the B surface antigen, and
  is denoted by IB
• Type O denotes having neither A or B surface
  antigen, and is denoted by IO
• There are several blood type combinations
  possible
• A
• B
• AB (Universal recipient)
• O (Universal donor)

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Blood Immunity

• A person can receive blood only when the donor's
  blood type does not contain any surface antigen
  the recipient does not. This is because the
  recipient has antibodies which will attack any
  foreign surface protein.
• Thus, Type AB can accept any blood types because
  it will not attack A or B surface antigens.
  However, a type AB person could only donate blood
  to another AB person. They are known as Universal
  Recipients.
• Also, Type O persons are Universal donors because
  they have NO surface antigens that recipients'
  immune systems can attack. Type O persons can
  ONLY receive blood from other type O persons.
• There is another blood type factor known as Rh.
• People are either Rh or Rh- based on a basic
  dominant/recessive mechanism.
• Not usually a problem except with pregnancy.
• It is possible that an Rh- mother can carry an
  Rh fetus and develop antibodies which will
  attack destroy the fetal blood
• This usually occurs with 2nd or 3rd pregnancies,
  and is detectable and treatable.

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Other Factors Incomplete Dominance

• Some alleles for a gene are not completely
  dominant over the others. This results in
  partially masked phenotypes which are
  intermediate to the two extremes.

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Other Factors Continuous Variation

• Many traits may have a wide range of continuous
  values. Eg. Human height can vary considerably.
  There are not just "tall" or "short" humans

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Other Factors

• Gene interaction
• Many biological pathways are governed by multiple
  enzymes, involving multiple steps. If any one of
  these steps are altered. The end product of the
  pathway may be disrupted.
• Environmental effects
• Sometimes genes will not be fully expressed owing
  to external factors. Example Human height may
  not be fully expressed if individuals experience
  poor nutrition.

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The Average American Phenotype
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Quiz

• http//anthro.palomar.edu/mendel/quizzes/mendqui1.
  htm
• Single gene traits found in humans
• Tongue rolling
• Hand clasp
• Free ear lobe
• Dimple chin
• Mid digital hair
• Widows peak
• Eye pigment
• Hitch hiker thumb
• Bent little finger