Mendelian Genetics

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Chapter 14

• Mendelian Genetics

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Important Terms

• Character--something that is inherited.
• Flower color
• Trait--a variant of a character.
• Purple flower vs. white flower
• True breeding--produces only one type of
  offspring.
• No variation of traits.

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Important Terms

• Hybridization--crossing of two variants of a true
  breeding plants. The hybrid contains genes from
  both parents which likely come out in the next
  generation.

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Important Terms

• P generation--Usually true breeding and start the
  experiment.
• F1 generation--1st filial which are hybrid
  offspring of the parents.
• F2 generation--2nd filial which is offspring of
  the hybrids. This is when we start to see the
  traits reappear from the P generation.

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Mendel

• By examining the P, F1 and F2 generations, Mendel
  was able to elucidate the patterns of inheritance.

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Mendel

• What made Mendels work so good was that he kept
  excellent records of what he did and the results
  of his experiments.

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Mendel

• At the time, people believe in a blending
  hypothesis. They believed that the traits of a
  particular organism would be blended together.
• Mendels experiments abolished this notion.

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Mendel

• Mendel crossed true-breeding purple flowers and
  true-breeding white flowers and the offspring
  (F1) were all purple.
• When he crossed the F1 purple flowers, he got
  purple and white in a 31 ratio.
• He determined that purple was dominant to white.

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Mendel

• The blending hypothesis was wiped out because
  none of the flowers were pale purple.
• He also gave rise to the term heritable factor
  which we now call genes. He said heritable
  factors must somehow determine flower color.

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Mendels 4 Part Model to Explain What He Saw

• 1. Alternative versions of genes account for
  variations in inherited characteristics.

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In Todays Terminology

• Each gene resides on a specific locus on a
  specific c-some. The DNA at this locus can vary
  in its sequence of nucleotides and thus its
  information.
• The sequence of nucleotides, in this case, can
  change the flower color.
• The alleles are due to variations in the DNA.

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Mendels 4 Part Model to Explain What He Saw

• 2. For each character, an organism inherits 2
  alleles, one from each parent.
• This was a remarkable deduction from Mendel
  considering he knew nothing about c-somes or
  ploidy.

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Mendels 4 Part Model to Explain What He Saw

• 3. If 2 alleles at a locus differ, then the
  dominant allele determines the organisms
  appearance while the recessive allele gets masked
  and no noticeable change in the organisms
  appearance can be seen.

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Mendels 4 Part Model to Explain What He Saw

• 4. The 2 alleles for a heritable characteristic
  segregate during gamete formation and end up in
  different gametes. This makes up what is known
  as the Law of Segregation.

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The Law of Segregation

• In terms of chromosomes, the homologous
  chromosomes are being separated and distributed
  to different gametes during meiosis.

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Law of Segregation

• If different alleles are present, there is a
  50/50 chance that the gamete will receive a copy
  of one gene vs. another.

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Law of Segregation

• If the alleles are the same, each gamete contains
  the same copy of the gene and it is said to be
  true-breeding for a particular trait.

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The Observed 31 Ratio

• Can the segregation of gametes account for the
  31 ratio Mendel observed?
• Using a Punnett square, you find the answer is
  yes.
• Examine the genotypes and the phenotypes.

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More Useful Terms

• Homozygous--organisms with identical alleles for
  a trait in question.
• Heterozygous--organisms with different alleles
  for a trait in question.
• Phenotype--the outward appearance of an organism.
• Genotype--the genetic makeup of an organism.

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A Test Cross

• Suppose we have a purple flower and we want to
  know if it is homozygous dominant or
  heterozygous, (recessive will be white).
• To do this, cross the organism with a homozygous
  recessive and observe the offspring. If any
  white are produced, the trait is said to be
  heterozygous, and will be produced in a 11 ratio.

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The Law of Segregation

• Applies to all genes on a particular chromosome.
  Says that all genes segregate independently of
  each other regardless of what phenotype they are
  carrying so long as each gamete contains one copy
  of each trait.

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Law of Segregation

• Mendel demonstrated this using a dihybrid cross.
• He wanted to see if the gametes contained genes
  of all possible combinations or if certain genes
  went with certain other genes.

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The Cross

• Plants producing yellow colored, round seeds were
  crossed with plants producing green colored,
  wrinkled seeds.
• If they assort independently, a 9331 ratio
  should be produced.
• If they dont assort independently, if they are
  somehow linked, a different ratio will be
  observed.

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Conclusions

• From the cross, Mendel concluded that no matter
  how many characteristics are observed, they
  always segregate independently of one another.

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The Law of Independent Assortment

• As a result of Mendels breeding experiment with
  dihybrid crosses, he arrived at what is known as
  the Law of Independent Assortment which says that
  all alleles of a gene pair will segregate
  independently of other pairs during gamete
  formation.
• This law only applies to genes residing on
  different chromosomes.

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Rules Regarding Probability

• The probability scale ranges from 0 to 1, zero is
  not going to happen, 1 is its certain to happen.
• The probability of all possible outcomes is 1,
  and all outcomes of a particular event are
  independent each other--they have no bearing on
  what has happened or what will happen.

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2 Rules

• Two rules help us determine the probability of
  chance events.
• 1. The multiplication rule.
• 2. The addition rule.

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The Multiplication Rule

• To determine the probable outcome of a chance
  event, multiply the probability of each possible
  outcome.
• Coin example 1/2 1/2 1/4

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The Multiplication Rule

• Another example Suppose we roll one die
  followed by another and want to find the
  probability of rolling a 4 on the first die and
  rolling an even number on the second die.
• P(4) 1/6
• P(even) 3/6
• The probability of rolling a 4 and an even is 1/6
  3/6 3/36, or 1/12.

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The Addition Rule

• Allows us to determine the probability of any
  mutually exclusive events by adding together
  their individual probabilities.

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The Addition Rule

• For instance
• Suppose you are going to pull one card out of a
  deck.
• What is the probability of pulling a king or an
  ace?
• P(King) 4/52
• P(Ace) 4/52
• The probability of pulling a King or an Ace is
  4/52 4/52, which is 8/52, or 2/13.
• There is a 2 in 13 chance of pulling a King or an
  Ace.

Each are mutually exclusive
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The Addition Rule

• So, how does this apply to us?
• Use a monohybrid heterozygous F2 cross to
  illustrate.
• What is the possibility of getting a heterozygous
  F2 offspring?
• 1/4 1/4 1/2
• 1/2 of the offspring should be heterozygous.

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Dominance

• There are varying degrees of dominance. Some
  characters are completely dominant to others.
  For instance, purple is completely dominant to
  white round is completely dominant to wrinkled.
• When you begin looking at things, there are
  varying forms of dominance.

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Complete Dominance

• Mendels peas showed complete dominance. One
  trait was completely dominant to another (purple
  to white).

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Codominance

• Another extreme is codominance where an organism
  has 2 different alleles that affect the phenotype
  in separate, distinguishable ways. A common
  example is with cystic fibrosis.
• CF causes the patients body to produce a thick,
  sticky mucous that clogs airways and ducts
  leading from the pancreas to the intestine. This
  causes a whole host of problems.

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Cystic Fibrosis and Codominance

• The CF gene is found on the long arm of c-some 7.
• Codes for CFTR protein.

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CFTR Function

• CFTR acts as an ion gate which allows for the
  movement of Cl- in and out of the cell.
• Patients with the CF gene make a dysfunctional
  protein which keeps the gate closed causing the
  Cl- to build up. The cell then produces a thick
  mucous in response to this causing the symptoms
  of the disease.

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Codominance at the Molecular Level

• Most people have 2 normal copies of the allele
  for CFTR and make a functional CFTR protein.
• People with CF have 2 mutant copies of the allele
  and produce only dysfunctional CFTR.
• Heterozygotes produce one good copy and one bad
  copy.

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Codominance at the Molecular Level

• These heterozygotes produce enough functional
  CFTR protein to allow for normal Cl- transport
  and no adverse effects seen. Thus, even though
  the genes are codominant, symptoms remain
  recessive on the physiological level.

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Incomplete Dominance

• Some alleles exhibit incomplete
  dominance--certain characteristics fall somewhere
  in between the phenotypes of the 2 homozygotes.
• For example The flowering time of Mendels peas
  and the color of certain flowers.

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Incomplete Dominance

• Mendel knew he had peas that flowered shortly
  after germination and some that took a long time
  to flower.
• When he crossed them, he found that their
  offspring produced flowers somewhere in between
  when the two homozygotes flowering time.

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Incomplete Dominance

• With pink snapdragons, a red and a white will
  produce a pink flower--incomplete dominance.
  Why is it not blending?

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Complete Dominance, Incomplete Dominance and
Codominance--A Summary

• If you look at the organismal level (outward
  phenotype based on alleles) vs. the biochemical
  level (the way the metabolism functions) vs. the
  molecular level (the proteins/enzymes that are
  made) can play a large role in determining
  complete dominance, incomplete dominance and
  codominance.

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Multiple Alleles

• Thus far we have been talking about 2 alleles
  that govern certain traits. Often times there
  are multiple alleles that govern traits within a
  population.
• For example
• 3 alleles which code for 4 different blood types.

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Pleiotropy Multiple Phenotypes

• Most genes exhibit what is known as pleiotropy
  which is where one gene has multiple phenotypic
  effects.
• Example CF and sickle cell anemia

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Gene Masking--Epistasis

• Epistasis occurs when one gene alters the
  phenotypic expression of another gene.
• This example occurs in the coat color of mice.
• Black, brown, albino

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Polygenic Inheritance

• The opposite of pleiotrophy (one gene, many
  characteristics) is polygenic inheritance which
  is the case where many genes act on a single
  characteristic.
• For example skin color is determined by at
  least 3 separately inherited genes. Variations
  of the genotype of these individuals produces all
  of the varieties of skin color we see.

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Genetic Disorders

• Many are recessive traits.
• Easily propogated because heterozygotes dont
  display outward characteristics--they are
  carriers.
• Tay-Sachs, CF, sickle-cell

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Genetic Disorders

• Some disorders come from dominant alleles.
• Dwarfism, Huningtons Disease.
• Lethal dominants are much less common because
  they are less likely to be passed through the
  gene pool--for obvious reasons.