Who is Gregor Mendel?

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Who is Gregor Mendel?

• Introduction to Mendelian genetics

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TRUE OR FALSE?

1. Girls inherit more traits from their mother than
   their father
2. You have inherited traits that are not apparent
3. Color blindness is more common in males than
   females
4. Identical twins are ALWAYS the same sex
5. A person can transmit genetic traits to their
   offspring which they themselves DO NOT show
6. The father determines the sex of a child
7. The total number of male births exceeds female
   births each year
8. Acquired characteristics, like mathematical
   skills, can be inherited
9. Fraternal twins are more closely relates to each
   other than to other siblings

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Answers

1. False
2. True
3. True
4. True
5. True
6. True
7. True
8. False
9. False

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Genetics

• The field of Biology devoted to understanding how
  characteristics are passed from parents to
  offspring

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Gregor Mendel

• Born in 1822 entered monastery at age 21
• Carried out experiments in the garden using pea
  plants.
• Cross pollinated pea plants with different
  characteristics examples
• seed shape
• seed color
• flower color
• plant height

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Gregor Mendel

• In the 19thcentury, Mendel studied heredity-which
  is the transmission of characteristics from
  parent to offspring
• Mendel is most famous for studying pea plants
• He studied what he called factors in pea plants
• Factors would be things like tall or short
  (height), or yellow or green (pod color)

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Gregor Mendel

• He decided to study pea plants because
• They are easy to study.
• They have many traits that exist in only in two
  forms.
• They produce a large number of offspring in one
  generation.

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Some of Mendels Factors
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Gregor Mendel

• First, Mendel grew true-breeding plants
• According to Mendel, true-breeding plants are
  plants that will always produce offspring with
  the same traits
• So a true-bred pea plant with purple flowers will
  only produce plants with purple flowers because
  it only has the factors for purple (not white).

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Gregor Mendel P generation

• Mendel bred two opposite true-breeding plants
• For example, he bred a true-breeding purple
  flower pea plant and a true-breeding white flower
  pea plant
• He called this his P generation parent
  generation

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Gregor MendelF1 generation

• All of the offspring of the P generation (which
  he called the F1 generation) turned out purple
• Mendel called purple flower color the dominant
  factor
• He hypothesized that when the dominant factor was
  present, the recessive factor(white color) did
  not show.

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Gregor MendelF2 generation

• Next, Mendel crossed the offspring from the F1
  generation (he called this the F2 generation)
• He observed that about 75 of the flowers were
  purple and about 25 were white
• This is equal to about a 31 ratio

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Mendel
P Generation(true-breeding parents)
Purple flowers
White flowers
F1 Generation(hybrids)
All plants had purple flowers
F2 Generation
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Mendels Real Results
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Genes

• Genes are the factors that control traits.
• Genes are made of DNA they are part of the
  chromosomes.

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Mendels Laws

• Keep in mind that Mendel knew nothing of Punnett
  squares, genes, alleles, or even DNA!!!
• All he could do was observe phenotypes and record
  ratios and other statistics
• He came up with 2 important laws as a result of
  his observations.

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Mendels LawsLaw of Segregation

• Mendel concluded that each plant gets two factors
  (alleles) for a characteristic and when the plant
  reproduces, these two factors separate or
  segregate. So
• Each gamete (sex cell) gives one factor (allele)
  AND therefore
• Each offspring gets one factor from each parent

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Law of SegregationPunnett Squares
Alleles separate
Alleles separate
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Mendels Laws Law of Independent Assortment

• Mendel did experiments using more than one trait
  (like height and seed color)
• He noticed that one trait did not influence the
  inheritance of another trait
• In other words, different factors separate
  independently of each other during the formation
  of gametes

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Mendels Laws Law of Independent Assortment

• Examples
• Pea plants can be short or tall
• Their seeds can be green or yellow
• Short plants can have green or yellow seeds
• Tall plants can have green or yellow seeds
• So the inheritance of one does not affect the
  inheritance of the other.
• Mendel noticed this with all the traits he
  studied

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Independent Assortment
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Mendels Laws

• Independent Assortment is not always true-
• If different genes are located on the same
  chromosome, then they will most likely be
  inherited together
• These are called Linked Genes

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What were Mendels factors in reality?

• We call these alleles today
• Alleles are alternative forms of a gene
• Alleles for flower color were purple and white
• The characteristics (like height) are caused by
  genes on DNA
• Genes are segments of DNA that code for one
  protein
• Each gene has 2 alleles, or versions (1 from mom
  and one from dad)

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What were Mendels factors in reality?

• The reason alleles come in pairs is because
  chromosomes come in pairs (homologous pairs)!!
• One allele on each chromosome!
• WHAT A COINCIDENCE!!!

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The Genetics of Mendels Experiments

• Some Vocab
• Dominant trait-masks the recessive
• Shown with capital letters
• Recessive trait-only shows if dominant is not
  present
• Shown with lower case letters
• Phenotype-physical appearance
• For example purple, wrinkled, tall, etc

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The Genetics of Mendels Experiments

• Some Vocab
• Genotype-genetic makeup
• This is usually abbreviated with letters like Gg,
  FF, or hh
• Genotypes for a trait are usually2 letters
  because you get 2 alleles (1 from mom and 1 from
  dad)
• Homozygous-two of the same alleles (like HH or
  hh)
• Heterozygous-two different alleles (like Hh)

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The Genetics of Mendels Experiments

• More vocab
• When an individual is heterozygous (1 dominant
  and 1 recessive trait)
• They are often called carriers because they
  carry the recessive gene even though they dont
  express the phenotype

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The Genetics of Mendels Experiments

• Mendels P generation had the genotypes FF (for
  purple) and ff (for white)
• True breeding is also homozygous
• FF is homozygous dominant
• ff is homozygous recessive

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The Genetics of Mendels Experiments

• Diploid having 2 copies of each gene, one from
  each parent.
• Gametes (sperm and egg, or pollen and ovule) are
  haploid only 1 copy of each gene.
• The zygote, the first cell of the offspring, is
  diploid because the sperm fertilizes the egg

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The Genetics of Mendels Experiments

• Probability is the likelihood that a particular
  event will occur.
• Shown as a percentage.
• Example the probability of getting a heads when
  you flip a coin is
• ½, or 50
• What is the probability of getting a 2 when you
  roll a die?
• 1 way to get it out of 6 possibilities
• 1/6 or 17

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The Genetics of Mendels Experiments

• We can show the results Mendel observed using a
  Punnett Square
• A Punnett Square shows possible genetic
  combinations in the zygotes
• Mendel crossed his true breeding purple and white
  flower pea plants
• We write this as FF x ff
• LETS DO THIS ON THE BOARD

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The Genetics of Mendels Experiments

• What Mendel did not know
• All of F1 pea plant flowers heterozygous (two
  different alleles), or Ff
• That is why they were all purple
• Remember dominant alleles mask recessive alleles
• So with one purple allele present and one white,
  only purple would show as it is dominant

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F2 generation

• LETS EXAMINE EACH RATIO FOR EACH CROSS
• F2 Generation
• What genotypes do you start with?
• How are they crossed?
• What are your results?
• What is the genotypic ratio (genes)?
• What is the phenotypic ratio of purple (F) to
  white (f)?

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Punnett Square Examples

• Lets do a Punnett square for BB x Bb
• B black fur in bunnies
• b white fur in bunnies
• Black fur is dominant
• What is the genotypic ratio?
• What is the phenotypic ratio?
• What are the chances for a white bunny?

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Punnett Square Examples

• Lets look at a heterozygous cross
• Bb x Bb
• What is the genotypic ratio?
• What is the phenotypic ratio?
• What are the chances for a white or black bunny?

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Predicting the Results of Heredity

• What do these ratios and percents mean?
• If we flip a coin, there is a 50 chance that it
  will land on heads. But it is still possible to
  get 5 tails in a row (although it is highly
  UNLIKELY!)
• The more times you flip it, the more likely your
  results will be 5050
• If Bb and Bb bunnies mate, there is a 14 chance
  the offspring will be white (this does NOT mean
  that they will or will not have white bunnies)
• If they have LOTS of children, about 25 of them
  will be white

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REMEMBER

• Homozygous dominant means 2 BIG letters
• Heterozygous means one big one little
• Homozygous recessive means 2 little letters
• If an organism shows the dominant trait, then the
  can be either heterozygous OR homozygous dominant

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Test Cross

• When genotypes are not know, a test cross can be
  performed to figure it out
• The organism with an unknown genotype is crossed
  with a homozygous recessive individual.
• Test crosses are often used in breeding (like dog
  breeding) to determine is organisms are really
  pure bred (homozygous) for desired
  characteristics

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Test Cross

• Problem
• Lets say you want to breed black bunnies and you
  do not want any white bunnies
• What would be the only parents genotypes to
  produce black bunnies?
• BB x BB
• There are 2 ways to know for sure which black
  bunnies are homozygous and which are
  heterozygous expensive genetic testing, or test
  crosses

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Test Cross

• Solution
• We take some black bunnies and mate them with
  white bunnies (homozygous recessive)
• Lets look at the Punnett Square results to see
  the possible results
• Remember, black bunnies can be either BB or Bb

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Test Cross Punnett Squares(try each cross)

• If a BB is crossed with bb, no white bunnies are
  produced
• If a Bb is crossed with bb, then white bunnies
  may be produced
• If a test cross produces white bunnies, we know
  the unknown genotype is Bb if not the genotype
  is BB
• The cross would be performed multiple times to be
  sure of the results

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Predicting Dihybrid Crosses

• When 2 traits are being looked at
• Lets do a cross between two heterozygous tall,
  heterozygous purple flowered pea plants
• So, TtFfx TtFf
• For each plant, we now look at genotype for color
  and height

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Predicting Dihybrid Crosses

• Instead of 2 possible gametes, there will be 4
• So, the Punnett Square will be 4 x 4
• Phenotypic Ratios
• Tall, purple tall, white short, purple
  short, white
• Keep same letters together, capitals 1st
• You will not be asked for genotypic ratios for
  dihybrid crosses
• What are the phenotypic ratios?
• LETS DO IT ON THE BOARD

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Complex Inheritance

• Mendel observed monogenic traits and no linked
  genesIts not usually that simple.

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Other Types of Inheritance

• Incomplete Dominance
• The phenotype of the heterozygote is intermediate
  between phenotypes of the dominant and recessive
  traits
• Example when a homozygous red carnation is
  crossed with a homozygous white carnations, then
  pink carnations are produced
• We usually dont use lower case letters in this
  type of inheritance because nothing is really
  dominant

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Incomplete Dominance

• RR Red
• RW pink
• WW white

Lets look at the cross on the board
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Other Types of Inheritance

• Codominance
• Occurs when both alleles for a trait are
  expressed in heterozygous offspring
• Codominant alleles are often symbolized with
  different letters

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Codominance

• BB Brown
• BW Roan
• WW White
• Notice both brown and white are present in the
  heterozygous genotype

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Codominance

• LETS EXAMINE THE PUNNETT SQUARE ON THE BOARD
• Roan x Roan
• BW x BW
• What are the ratios for each phenotype?

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Other Types of Inheritance

• Multiple Alleles
• Genes with 3 or more alleles (or variations)
• Human blood type shows codominance and it has
  multiple alleles-A, B, and O

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Blood Type

• Human blood types have 3 alleles A, B, and O.
• Each person still only gets 2 alleles, but there
  are 3 possibilities
• O is recessive to A and B,
• A and B are codominant
• Genotype AO or AA A blood
• Genotype BO or BB B blood
• Genotype OO O blood
• Genotype AB AB blood (both alleles expressed)

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Blood Type
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Terminology Genotype

• Heterozygous B BO
• Heterozygous A AO
• Homozygous recessive OO
• Homozygous A AA
• HomozygousB BB
• AB(technically heterozygous) AB

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Codominance Punnett Square

• LETS EXAMINE THE PUNNETT SQUARE ON THE BOARD
• Heterozygous A with Heterozygous B
• AO x BO
• What are the ratios for each phenotype?

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EYE COLOR (MULTIPLE ALLELES)
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Other Types of Inheritance

• Sex-Linked Genes and Traits
• Remember sex chromosomes are the chromosomes that
  determine the sex of an organism
• So these are traits/genes carried on sex
  chromosomes
• These traits are symbolized using a superscript
  on the X or Y, such as Xr or XR

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Other Types of Inheritance

• Sex-Linked Genes and Traits Examples
• In fruit flies, the gene for eye color is on the
  X chromosome. Red (XR) is dominant, white (Xr) is
  recessive.
• To have white eyes, females must have the
  genotype XrXr, or in other words TWO white
  alleles
• To have white eyes, males must have the genotype
  XrY, or in other words ONE white allele
• This is why X chromosome sex-linked traits are
  more common in males

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Try the Punnett Square

• Homozygous red eyed female x white eyed male
• XRXR x XrY
• What are the ratios for each phenotype?

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Epistasis

• Epistasis One gene masks the expression of a
  different gene for a different trait.

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By the way (Agouti is like a guinea pig)

• In the previous example
• Agouti has either black or brown fur
• If they are homozygous recessive for c allele
  (albino gene), then black and brown are not
  expressed and agouti is albino

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Other Types of Inheritance

• Polygenic Inheritance
• Traits that are controlled by more than one gene
  
• Most human traits are polygenic
• Examples are height, skin color, eye color, and
  hair color

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Other Types of Inheritance

• Complex Characters
• Characters that are influenced by genetics AND
  the environment
• Skin color and height are examples

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Other Types of Inheritance

• Sex-Influenced Traits
• Traits in which males and females show different
  phenotypes even though they have the same
  genotypes
• Baldness is an example- it is dominant in men,
  but recessive in women
• The differences are mainly due to males and
  females producing different hormones (chemical
  signals)

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Other Types of Inheritance

• Single Allele Traits
• Traits where there is only one allele
• If you have the allele you have the trait-there
  is no recessive
• Huntingtons disease is an example

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Pedigrees

• Another way to show heredity.

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Pedigree is a chart or family tree that tracks
which members of a family have a particular trait.
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Pedigrees

• In pedigrees, carriers have one copy of the
  recessive allele
• So they CARRY the trait, but they do not show it
• Pedigrees can be used to make predictions about
• Future offspring
• The genotype of individuals in the pedigree

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Pedigrees

• The first pedigree tracks the widows peak, so
  the filled in shapes have a widows peak
• Widows peak is a dominant trait
• Carriers are not always shown on pedigrees
• Think about what alleles their parents can give
  them

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First generation(grandparents)
Ww
Ww
ww
ww
Second generation(parents plus aunts and uncles)
Ww
Ww
Ww
ww
ww
ww
Third generation (two sisters)
ww
WW
or
Ww
ww no widows peak WW widows peak
Dominant trait (widows peak)
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Chromosome Mutations

• Chromosome mutations involve changes in the
  structure of a chromosome or the loss or gain of
  a chromosome.
• Deletion The loss of a piece of chromosome due
  to breakage
• Inversion A chromosomal segment breaks off,
  flips around, and reattaches
• -Missence A change in chromosomal arrangement
  by insertion of DNA segment

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Translocation-A piece of chromosome breaks off
and reattached to a nonhomologous chromosome
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Chromosome Mutations

• Nondisjunction-When a chromosome fails to detach
  from its homologue during meiosis, so one gamete
  gets an extra chromosome
• Instead of a haploid number (n) or diploid (2n),
  the gamete has 3 chromosomes (3n)

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Chromosome Mutations (Examples)

• Downs syndrome
• Nondisjunction of chromosome 21 in the egg cell
  produces 3 copies of chromosome 21
• Symptoms include heart defects, stunted growth,
  mental retardation
• Cystic fibrosis
• Can be caused by several mutations on chromosome
  7 (insertion, missence)
• Symptoms include problems with respiratory and
  digestive systems

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Chromosome Mutations (Examples)

• Klinefelters syndrome
• A male receives an extra X chromosome (XXY)
  because of nondisjunction of egg cell
• Symptoms Boyish, rounded look (despite age) and
  often infertility
• Trisomy 18 (Edwards syndrome)
• Nondisjunction of chromosome 18 results in 3
  copies of chromosome 18
• Much more severe problems than Downs syndrome.
  Only 10 of births survive to their first
  birthday