Genetic Crosses and Probability

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Genetic Crosses and Probability

• Chapter 9
• Sec. 9.2

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Genotype and Phenotype

• Genotype- the genetic makeup of an organism
  (Represented by alleles, capital letters for
  dominant and 2 lowercase letters for recessive)
  Ex. PP, Pp, or pp
• Phenotype- the observable, physical appearance of
  an organism (results from the genotype)
• Ex. The phenotype of a PP or Pp is Purple
• The phenotype of a pp is white

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Genetics Vocabulary continued

• Homozygous- when both alleles of a pair are the
  same for a gene
• Homozygouse Dominant- PP
• Homozygous Recessive- pp
• Heterozygous- when the 2 alleles in the pair are
  different for a gene
• Heterozygous for flower color Pp

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What is Probability?

• Probability is the likelihood that a particular
  event will occur.
• It does not ALWAYS happen (think Deal or No Deal)
• Can be written as a decimal, percentage, ratio,
  or fraction
• How do we use the principles of probability
  in our daily lives?
• Ex.
• Horse racing
• NCAA March Madness
• Superbowl Coin Toss
• Atlantic City

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Determining probability

• Number of times the event is expected
• Number of times it could have happened
• What is the probability that you picked an odd
  number when picking from 1-10? There are five odd
  numbers between 1 and 10. The probability is
  55.
• Or you can express it as a fraction 5/10. Since
  it's a fraction, why not reduce it? The
  probability that you will pick an odd number is
  1/2.
• Probability can also be expressed as a
  percent...1/250 Or as a decimal...1/250.5

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The Principles of Probability

• The Principles of probability can be used to
  predict the outcomes of genetic crosses
• Alleles segregate by complete randomness
• Similar to a coin flip!

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What is a Punnett Square?

• A punnett square is a diagram showing the gene
  combinations that might result from a genetic
  cross
• The letters in punnett squares represent alleles.
  
• A capital letter is used to show the dominant
  alleles.
• A lower case letter is used to show the recessive
  alleles.
• Monohybrid cross- a cross between individuals
  that involve one pair of traits

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How do you fill in a Punnett Square?

• Use the genotype to determine the gametes
  formed. Ex. a Gg parent will have 2 gametes
  because of segregation G or g
• Parent 1 gametes on the top, parent 2 gametes on
  the side
• If both parents are heterozygous for green color

Parent 1 Parent 1
G g
G g
G g
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How do you fill in a Punnett Square?
Parent 1 Parent 1
G g
Parent 2 G G g
Parent 2 g G g
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How do you fill in a Punnett Square?
Parent 1 Parent 1
G g
Parent 2 G GG Gg
Parent 2 g Gg gg
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How do you fill in a Punnett Square?
Parent 1 Parent 1
G g
Parent 2 G GG Gg
Parent 2 g Gg gg
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What do the alleles mean?

• Homozygous Dominant
• Homozygous recessive
• Heterozygous Dominant

• ? GG
• ? gg
• ? Gg

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To find the ratio of offspring in a punnett
square

• Count the number of different offspring
• For a genotypic ratio count the different
  genotypes
• For a phenotypic ratio count the different
  phenotypes

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Genotypic ratio
Parent 1 Parent 1
G g
Parent 2 G GG Gg
Parent 2 g Gg gg

• 1 GG
• 2 Gg
• 1 gg
• So the genotypic ratio is 12 1

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Phenotypic ratio

• 3 green plants
• 1 yellow plant
• So the ratio is 31

Parent 1 Parent 1
G g
Parent 2 G GG Gg
Parent 2 g Gg gg
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Ex 1 Homozygous x HomozygousLets cross a pure
bred purple colored flower and a white flower.
Purple is a dominant color and white is a
recessive color.

• Cross PP x pp P P
• (purple x white)
• p
• p

Pp Pp
Pp Pp
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Ex 2 Homozygous x HeterozygousA guinea pig who
is homozygous dominant for black fur crosses with
a guinea pig who is heterozygous for this trait
(black fur)

• Cross BB x Bb B B
• B
• b

BB BB
Bb Bb
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Ex 3 Heterozygous x HeterozygousIn rabbits, the
allele for black coat color is dominant (B) over
the allele for brown coat color (b). Cross 2
rabbits that are heterozygous for coat color
B b

• Cross Bb x Bb
• B
• b

BB Bb
Bb bb
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Design your own peas.
An interactive Pea Breading experiment http//www
.sonic.net/nbs/projects/anthro201/exper/ Choose
two plants to breed and see the resulting
possibilities for the next generation
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Independent Assortment

• Does the segregation of one pair of alleles
  affect the segregation of another pair of
  alleles?
• Does the gene that determines round or wrinkled
  peas have anything to do with pea seed color?
• NO!
• Genes for different traits can separate
  independently during the formation of gametes
• So one pair of alleles do not influence each
  others inheritance.

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Complete Dominance

• In Mendelian traits if you are homozygous
  dominant OR heterozygous you will show the
  dominant trait
• This is called having complete dominance
• Example, Rolling your tongue
• RR and Rr can roll their tongue
• rr cannot roll their tongue

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Exceptions to the dominance rules

• Incomplete dominance
• Codominance

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Incomplete Dominance- The phenotypes MIX when the
genotype is heterozygous. Two or more alleles
contribute to phenotype, resulting in a phenotype
in the middle between the dominant and recessive
trait.
R R

• Neither allele is completely dominant over the.
• Ex. Four oclock flowers
• RRRED (when all red flowers cross?RED)
• rr WHITE (when all white flowers cross ?WHITE)
  or rr
• When a RED crosses with a white we get ? PINK or
  Rr
• Which allele is dominant? NONE!
• Incomplete dominance is like mixing paint, you
  get a blend of the 2 colors

Rr Rr
Rr Rr
r r
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Codominance- when both phenotypes/alleles for a
gene are shown in heterozygous offspring

• Both alleles of the gene contribute to the
  phenotype of the organism.
• Neither allele is dominant or recessive
• Codominance is like mixing glitter together, you
  can still see the individual glitter colors
• Example Horses
• The genes for white and red coat color is called
  ROAN
• Homozygous Dominant RRred coat
• Homozygous Recessive RRwhite coat
• Heterozygous RR roan
  (both red and white
  hairs can be seen)

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Dihybrid Crosses

• These are done when we want to determine the
  inheritance of 2 traits at the same time. For
  example, pea color and pea texture
• If the parents genotypes are heterozygous for
  both traits (use Y for color and R for texture)
• Parents genotype is YyRr
• Note Remember to use 1 letter for EACH trait

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The heterozygous parents will give off gametes

• YyRr parents will give off 4 different gametes
  because of independent assortment and segregation
• YR Yr yR yr

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Dihybrid Crosses

• Cross RrYy X RrYy
• Dihybrid Cross a cross involving 2 different
  traits
• Pea Shape
• Round (R) Dominant
• Wrinkled (r) recessive
• Pea Color
• Yellow (Y) Dominant
• Green (y) Recessive

RY Ry rY ry
RRYY RRYY RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
RY Ry rY ry
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So when we cross 2 heterozygous parents
you get the same ratio all the time 9331