Introduction to Genetics

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Introduction to Genetics

• Biology

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Gregor Mendel

• Genetics the study of patterns of inheritance
  and how traits or characteristics are passed on
  from parent to offspring
• Gregor Mendel the father of genetics. He used
  pea plants to study patterns
• of inheritance.

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Self-Pollination vs. Cross Pollination

• Mendel controlled the pollination of pea plants.
• Pea plants normally reproduce by self pollination
  (male/female parts from same plant unite)
• Mendel cross-pollinated plants by removing the
  anthers (male part of plant) and pollinating the
  stigma (female) part of another plant

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Pea Plants

• Mendel studied seven different traits in pea
  plants
• Trait a specific characteristic that varies
  from one individual to the next.
• Each trait had two contrasting characteristics
  (alleles)
• For example
• Seed Shape smooth/wrinkled
• Seed Color yellow/green
• Plant Height tall/short

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Mendels Vocabulary

• True-breeding plants that produce only like
  offspring when self-pollinated.
• Hybrid cross between two organisms of different
  traits
• P1 Generation The original pair of organisms in
  a series of crosses
• F1 Generation First generation of offspring as
  a result of P1 cross.

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Mendels Work

• Mendel crossed true-breeding short plants with
  true-breeding tall pea plants
• 100 of the offspring were tall
• Mendel crossed F1 plants (tall plants) to see if
  recessive trait reappeared
• 25 of offspring were short 75 were tall in F2
  generation

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Section 11-1
P Generation
F1 Generation
F2 Generation
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short
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Section 11-1
P Generation
F1 Generation
F2 Generation
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short
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Section 11-1
P Generation
F1 Generation
F2 Generation
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short
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Seed Shape
Flower Position
Seed Coat Color
Seed Color
Pod Color
Plant Height
Pod Shape
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
Round
Yellow
Gray
Smooth
Green
Axial
Tall
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Mendels Conclusions

• Every individual receives two factors (alleles)
  for each trait, one from mom and one from dad.
• Factors can sometimes be dominant or recessive
• Dominant one factor masks the presence of
  another
• Recessive factor that is masked by the presence
  of a dominant allele
• Law of Segregation pair of factors (alleles)
  are segregated/separated during the formation of
  gametes

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Mendels Conclusions (con.)

• Law of Independent Assortment Alleles for
  different characteristics are distributed to
  gametes independently
• Alleles/Traits are not linked to one another

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Probability

• Principle of Probability the likelihood that a
  particular event will occur
• Probability is identified as a ratio, fraction,
  or percent
• 50
• ½
• 12

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Probability

• Example Coin Flip
• 50 chance of getting heads
• Could get heads 10 consecutive flips
• Over many trials results will be near 50
• Past coin flips do not affect future coin flips

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Punnett Squares

• Diagrams that apply the principles of probability
  to predict the outcome of genetic cross
• Uppercase letters represent dominant alleles Ex.
  R
• Lowercase letters represent recessive alleles
• Ex. r

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Punnett Squares (con.)

• Homozygous organisms with two of the same
  alleles for a trait (RR or rr)
• Heterozygous organisms with two different
  alleles for a trait (Rr)
• Genotype The genetic makeup of an organism (RR,
  Rr, rr)
• Phenotype The physical characteristics of an
  organism (Round)

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How to use a Punnett Square

• Monohybrid Cross- one-trait cross
• Identify the genotype for each parent
• Write the alleles from one parent on the top and
  the alleles for the other parent on the left side
  of the square
• Fill in the table like a multiplication table,
  capital letter always goes first

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Ex. 1
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Ex. 2

• RR x rr

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Analyzing results

• Genotypic ratio-compares possible genotypes
• homozygous dominant heterozygous
  homozygous recessive
• In Ex. 1 it is 121
• In Ex. 2 it is 040

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Analyzing results (Cont)

• Phenotypic ratio-compares possible phenotypes
• with Dominant trait with recessive trait
• In Ex. 1 it is 31
• In Ex. 2 it is 40

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Analyzing results (Cont)

• Percentages- chance of something occuring
• In Ex. 1 there is a 75 chance the plant will be
  tall and a 25 chance the plant will be short.

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Patterns of Inheritance

• Complete Dominance In the heterozygous
  individual, only the dominant allele is
  expressed, the recessive allele is present but
  unexpressed.

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Ex. In pea plant green pods are dominant over
yellow pods. Crosstwo pea plant that are
heterozygous for pod color.

• What is the phenotypic ratio?
• What is the genotypic ratio?

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ANSWER

• Genotypic ratio 121
• Phenotypic ration is 31

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Exceptions to Mendels Rules Not all traits are
clearly dominant or recessive

• Incomplete Dominance One allele is not
  completely dominant over another
• Example Japanese Four OClock Plant
• RR Red Flowers
• RW Pink Flowers
• WW White Flowers

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Incomplete Dominance in Four OClock Flowers
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Incomplete Dominance in Four OClock Flowers
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Ratios

• Find genotypic ratio the same way we did for
  complete dominance
• Now there are 3 phenotypes so the ratio is
• Dom Heterozygous Rec
• Find Genotypic ratio and phenotypic ratio for the
  previous problem
• Genotypic ratio 040
• Phenotypic ratio 040

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• Codominance In a heterozygote both alleles are
  dominant and are expressed Example coat color in
  cattle
• RR Red Coat
• RW Roan Coat
• WW White Coat

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RR Red
WW White
RW Roan
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Ex. Cross 2 roan coat cows.

• Genotypic Ratio- RR RW WW
• 121
• Phenotypic Ratio- red roan white
• 121

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Dihybrid Cross

• Cross involving two traits (hair color eye
  color or pod color pod shape)
• Traits assort independently of each other
• Mendel used to discover the principle of
  independent assortment

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Dihybrid Cross
Go to Section
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How to use a Dihybrid Cross

• Identify the 2 traits
• Assign a capital letter to the dominant form of
  the first trait and the same lower case letter
  for the recessive form.
• Choose a different letter and do the same for
  the second trait.
• Identify the genotype for each parent

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• 5. Identify the gametes for each parent
• Combine
• 1st letter with 3rd letter
• 1st letter with 4th letter
• 2nd letter with 3rd letter
• 2nd letter with 4th letter
• 6. Fill in the Punnett Square
• -ABC order
• -keep like letters together
• -capital always goes first

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Ex. RrTt x RrTt
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Analyze Using Phenotypic Ratio

• 1st trait dominant, 2nd trait dominant
• 1st trait dominant, 2nd trait recessive
• 1st trait recessive, 2nd trait dominant
• 1st trait recessive, 2nd trait recessive
• In previous example, the phenotypic ratio is
  9331

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Sex-Linked Traits

• Sex-linked traits are determined by genes found
  only on the sex X chromosome
• REMEMBER
• XX-Females Xy-Males
• EX. Red-Green Colorblindess

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GENOTYPES

• Possible female Genotypes
• XBXB-normal
• XBXb-normal (carrier)
• XbXb-colorblind
• Possible male genotypes
• Xby-normal
• Xby-colorblind

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Ex. Cross a female carrier with a normal male

• What is the chance they will have a colorblind
  son?
• 50

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Multiple Alleles

• Multiple Alleles Having more than 2 alleles for
  a trait.
• Ex. Human blood type

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Human Blood Type

• 3 alleles A, B, and O
• A and B are codominant
• O is recessive

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Possible Blood Type Combinations

• GENOTYPES PHENOTYPES
• AA-homozygous dominant Type A
• BB-homozygous dominant Type B
• AO-heterozygous Type A
• BO-heterozygous Type B
• AB-codominant Type AB
• OO-homozygous recessive Type O

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EX. Cross a mother who has type O blood with a
father who has typeAB blood. What are the
possiblephenotypes?

• Answer Type A and Type B (both are heterozygous)

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Polygenic Traits

• Polygenic Traits traits can also be controlled
  by more than one gene
• Example Skin color is controlled by 3 to 6
  genes that control melanin production

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Pedigrees

• Pedigree diagram that shows how a trait is
  inherited over several generations.
• Symbols
• Circle female
• Square male
• Diamond unknown sex

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Pedigrees (cont)

• Shaded in shape person exhibits the trait
• Half shadedthe person is a carrier
• Not shadedthe person is unaffected
• Marriage line-horizontal line from male to female
  
• Child line-vertical line which extends from the
  marriage line

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Karyotypes

• Karyotype-picture of an individuals chromosomes
• -Review of chromosome structure
• a-centromere
• b-sister chromatids
• -

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• -In normal human body cells, there are 2 copies
  of each chromosome which makes 23 pairs or 46
  total chromosomes
• -Each pair of chromosomes are called homologous
  chromosomes

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• www.hhmi.org

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Karyotype (Cont)

• In a Karyotype, 1-22 pairs are called autosomes.
• The 23rd pair are called sex chromosomes.
• XX-female, Xy-male
• Gender is determined by sex chromosomes
• Use a Punnett Square for the following cross
• XX (mom) x Xy (dad)
• What is the chance for a girl? Boy?
• 50, 50

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Karyotype (cont)

• Karyotypes can be analyzed to find genetic
  disorders.
• Ex. Downs Syndrome (trisomy 21)

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• www.health.gov