Mendels Principles of Heredity

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Mendels Principles of Heredity
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Homework Problem Set 1

• 2-1
• 2-3
• 2-4
• 2-9
• 2-10
• 2-13

• 2-14
• 2-17
• 2-18
• 2-20
• 2-23
• 2-25

Due Date Monday, Jan. 19, 2004
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Key Experimental Features

• The garden pea
• Self-fertile but also easy to cross
• Large number of individuals in small space
• Relatively short life cycle
• Examined discrete traits
• Purple White, Yellow Green

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Key Experimental Features

• Pure-Breeding Lines
• Used to produce hybrids by crossing
• Large numbers of plants
• Ensured better quantitative analysis

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              Mendelian Genetics Results from
Mendel's Experiments
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Terms and Results Found in the Table

• 1. Phenotype literally means "the form that is
  shown" it is the outward, physical appearance of
  a particular trait
• Mendel's pea plants exhibited the following
  phenotypes
• round or wrinkled seed phenotype
• yellow or green seed phenotype
• red or white flower phenotype
• tall or dwarf plant phenotype

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Terms and Results Found in the Table

• 2. What is seen in the F1 generation? We always
  see only one of the two parental phenotypes in
  this generation. But the F1 possesses the
  information needed to produce both parental
  phenotypes in the following generation. The
  phenotype of the F1 is dominant to the trait that
  disappears in the F1.

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Terms and Results Found in the Table

• 3. The F2 generation always produced a 31 ratio
  where the dominant trait is present three times
  as often as the recessive trait.

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Terms and Results Found in the Table

• 4. Mendel coined two new terms.
• Dominant - the allele that expresses itself at
  the expense of an alternate allele the phenotype
  that is expressed in the F1 generation from the
  cross of two pure lines
• Recessive - an allele whose expression is
  suppressed in the presence of a dominant allele
  the phenotype that disappears in the F1
  generation from the cross of two pure lines and
  reappears in the F2 generation

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Mendel's Conclusions

• 1. The hereditary determinants are of a
  particulate nature. These determinants were later
  called genes.
• 2. Each parent has two alleles in each cell for
  each trait studied. The F1 from a cross of two
  pure lines contains one allele for the dominant
  phenotype and one for the recessive phenotype.
  These two alleles comprise the allelic pair.
• 3. One member of the allelic pair segregates into
  a gamete, thus each gamete only carries one
  member of the gene pair.
• 4. Gametes unite at random and irrespective of
  the other allele involved.

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Other Important Genetic Terms

• Allele - one alternative form of a given gene
  pair tall and dwarf are the alleles for the
  height of a pea plant more than two alleles can
  exist for any specific gene, but only two of them
  will be found within any individual
• Allelic pair - a combination of two alleles
• Genotype - the specific allelic combination for a
  certain gene or set of genes

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Other Important Genetic Terms

• Homozygote - an individual which contains only
  one allele at the allelic pair for example DD is
  homozygous dominant and dd is homozygous
  recessive pure lines are homozygous for the gene
  of interest
• Heterozygote - an individual that contains two
  different alleles at the allelic pair for
  example the Dd heterozygote

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Mendelian Genetics Symbolic representation of
the cross between tall and short pea plants
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The following Punnett square shows the F2
generation created by selfing the F1 plants.
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Mendelian Genetics The Punnett Square allows us
to visualize specific genetic ratios.
Mendel's Law of Segregation two alleles for each
trait separate during gamete formation and then
unite at random at fertilization
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Mendelian Genetics F3 F4 confirmation of
Mendel's first law Mendel confirmed his
hypothesis by selfing the F2 plants. If his law
was correct he could predict the results.
F1 Yy Self F2 YY Yy Yy yy Self F3 YY
YY Yy Yy yy YY Yy Yy
yy yy Self F4 YY YY Yy Yy yy YY Yy
Yy yy yy
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Monohybrid cross - a cross between parents that
differ at a single gene pair (usually AA x aa)
Monohybrid - the offspring of two parents that
are homozygous for alternate alleles of a gene
pair The monohybrids are then selfed or allowed
to intermate. Monohybrids are good for
describing the relationship between alleles. It
is the phenotype of the heterozygote which
permits us to determine the relationship of the
alleles.
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Testcross - the cross of any individual to a
homozygous recessive parent used to determine if
the individual is homozygous dominant or
heterozygous Cross A YY X yy (all
Yellow) Cross B Yy X yy (11 yellowgreen)
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Rules of Probability

• Product Rule 2 or more independent events
  product of the probabilities
• Gg X Gg
• Probability of GG progeny ¼
• ½ X ½ ¼

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Rules of Probability

• Sum Rule Probability of either of 2 mutually
  exclusive events occurring sum of the
  probabilities
• Gg X Gg
• Probability of Gg progeny 2/4 or 1/2
• ¼ ¼ 2/4
• G comes from either parent and
• g comes from either parent

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Dihybrid Crosses

• Two pairs of alleles
• Reveal the Law of Independent Assortment
• Each pair of alleles assorts independently during
  gametic formation.

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Mendel's Law of Independent Assortment Dihybrid
cross a cross between two parents that differ by
two pairs of alleles (AABB x aabb) Dihybrid an
individual heterozygous for two pairs of alleles
(AaBb)
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Mendel's Law of Independent Assortment (cont.)
Gene Symbols Seed Color Yellow Y Green y
Seed Shape Round R Wrinkled r Let's now
look at a Punnett square describing the cross
using our gene symbols.
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Law of Independent Assortment

• During Gamete Formation, different pairs of
  alleles segregate independently

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Probability of Phenotypes

• Yellow round ¾ X ¾ 9/16
• Yellow wrinkled ¾ X ¼ 3/16
• Green round ¾ X ¼ 3/16
• Green wrinkled ¼ X ¼ 1/16

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Still Obeys 1st Law

• Ratio of yellow to green 124 or 31
• Ratio of round to wrinkled 124 or 31

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Testcross confirmation of Mendel's 2nd law
Example of the yellow, round seeded F1.
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Punnett Square For Testcross
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• The phenotypic ratio of the test cross is
• 1 Yellow, Round Seed
• 1 Yellow, Wrinkled Seed
• 1 Green, Round Seed
• 1 Green, Wrinkled Seed

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Testcross confirmation of Mendel's 2nd law
Other examples of the yellow, round seeded F1.

• Testcross 1 Y_R_ X yyrr
• Results all yellow, round seed
• What is the genotype of tested individual?

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Testcross confirmation of Mendel's 2nd law
Other examples of the yellow, round seeded F1.

• Testcross 1 Y_R_ X yyrr
• Results 1 yellow, round 1 yellow, wrinkled
• What is the genotype of tested individual?

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Multihybrid Crosses

• Aa Bb Cc Dd X Aa Bb Cc Dd
• What proportion of progeny AA bb Cc Dd ?

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Aa Bb Cc Dd X Aa Bb Cc Dd

• 2 different alleles per gene
• 4 genes
• So 24 16 different gametes from each
• Punnett Square 16 X 16 or 256 boxes

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AA bb Cc Dd Probability of above genotype?

• Remember monohybrid cross 121 genotype
• 1/4 2/4 1/4
• Next multiply probability of each random event
• 1/4 X 1/4 X 2/4 X 2/4 4/256 1/64

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Aa Bb Cc Dd X Aa Bb Cc Dd Probability of
Phenotype? A_ bb C_ D_

• Remember monohybrid cross 121 genotype
• 1/4 2/4 1/4
• Next use sum rule to add probabilities of each
  gene
• AA or Aa 1/4 2/4 3/4
• bb 1/4
• CC or Cc 1/4 2/4 3/4
• DD or Dd 1/4 2/4 3/4

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3/4 X 1/4 X 3/4 X 3/4 27/256 Prediction
A_ bb C_ D_ occurs 27 out of 256 times.
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Probability of Each?
Aa Bb cc dd? A_ B_ cc dd?