Mendel and Inheritance

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Mendel and Inheritance

• MUPGRET Workshop
• December 4, 2004

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Genetic variation

• In the beginning geneticists studied differences
  they could see in plants.
• These differences are called morphological
  differences.
• Individual variants are referred to as
  phenotypes, ex. tall vs. short plants or red vs.
  white flowers.

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Trait

• A broad term encompassing a distribution of
  phenotypic variation.
• Example
• Trait Disease resistance
• Phenotype resistant vs. susceptible
• Morphological differences associated with the
  trait might include fungal infection, fungal
  growth, sporulation, etc.

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Mendel

• Monk at the St. Thomas monastery in the Czech
  Republic.
• Performed several experiments between 1856 and
  1863 that were the basis for what we know about
  heredity today.
• Used garden peas for his research.
• Published his work in 1866.

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Mendel

• Results are remarkably accurate and some have
  said they were too good to be unbiased.
• His papers were largely ignored for more than 30
  years until other researchers appreciated its
  significance.

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Garden Pea

• Pisum sativum
• Diploid
• Differed in seed shape, seed color, flower color,
  pod shape, plant height, etc.
• Each phenotype Mendel studied was controlled by a
  single gene.

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Terms

• Wild-type is the phenotype that would normally be
  expected.
• Mutant is the phenotype that deviates from the
  norm, is unexpected but heritable.
• This definition does not imply that all mutants
  are bad in fact, many beneficial mutations have
  been selected by plant breeders.

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Advantages of plants

• Can make controlled hybrids.
• Less costly and time consuming to maintain than
  animals.
• Can store their seed for long periods of time.
• One plant can produce tens to hundreds of progeny.

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Advantages of plants

• Can make inbreds in many plant species without
  severe effects that are typically seen in
  animals.
• Generation time is often much less than for
  animals.
• Fast plants (Brassica sp.)
• Arabidopsis

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Allele

• One of two to many alternative forms of the same
  gene (eg., round allele vs. wrinkled allele
  yellow vs. green).
• Alleles have different DNA sequences that cause
  the different appearances we see.

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Principle of Segregation(Mendels First Law)
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Important Observations

• F1 progeny are heterozygous but express only one
  phenotype, the dominant one.
• In the F2 generation plants with both phenotypes
  are observed?some plants have recovered the
  recessive phenotype.
• In the F2 generation there are approximately
  three times as many of one phenotype as the
  other.

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Mendels Results
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3 1 Ratio

• The 3 1 ratio is the key to interpreting
  Mendels data and the foundation for the the
  principle of segregation.

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The Principle of Segregation

• Genes come in pairs and each cell has two copies.
• Each pair of genes can be identical (homozygous)
  or different (heterozygous).
• Each reproductive cell (gamete) contains only one
  copy of the gene.

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Mendels Principle of Segregation

• In the formation of gametes, the paired
  hereditary determinants separate (segregate) in
  such a way that each gamete is equally likely to
  contain either member of the pair.
• One male and one female gamete combine to
  generate a new individual with two copies of the
  gene.

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Round vs. Wrinkled
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Round vs. wrinkled

• The SBEI causes the round vs. wrinkled phenotype.
• SBEI starch-branching enzyme
• Wrinkled peas result from absence of the branched
  form of starch called amylopectin.
• When dried round peas shrink uniformly and
  wrinkled do not.

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Round vs. wrinkled

• The non-mutant or wild-type round allele is
  designated W.
• The mutant, wrinkled allele is designated w.
• Seeds that are Ww have half the SBEI of wild-type
  WW seeds but this is enough to make the seeds
  shrink uniformly.
• W is dominant over w.

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Round vs. wrinkled

• An extra DNA sequence is present in the wrinkled
  allele that produces a non-functional SBEI and
  blocks the starch synthesis pathway at this step
  resulting in a lack of amylopectin.

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A Molecular View
Parents
F1
F2 Progeny
WW ww Ww ¼WW ¼Ww ¼wW ¼ww
1 2 1 Genotype 3 1 Phenotype
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Dihybrid crosses reveal Mendels law of
independent assortment

• A dihybrid is an individual that is heterozygous
  at two genes
• Mendel designed experiments to determine if two
  genes segregate independently of one another in
  dihybrids
• First constructed true-breeding lines for both
  traits, crossed them to produce dihybrid
  offspring, and examined the F2 for parental or
  recombinant types (new combinations not present
  in the parents).

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Mendel and two genes
Round Yellow
Wrinkled Green
x
All F1 Round, Yellow
Wrinkled Yellow 101
Wrinkled Green 32
Round Yellow 315
Round Green 108
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Dihybrid cross produces a predictable ratio of
phenotypes

• genotype phenotype number
  phenotypic ratio
• Parent Y_R_ 315 9/16
• Recombinant yyR_ 108 3/16
• Recombinant Y_rr 101
  3/16
• Parent yyrr 32 1/16
• Ratio of yellow (dominant) to green
  (recessive)31 (124)
• Ratio of round (dominant) to wrinkled
  (recessive)31 (124)

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Ratio for a cross with 2 genes

• Crosses with two genes are called dihybrid.
• Dihybrid crosses have genetic ratios of 9331.

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Mendel and two genes
Wrinkled Yellow 101
Wrinkled Green 32
Round Yellow 315
Round Green 108
Yellow 416 Green 140
Round 423 Wrinkled 133
Each gene has a 3 1 ratio.
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Summary of Mendel's work

• Inheritance is particulate - not blending
• There are two copies of each trait in a germ cell
• Gametes contain one copy of the trait
• Alleles (different forms of the trait) segregate
  randomly
• Alleles are dominant or recessive - thus the
  difference between genotype and phenotype
• Different traits assort independently

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Rules of Probability
Independent events - probability of two events
occurring together What is the probability that
both A and B will occur? Solution determine
probability of each and multiply them
together. Mutually exclusive events -
probability of one or another event occurring.
What is the probability of A or B
occurring? Solution determine the probability
of each and add them together.
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PRODUCT RULE
From James Birchler
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SUM RULE
Mutually exclusive ways!
From James Birchler
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From James Birchler
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All Dominant
Dominant
Recessive
All Recessive
From James Birchler
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Punnett Square method - 24 16 possible gamete
combinations for each parent Thus, a 16 ? 16
Punnett Square with 256 genotypes Thats one big
Punnett Square!
Loci (Genes) Assort Independently - So we can
look at each locus independently to get the
answer.
Branch diagrams are also convenient tools