Mendel and Inheritance

1
Mendel and Inheritance

• MUPGRET Workshop
• June 13, 2005

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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.
• Notice that 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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Principle of Segregation
X
Parental Lines
Round
Wrinkled
All round F1 progeny
Self-pollinate
3 Round 1 Wrinkled
Round 5474
Wrinkled 1850
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Mendels Results
Parent Cross F1 Phenotype F2 data
Round x wrinkled Round 5474 1850
Yellow x green Yellow 6022 2001
Purple x white Purple 705 224
Inflated x constricted pod Inflated 882 299
Green x yellow pod Green 428 152
Axial x terminal flower Axial 651 207
Long x short stem Long 787 277
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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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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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Principle of Segregation

• Either copy of the gene is equally likely to be
  included in a gamete.
• One male and one female gamete combine to
  generate a new individual with two copies of the
  gene.

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Allele

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

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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.

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Principle of Segregation
X
Parental Lines
Round (WW)
Wrinkled (ww)
All round F1 progeny (Ww)
Self-pollinate
3 Round 1 Wrinkled
Round (WW Ww) 5474
Wrinkled (ww) 1850
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Punnett Square
A (½) a (½)
A (½) AA (½ x ½ ¼) Aa (½ x ½ ¼)
a(½) Aa (½ x ½ ¼) aa (½ x ½ ¼)
Male
Female
¼ AA ½ Aa ¼ aa
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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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Chi-Squared Analysis

• Tests if your observations are statistically
  different from your expectation.
• For example does the Mendel data fit the 31
  hypothesis?
• Chi-squared
• ?(observed-expected)2/expected

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Testcross and Backcross
x
Parents
WW
ww
F1
x
W w
w Ww ww
w Ww ww
ww
Ww
Testcross Progeny
ww
Ww
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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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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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Punnett Square
Yellow ¾ Green ¼
Round ¾ Round, Yellow ¾ x ¾ 9/16 Round, Green ¾ x ¼ 3/16
Wrinkled ¼ Wrinkled, Yellow ¼ x ¾ 3/16 Wrinkled, Green ¼ x ¼ 1/16
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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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Principle of Independent Assortment
Ww Gg
F1
Gametes Frequencies
wG ¼
wg ¼
WG ¼
Wg ¼
If a gamete contains W the probability that it
contains G is equal to the probability that it
contains g.
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¼ WG ¼ Wg ¼ wG ¼ wg
¼ WG WW GG 1/16 WW Gg 1/16 WwGG 1/16 WwGg 1/16
¼ Wg WWGg 1/16 WWgg 1/16 WwGg 1/16 Wwgg 1/16
¼ wG WwGG 1/16 WwGg 1/16 wwGG 1/16 wwGg 1/16
¼ wg WwGg 1/16 Wwgg 1/16 wwGg 1/16 wwgg 1/16
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Phenotypes

• W Round
• w Wrinkled
• W is dominant to w.
• G Yellow
• g Green
• G is dominant to g.

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F2 Progeny
Genotype Phenotype
1/16 WWGG 2/16 WWGg 2/16 WwGG 4/16 WwGg 9/16 Round Yellow
1/16 wwGG 2/16 wwGg 3/16 Wrinkled Yellow
1/16 WWgg 2/16 Wwgg 3/16 Round Green
1/16 wwgg 1/16 Wrinkled Green
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Principle of Independent Assortment

• Segregation of the members of any pair of alleles
  is independent of the segregation of other pairs
  in the formation of reproductive cells.

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Summary of Mendel

• Inherited traits are controlled by the alleles
  present in the reproductive cells that fuse to
  form the embryo.
• In a diploid, progeny inherit one allele from the
  mother and one from the father.
• Differences in the DNA sequence of two alleles
  for a gene may result in different phenotypes.

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Summary

• The phenotype is the same if the gene is
  inherited from the mother or from the father.
• One allele from the diploid is inherited in each
  reproductive cell.