Mendel, Genes, and Inheritance

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Mendel, Genes, and Inheritance

• Chapter 12

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Gregor Mendel

• Austrian Monk with a strong background in plant
  breeding and mathematics
• Using pea plants, found indirect but observable
  evidence of how parents transmit genes to
  offspring

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Mendels Work

• Bred pea plants
• cross-pollinated true breeding parents (P)
• raised seed then observed traits (F1)
• filial
• allowed offspring to cross-pollinate observed
  next generation (F2)

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Mendels Data
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Mendels Experiments
true-breeding purple-flower peas
true-breeding white-flower peas
X
P
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Genes

• Sequences of DNA that contain information about
  specific traits, by coding for individual
  proteins
• Passed from parents to offspring
• Each has a specific location (locus) on a
  chromosome

Alleles

• Different versions of the same trait

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Genes and inheritence

• For each characteristic (gene), a diploid
  organism inherits 2 sets of alleles
• 1 from each parent
• Homologous chromosomes
• Pairs of chromosomes that contain the same traits
  (genes)
• For each trait there are two alleles (one on each
  homologous chromosome)

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Genetic Terms
purple-flower allele white-flower allele are 2
DNA variations at flower-color locus
different versions of gene on homologous
chromosomes
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Mendels Findings Dominance

• Some alleles for a given traits mask others
• Dominant allele
• Fully expressed in a hybrid
• Designated by a capital letter
• e.g. P Purple allele
• Recessive allele
• no noticeable effect in a hybrid
• Designated by a lowercase letter
• E.g. p White allele

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Genotype vs. phenotype

• Phenotype
• Description of an organisms trait
• visible characteristic
• Genotype
• Description of an organisms genetic makeup i.e.
  which alleles are present in the organism
• Alleles may be the same or they may be different

• Homozygous same alleles PP, pp
• Heterozygous different alleles Pp

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Looking closer at Mendels work
true-breeding purple-flower peas
true-breeding white-flower peas
X
P
PP
pp
Phenotype
100 purple-flower peas
100 purple-flower peas
F1 generation (hybrids)
100
Pp
Pp
Pp
Pp
25 white-flower peas
75 purple-flower peas
75 purple-flower peas
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F2 generation
?
?
?
?
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Punnett squares
genotype
phenotype

• Pp x Pp

PP
Pp
Pp
pp
31
121
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Phenotype vs. genotype

• 2 organisms can have the same phenotype but have
  different genotypes

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Dominant phenotypes

• It is not possible to determine the genotype of
  an organism with a dominant phenotype by looking
  at it.
• So how can you figure it out?

PP?
Pp?
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Test cross

• Cross-breed the dominant phenotype unknown
  genotype with a homozygous recessive (pp) to
  determine the identity of the unknown allele

x
is itPP or Pp?
pp
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Test cross
PP
pp
Pp
pp
p
p
p
p




Pp
Pp
Pp
Pp
P
P
P
Pp
Pp
p
pp
pp
505011
100
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Mendels laws of heredity (1)

• Law of segregation
• when gametes are produced during meiosis,
  homologous chromosomes separate from each other
• each allele for a trait is packaged into a
  separate gamete

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Law of Segregation

• What meiotic event creates the law of
  segregation?

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Monohybrid cross

• Some of Mendels experiments followed the
  inheritance of single characters
• flower color
• seed color
• monohybrid crosses

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Dihybrid cross

• Other of Mendels experiments followed the
  inheritance of 2 different characters
• seed color and seed shape
• Dihybrid crosses

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Dihybrid cross
true-breeding yellow, round peas
true-breeding green, wrinkled peas
P
x
YYRR
yyrr
Y yellow R round
y green r wrinkled
YyRr
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Dihybrid Cross

• How are the alleles on different chromosomes
  handed out?
• together or separately?

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Dihybrid cross
YyRr
YyRr
x
YR
Yr
yR
yr




YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
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Mendels laws of heredity (2)

• Law of independent assortment
• each pair of alleles segregates into gametes
  independently
• 4 classes of gametes are produced in equal
  amounts
• YR, Yr, yR, yr
• only true for genes on separate chromosomes

YyRr
Yr
Yr
yR
yR
YR
YR
yr
yr
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Law of Independent Assortment

• What meiotic event creates the law of
  independent assortment?

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• The chromosomal basis of Mendels laws
• Trace the genetic events through meiosis,
  gamete formation fertilization to offspring

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Review Mendels laws of heredity

• Law of segregation
• each allele segregates into separate gametes
• Law of independent assortment
• Observable in dihybrid (or more) cross
• 2 or more traits
• each pair of alleles for genes on separate
  chromosomes segregates into gametes independently
• Each gamete carries one allele of each trait

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Extending Mendelian genetics

• Mendel worked with a simple system
• peas are genetically simple
• most traits are controlled by a single gene
• each gene has only 2 alleles, 1 of which is
  completely dominant to the other
• The relationship between genotype phenotype
  is rarely that simple

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Incomplete dominance

• Heterozygotes show an intermediate phenotype
• RR red flowers
• rr white flowers
• Rr pink flowers
• make 50 less color

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Incomplete dominance
X
true-breeding red flowers
true-breeding white flowers
P
100 pink flowers
F1 generation (hybrids)
100
121
F2 generation
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Incomplete dominance
genotype
phenotype

• CRCW x CRCW

CRCR
CRCW
CRCW
CWCW
121
121
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Codominance

• BOTH alleles express in heterozygotes
• Example In chickens, feather color trait has two
  alleles
• B Black feathers
• W White feathers
• What is phenotype of BB, WW?
• What is the phenotype of BW?

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Codominance

WW
BB
WB
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Multiple Alleles

• More than two possible alleles for one trait
• Example in ABO blood group, 3 possible alleles
• IA, IB, i
• 4 blood types A, B, O, AB
• How?

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Pleiotropy

• Most genes are pleiotropic
• one gene affects more than one phenotypic
  character
• wide-ranging effects due to a single gene
• dwarfism (achondroplasia)
• gigantism (acromegaly)

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Epistasis

• One gene masks another
• coat color in mice 2 genes
• pigment (C) or no pigment (c)
• more pigment (blackB) or less (brownb)
• cc albino, no matter B allele
• 9331 becomes 934

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Epistasis in Labrador retrievers

• 2 genes E B
• pigment (E) or no pigment (e)
• how dark pigment will be black (B) to brown (b)

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Polygenic inheritance (continuous variation)

• Some phenotypes determined by additive effects of
  2 or more genes on a single character
• phenotypes on a continuum
• human traits
• skin color
• height
• weight
• eye color
• intelligence
• behaviors

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Environmental influence

• The expression of some genes can be influenced by
  the environment
• for example coat color in Himalayan rabbits and
  Siamese cats
• an allele produces an enzyme that allows pigment
  production only at temperatures below 30oC

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Environmental influence
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Acknowledgement

• Much of this presentation was modified from the
  Biology Zone website
• http//bio.kimunity.com/