Transcript Variations to Mendel`s Laws

Variations to Mendel’s
Laws
Extensions and Exceptions
Alterations to Mendel’s Ratios

In these cases, genotypic ratio is as Mendel
predicted but phenotypic ratio is altered
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Lethal allele combinations
Multiple alleles
Different dominance relationships
Epistasis
Penetrance and expressivity
Pleiotropy
Genetic heterogeneity
Phenocopies
Lethal Allele Combinations
Multiple Alleles
 Give
a range of phenotypes
 Each of us has 2 alleles for any given
gene

1 from mom and 1 from dad
 There
can be many different alleles for a
gene

Different alleles are formed by mutations
Example of Multiple Alleles -PKU

Phenylketonuria (PKU)

Enzyme that breaks down phenylalanine is deficient
• Phenylalanine accumulates
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There are hundreds of possible alleles
Allelic combinations give rise to different phenotypes
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Severe mental retardation
Moderate PKU
Mild PKU
Asymptomatic PKU
Different Dominance Relationship –
Incomplete dominance

The heterozygous
phenotype is
intermediate between
the homozygous
dominant and the
homozygous
recessive
 Blended phenotype
Different Dominance Relationship –
codominance

Phenotypes of both
alleles are expressed
Codominance –Blood Type
Codominance –Blood Type
Epistasis
 One
gene affects the expression of
another

Example: If a dog has the hairless gene, the
genes that affect hair color will not be
expressed
Penetrance and Expressivity
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Describe degrees of inheritance
Due to multifactorial inheritance
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Penetrance =all-or-none expression of a gene
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Expression of a gene is influenced by other genes
and by environment
100% of the people who inherit mutant amyloid
precursor protein (mAPP) develop Alzheimer disease
so mAPP is 100% penetrant
Expressivity =severity of gene expression
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Polydactyly has variable expressivity
• Some have an extra toe and others have an extra toe and an
extra finger.
Pleiotropy

Gene affects several functions
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Phenotype is varied
Example: Porphyria variegata
Genetic Heterogeneity
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Different genes produce the same phenotype
 Example:
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Hearing loss may be due to one of 132 different
genes that follow autosomal recessive inheritance
B, b =gene for hearing loss type 1
R, r =gene for hearing loss type 2
BBrr
bbRR
deaf
deaf
BbRr
NOT deaf
Phenocopy

An environmentally caused trait that
appears to be inherited
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Example: Phocomelia is a rare genetic
disorder whose effects are mimicked by
the teratogen, thalidomide
Example: AIDs transmission from mother
to offspring
Mitochondrial Genes
Mitochondrial Inheritance Pattern
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Mitochondrial genes are passed from mothers to
offfspring.
 Only females pass on the genes
The 37 Mitochondrial Genes
 24
encode proteins important for protein
synthesis
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Mutations can have devastating effects
 13
encode proteins needed for energy
production
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Mutations often affect skeletal muscle and
cause fatigue
Heteroplasmy

A mutation can occur in one mitochondrial DNA
ring and not another.
 When the mitochondria divide, different batches
of daughter mitochondria are produced (some
with the mutation, some without)
 It is therefore possible to have mutant
mitochondrial DNA in some tissues but not
others
 Causes variation is expressivity of a
mitochondrial disease depending on which
tissues/organs have cells with mutated
mitochondrial DNA
Linkage
 Two
genes on the same chromosome may
“co-segregate”
 Example:
Dihybrid cross of pea plants with
purple flowers (Pp) and long pollen grains
(Ll)
Parents
P
p
L
l
Genotype PpLl
Genes not linked
Self-cross
Figure 5.10
P
L
p
l
Genotype PpLl
Genes linked
Self-cross
Parents
P
p
L
l
Genotype PpLl
Genes not linked
Self-cross
P
L
p
l
Genotype PpLl
Genes linked
Self-cross
F1
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Parents
P
p
L
P
L
l
Genotype PpLl
Genes not linked
Self-cross
F1
Male
gametes
Genotype PpLl
Genes linked
Self-cross
Female gametes
PL
pl
Female gametes
PL
Pl
pL
pl
PL
Pl
pL
pl
p
l
Male
gametes
PL
pl
Parents
P
p
L
l
Genotype PpLl
Genes not linked
Self-cross
F1
Female gametes
PL
Pl
pL
pl
PL
Pl
Male
gametes
pL
pl
P
L
PPLL PPLl PpLL PpLl
PPLl PPll PpLl Ppll
PpLL PpLl ppLL ppLl
PpLl Ppll ppLl ppll
p
l
Genotype PpLl
Genes linked
Self-cross
Female gametes
PL
pl
Male PL PPLL PpLl
gametes
pl PpLl ppll
Parents
P
p
L
l
Genotype PpLl
Genes not linked
Self-cross
F1
Female gametes
PL
Pl
pL
pl
PL
Pl
PPLL PPLl PpLL PpLl
PPLl PPll
PpLl Ppll
Male
gametes
PpLL PpLl ppLL ppLl
pL
pl
PpLl Ppll ppLl ppll
Phenotypic ratio 9:3
P
L
p
l
Genotype PpLl
Genes linked
Self-cross
Female gametes
PL
pl
Male PL PPLL PpLl
gametes
pl PpLl ppll
Phenotypic ratio 3:
Parents
P
p
L
l
Genotype PpLl
Genes not linked
Self-cross
F1
Female gametes
PL
Pl
pL
pl
PL
Pl
PPLL PPLl PpLL PpLl
PPLl PPll
PpLl Ppll
Male
gametes
PpLL PpLl ppLL ppLl
pL
pl
PpLl Ppll ppLl ppll
Phenotypic ratio 9:3:3
P
L
p
l
Genotype PpLl
Genes linked
Self-cross
Female gametes
PL
pl
Male PL PPLL PpLl
gametes
pl PpLl ppll
Phenotypic ratio 3:
Parents
P
p
L
l
Genotype PpLl
Genes not linked
Self-cross
F1
Female gametes
PL
Pl
pL
pl
PL PPLL PPLl PpLL PpLl
Pl
PPLl PPll
PpLl Ppll
Male
gametes
pL PpLL PpLl ppLL ppLl
pl
PpLl Ppll ppLl ppll
Phenotypic ratio 9:3:3:1
P
L
p
l
Genotype PpLl
Genes linked
Self-cross
Female gametes
PL
pl
Male PL PPLL PpLl
gametes
pl PpLl ppll
Phenotypic ratio 3:1
Crossing Over May Disrupt Linkage
Linkage Maps

The farther apart 2
genes are, the more
likely their linkage will
be disrupted during
crossing over

% recombination tells
us the relative
location of the genes