variations to mendel’s laws extensions and exceptions
TRANSCRIPT
Variations to Mendel’s Variations to Mendel’s LawsLaws
Extensions and ExceptionsExtensions and Exceptions
Alterations to Mendel’s RatiosAlterations to Mendel’s Ratios
In these cases, genotypic ratio is as Mendel In these cases, genotypic ratio is as Mendel predicted but phenotypic ratio is alteredpredicted but phenotypic ratio is altered Lethal allele combinationsLethal allele combinations Multiple allelesMultiple alleles Different dominance relationshipsDifferent dominance relationships EpistasisEpistasis Penetrance and expressivityPenetrance and expressivity PleiotropyPleiotropy Genetic heterogeneityGenetic heterogeneity PhenocopiesPhenocopies
Lethal Allele CombinationsLethal Allele Combinations
Multiple AllelesMultiple Alleles
Give a range of phenotypesGive a range of phenotypes Each of us has 2 alleles for any given Each of us has 2 alleles for any given
genegene 1 from mom and 1 from dad1 from mom and 1 from dad
There can be many different alleles for a There can be many different alleles for a genegene Different alleles are formed by mutationsDifferent alleles are formed by mutations
Example of Multiple Alleles -PKUExample of Multiple Alleles -PKU
Phenylketonuria (PKU)Phenylketonuria (PKU) Enzyme that breaks down phenylalanine is deficientEnzyme that breaks down phenylalanine is deficient
• Phenylalanine accumulatesPhenylalanine accumulates There are hundreds of possible allelesThere are hundreds of possible alleles Allelic combinations give rise to different phenotypesAllelic combinations give rise to different phenotypes
• Severe mental retardationSevere mental retardation• Moderate PKUModerate PKU• Mild PKUMild PKU• Asymptomatic PKUAsymptomatic PKU
Different Dominance Relationship –Different Dominance Relationship –Incomplete dominanceIncomplete dominance
The heterozygous The heterozygous phenotype is phenotype is intermediate between intermediate between the homozygous the homozygous dominant and the dominant and the homozygous homozygous recessiverecessive
Blended phenotypeBlended phenotype
Different Dominance Relationship –Different Dominance Relationship –codominancecodominance
Phenotypes of both Phenotypes of both alleles are expressedalleles are expressed
Codominance –Blood TypeCodominance –Blood Type
Codominance –Blood TypeCodominance –Blood Type
EpistasisEpistasis
One gene affects the expression of One gene affects the expression of anotheranother Example: If a dog has the hairless gene, the Example: If a dog has the hairless gene, the
genes that affect hair color will not be genes that affect hair color will not be expressedexpressed
Penetrance and ExpressivityPenetrance and Expressivity Describe degrees of inheritanceDescribe degrees of inheritance Due to multifactorial inheritanceDue to multifactorial inheritance
Expression of a gene is influenced by other genes Expression of a gene is influenced by other genes and by environmentand by environment
Penetrance =all-or-none expression of a genePenetrance =all-or-none expression of a gene 100% of the people who inherit mutant amyloid 100% of the people who inherit mutant amyloid
precursor protein (mAPP) develop Alzheimer disease precursor protein (mAPP) develop Alzheimer disease so mAPP is 100% penetrantso mAPP is 100% penetrant
Expressivity =severity of gene expressionExpressivity =severity of gene expression Polydactyly has variable expressivityPolydactyly has variable expressivity
• Some have an extra toe and others have an extra toe and an Some have an extra toe and others have an extra toe and an extra finger.extra finger.
PleiotropyPleiotropy
Gene affects several functionsGene affects several functions Phenotype is variedPhenotype is varied
Example: Porphyria variegataExample: Porphyria variegata
Genetic HeterogeneityGenetic Heterogeneity
Different genes produce the same phenotypeDifferent genes produce the same phenotype Example:Example:
Hearing loss may be due to one of 132 different Hearing loss may be due to one of 132 different genes that follow autosomal recessive inheritancegenes that follow autosomal recessive inheritance
B, b =gene for hearing loss type 1B, b =gene for hearing loss type 1R, r =gene for hearing loss type 2R, r =gene for hearing loss type 2
BBrr bbRR
BbRr
deaf deaf
NOT deaf
PhenocopyPhenocopy
An environmentally caused trait that An environmentally caused trait that appears to be inheritedappears to be inherited
Example: Phocomelia is a rare genetic Example: Phocomelia is a rare genetic disorder whose effects are mimicked by disorder whose effects are mimicked by the teratogen, thalidomidethe teratogen, thalidomide
Example: AIDs transmission from mother Example: AIDs transmission from mother to offspringto offspring
Mitochondrial GenesMitochondrial Genes
Mitochondrial Inheritance PatternMitochondrial Inheritance Pattern
Mitochondrial genes are passed from mothers to Mitochondrial genes are passed from mothers to offfspring.offfspring.
Only females pass on the genesOnly females pass on the genes
The 37 Mitochondrial GenesThe 37 Mitochondrial Genes
24 encode proteins important for protein 24 encode proteins important for protein synthesissynthesis Mutations can have devastating effectsMutations can have devastating effects
13 encode proteins needed for energy 13 encode proteins needed for energy productionproduction Mutations often affect skeletal muscle and Mutations often affect skeletal muscle and
cause fatiguecause fatigue
HeteroplasmyHeteroplasmy A mutation can occur in one mitochondrial DNA A mutation can occur in one mitochondrial DNA
ring and not another.ring and not another. When the mitochondria divide, different batches When the mitochondria divide, different batches
of daughter mitochondria are produced (some of daughter mitochondria are produced (some with the mutation, some without)with the mutation, some without)
It is therefore possible to have mutant It is therefore possible to have mutant mitochondrial DNA in some tissues but not mitochondrial DNA in some tissues but not othersothers
Causes variation is expressivity of a Causes variation is expressivity of a mitochondrial disease depending on which mitochondrial disease depending on which tissues/organs have cells with mutated tissues/organs have cells with mutated mitochondrial DNAmitochondrial DNA
LinkageLinkage
Two genes on the same chromosome may Two genes on the same chromosome may “co-segregate”“co-segregate”
Example: Dihybrid cross of pea plants with Example: Dihybrid cross of pea plants with purple flowers (Pp) and long pollen grains purple flowers (Pp) and long pollen grains (Ll)(Ll)
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross Self-cross
p
L l
P
L
p
l
Figure 5.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross
F1
Self-cross
p
L l
P
L
p
l
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross
F1
Self-cross
p
L l
P
L
p
l
Female gametesPL Pl pL pl
Malegametes
PL
Pl
pL
pl
Female gametesPL pl
Malegametes
PL
pl
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross
F1
Self-cross
p
L l
P
L
p
l
Female gametesPL Pl pL pl
PPLL PPLl PpLL PpLl
PPLl PPll PpLl Ppll
PpLL PpLl ppLL ppLl
PpLl Ppll ppLl ppll
Malegametes
PL
Pl
pL
pl
Female gametesPL pl
PPLL PpLl
PpLl ppll
Malegametes
PL
pl
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross
F1
Phenotypic ratio 3:
Self-cross
p
L l
P
L
p
l
Female gametesPL Pl pL pl
PPLL PPLl PpLL PpLl
PPLl PPll PpLl Ppll
PpLL PpLl ppLL ppLl
PpLl Ppll ppLl ppll
Malegametes
PL
Pl
pL
pl
Female gametesPL pl
PPLL PpLl
PpLl ppll
Malegametes
PL
pl
Phenotypic ratio 9:3
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross
F1
Phenotypic ratio 3:
Self-cross
p
L l
P
L
p
l
Female gametesPL Pl pL pl
PPLL PPLl PpLL PpLl
PPLl PPll PpLl Ppll
PpLL PpLl ppLL ppLl
PpLl Ppll ppLl ppll
Malegametes
PL
Pl
pL
pl
Female gametesPL pl
PPLL PpLl
PpLl ppll
Malegametes
PL
pl
Phenotypic ratio 9:3:3
Parents P
Genotype PpLlGenes not linked
Genotype PpLlGenes linked
Self-cross
F1
Phenotypic ratio 3:1
Self-cross
p
L l
P
L
p
l
Female gametesPL Pl pL pl
PPLL PPLl PpLL PpLl
PPLl PPll PpLl Ppll
PpLL PpLl ppLL ppLl
PpLl Ppll ppLl ppll
Malegametes
PL
Pl
pL
pl
Female gametesPL pl
PPLL PpLl
PpLl ppll
Malegametes
PL
pl
Phenotypic ratio 9:3:3:1
Crossing Over May Disrupt LinkageCrossing Over May Disrupt Linkage
Linkage MapsLinkage Maps
The farther apart 2 The farther apart 2 genes are, the more genes are, the more likely their linkage will likely their linkage will be disrupted during be disrupted during crossing overcrossing over
% recombination tells % recombination tells us the relative us the relative location of the geneslocation of the genes