Download Extensions and Exceptions to Mendel`s Laws Sponge

Sponge Questions
Extensions and Exceptions
to Mendel’s Laws
Genetics Notes
• Is it possible for a type O child to be born from a
mating between a type-A parent and a type-B
parent? Between a type AB parent and a type O
parent?
• What are the chances that a woman who s Rh- will
have an Rh+ child if her mate is heterozygous?
• In the famous Charlie Chaplin paternity case in the
1940’s, Chaplin was accused of fathering an
illegitimate child. The baby’s blood was B, the
mother’s A and Chaplin’s O. If you had been the
judge, how would you have decided the case?
Exceptions to Mendel’s Law
Mendel chose traits in peas that showed two distinct
forms.
Not all genes exhibit such simple inheritance.
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Alleles interact
Gene interaction
Non-nuclear genes
Segregation of genes on same chromosome
Lethal alleles
Lethal alleles
Some allele combinations are lethal.
Mexican hairless dogs result from a mutation in a
gene that shows lethality
– hh
– Hh
hairy
hairless
– HH
dies
the wildtype trait
one mutation present creates a
visible phenotype
two mutation are lethal
Multiple Alleles
• A gene may exist in more than two allelic
forms in a population.
• Genes can mutate in many ways at any
nucleotide in their DNA sequence.
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Multiple alleles: coat color in rabbits
Grey
CC or Ccch or Cch or Cc
Himalayan
chc or c hc h
The heterozygous
phenotype is
typically
intermediate to the
homozygous
phenotype
Chinchilla
cchcch
Light grey
cchch or c chc
Incomplete dominance
Albino
cc
Codominant alleles are observed
simultaneously
Codominant Alleles
The ABO gene encodes a cell surface
protein.
• Allele A makes A protein
• Allele B makes B protein
• Allele O makes no protein
Alleles A and B can be present on the cell
surface at the same time.
• Alleles A and B are codominant.
• Allele O is recessive to both A and B
alleles
Blood Types Example
Epistasis
when one gene affects
the expression of a
Epistasis
- Condition results
second gene.
when one gene masks
another
- H gene is epistatic to the
ABO gene.
- H protein attaches the A or
B protein to the cell surface.
- hh genotype = no H protein
- All ABO genotypes appear as type
O
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Variable expressivity
- Expressivity of a phenotype is the severity,
degree and/or extent of the expression of a trait
FF or Ff
all show mild, moderate or
profound deafness
Incomplete penetrance
- Penetrance refers to the all or none expression of a
geneotype
- Occurs when the disease phenotype is not always
observed among individuals carrying the diseaseassociated genotype.
DD or Dd
80% polydactyly
DD or Dd
20% no polydactyly
Phenocopy
A trait caused by the environment that mimics an
inherited condition
Exposure to teratogens
• Thalidomide causes limb defects akin to rare
inherited phocomelia
– A birth defect in which the upper portion of a limb is
absent or poorly developed, so that the hand or foot
attaches to the body by a short, flipperlike stump
Infection
• Rubella in pregnant mothers causes deafness
mimicking inherited forms of deafness
Mitochondrion
• Organelle providing
cellular energy
• Contains small circular
DNA
• No crossing over or
DNA repair
• Many copies of the
mitochondrial genome
per cell
• 37 genes, no histones,
no introns
• Maternal inheritance
Pleiotropy
• One gene controls or
influences the
expression of many
symptoms
in a disorder. These
symptoms may be
variably expressed
• Occurs when a single
protein affects different
parts of the body or
participates in the
different biochemical
processes
King George III - Photo
Recessive
Inheritance
© North Wind Picture Archives
of porphyria variegata
Genetic Heterogeneity
Individuals with identical phenotypes may
reflect different genetic causes.
– Deafness
– Albinism
– Cleft palate
– Poor blood clotting
Mitochondrial Inheritance
• Mitochondria and their genome are transmitted
from a mother to all of her offspring.
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Heteroplasmy
Linkage
• Is the term indicating
that two genes are not
transmitted
independently.
Why?
• Two genes physically
near each other on a
chromosome will not
assort randomly in
meiosis.
• There are many
copies of the
mitochondrial DNA
• Heteroplasmy is the
condition in which
mitochondrial DNA
sequence is not the
same in all copies
Linkage
Unlinked :
4 type of gametes
PL, Pl, pL, pl
Tightly linked:
2 types of gametes
PL and pl
NOTE for LINKAGE which two types are observed (PL and pl OR Pl
and pL) depends on which alleles are on the same chromosome in
the parent!
Recombination
• When chromosomes
recombine new
combinations of alleles
are created
• Parental chromosomes
have the alleles present
in the original
configuration
• Recombinant
chromosomes have
new combinations of
alleles
Recombination
The frequency of recombination between two genes is
directly related to the physical distance between the genes
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Inheritance of linked genes
The genes for Rh factor (R) and anemia (E) are linked, but some
recombination occurs between the two genes
• A linkage map is a diagram indicating the relative distance between
genes.
• 1% recombination = 1 map unit = 1 centiMorgan (cM)
• Map distances are additive.
Linkage Disequilibrium
The non-random association between alleles at two
locations on a chromosome is called linkage
disequilibrium
Two genes, A and B, exist in a population.
• If the frequency of chromosomes with
AB=Ab=aB=ab then the genes are in equilibrium
• If the frequency of one allele of gene A is seen
more frequently with a particular allele of gene B,
then the genes are in linkage disequilibrium
Haplotype
Karyotype
A haplotype is the set of alleles inherited on one chromosome
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