Non-Mendelian Genetics
Be sure to review Mendel’s 3 Laws!
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Today, we know that genes often do not follow
Mendel’s Laws all the time! There are
exceptions…..
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Incomplete dominance
Co-dominance
Epistasis
Polygenics
Sex influenced
Sex linkage
Epigenetics
Pleiotropy
Barr Bodies
Domestication
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Reduced Penetrance
Variable Expressivity
Incomplete dominance
• Characterized by a blending of traits. Phenotype of
offspring falls somewhere between each parent, when
genotype is heterozygotic. These intermediate
offspring are called roans.
• EX: Red horses (RR) crossed with white horses (rr)
produce Rr roans that are “pink” in color.
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Co-dominance
• Characterized by the
expression of both
alleles. Shared
dominance. Even the
recessive allele affects
the organism.
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EX: Blood type - A
and B are both
dominant!
IAIA or IAi= A type
IAIB = AB type
IBIB or IBi = B type
ii = O type
Marker proteins
on cell membrane
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Epistasis
• Characterized where one set of genes (2 alleles)
controls a totally separate set of genes (2 other alleles)
Often the controlling alleles block expression of the
other alleles. Can results in general conditions called
Somatic mosaicism, Ex: Heterochromia iridum Or can
result in full blocking of entire gene sequence Ex:
albinism
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Epistasis
Ex: Albinism. If an organisms carries the albino gene (c)
no matter what genes that organism has for color, the
organism will be albino.
So a mouse with a
B=black coat genotype of BBcc
will have NO
c= albino
color!
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Albinism
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Albinism
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Albinism
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Albinism
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Polygenic traits
Characterized by many genes are needed to affect a
single trait.
How would the alleles be written?
Ex: Eye color
EX: Hair color
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Sex influenced
Genes that are expressed differently based on hormones
produced by the sex chromosomes X or y.
EX: patterned baldnessSexual dimporphism
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Sex linkage
Characterized by genes carried on the X or Y
chromosomes.
• If the genes are on the • Those carried on the X
y, they are called
are more abundant
holandric. Ex: Ear hair • EX: Muscular Dystrophy
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• EX: Hemophilia
• XhXh or XhY= hemophilia.
• Gender specific heterozygosis
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Why?
• X and y are not the same molecular mass/size
• Y chromosome has one job, change female
default into male phenotype using hormones
• X carries over 1,000 genes
• y carries 78
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Epigenetics
Phenotype that changes as genes are exposed
to external stimuli, like temperature.
Ex: Foxes or Siamese cats
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Pleiotropy
One set of genes control multiple outcomes.
EX: White coat color & ability to hear. Or
fizzle-feather gene and egg production.
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Barr Bodies
• Since females have 2 X chromosomes, if each
carries a regular dominant genes there could
be a conflict in expression.
• So one set of genes is chemically blocked
through a process called methylation to
prevent conflict of expression. Areas that are
knocked out are called Barr bodies.
• Ex: calico cats
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Domestication
• Human impacts on critical development
results in phenotypic outcomes not seen in
nature. Happens as sets of genes are
controlled artificially. Ex: pie-balding, curled
tail, droopy ears
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Reduced penetrance
• Some people with a mutation do not develop
features/symptoms of the disorder, the condition is
said to have reduced (or incomplete) penetrance.
• EX: Reduced penetrance often occurs with familial
cancer syndromes. For example, many people with a
mutation in the BRCA1or BRCA2gene will develop
cancer, but some people will not.
• Makes genetic predictors and treatment difficult.
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Variable Expressivity
• Variable expressivity refers to the range of signs
and symptoms that can occur in different
people with the same genetic mutation.
• EX: the features of Marfan Syndrome vary
widely—some people have only mild symptoms
while others experience life-threatening
complications even though all affected people
with this disorder have a mutation in the same
gene (FBN1).
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Gene mapping
There are 3 main types of gene maps based on the details
the “map” illustrates
A) Pedigree
B) Karyotype
C) Cytological
Gene maps are used to track and predict genetic traits
and disorders.
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Pedigree Maps are used to illustrate a known
phenotype through families. They do not illustrate
chromosomes or gene sequences.
Circles=Female, Squares=Males
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Karyotype, or Chromosomal Map, shows
the overall large chromosome structure, but
not gene sequence details-Autosomal versus
sex chromosomes
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Cytological maps - illustrate actual gene
sequences of the DNA
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• Which map would be used to show a
purebred dog’s ancestry?
• Which map would be used to show that a
non-disjunction has occurred in a human?
• Which map would be used to tell a person
that they have the gene for a specific
disorder.
• Be sure you know WHY, be ready to share
your answer
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