Post on 10-Feb-2020
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Non-Mendelian Genetics
and Gene Mapping
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…..
• Incomplete dominance
• Co-dominance
• Epistasis
• Polygenics
• Sex influenced
• Sex linkage
• Epigenetics
• Pleiotropy
• X-Inactivation
• Reduced Penetrance & Variable Expressivity
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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: Wavy hair is the product of sharing alleles for curly
and straight hair. Or roan horses that appear “pinkish”
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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)
at an entirely different loci on a different chromosome.
• One set of 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 has the albino gene set (cc)
no matter what genes for pigmentation that organism
has for coat color, the organism will be albino.
B=black coat
c= albinoSo a mouse with a
genotype of BBcc
will have NO
color!
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Polygenic traits
Characterized by the need for multiple alleles
(genes) for the expression of a single trait.
Ex: Eye color
EX: Hair color
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Sex influenced
Genes that are expressed differently based on hormones
produced by the sex chromosome combinations XX or
Xy. (testosterone/estrogen levels) Results in sexual
dimorphism
EX: patterned baldness or horn/antler development
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X and y sizing
Y chromosome has one job, change female default into male phenotypeX carries over 1,000 genesy carries 78
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Sex linkage
• If the genes are on the
y, they are called
holandric. Ex: Ear hair
• Those carried on the X
are more abundant
• EX: Muscular Dystrophy
Characterized by alleles (genes) physically located
on the X or y chromosomes.
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• EX: Hemophilia
• XhXh or XhY= hemophilia.
• Gender specific heterozygocity
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Epigenetics
Characterized by phenotypic changes in
expression as those genes are exposed to
external stimuli.
Examples:
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Domestication
• Human impacts on critical development results in epigenetic outcomes not seen in nature: Ex: pie-balding, curled tail, droopy ears
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Stress and other factors?
• Ghost in your genes???
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Pleiotropy
Characterized when 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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X-Inactivation• 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.
• Ex: calico cats
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Reduced penetrance
• Refers to the proportion of people with a particular genetic change (such as a mutation in a specific gene) who exhibit signs and symptoms of a genetic disorder.
• If some people with the mutation do not develop features of the disorder, the condition is said to have reduced (or incomplete) penetrance.
• Reduced penetrance often occurs with familial cancer syndromes. For example, many people with a mutation in the BRCA1 or BRCA2 gene will develop cancer, but some people will not.
• Makes genetic predictors and treatment difficult.
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Variable Expressivity
• Although some genetic disorders exhibit little variation, most have signs and symptoms that differ among affected individuals. Variable expressivity refers to the range of signs and symptoms that can occur in different people with the same genetic mutation.
• For example, 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).
• As with reduced penetrance, variable expressivity is probably caused by a combination of genetic, environmental, and lifestyle factors, most of which have not been identified.
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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