Download Chapter 06 Lecture Outline

10/19/15
Simple Mendelian inheritance describes inheritance
patterns that obey
•  The Law of Segregation
•  The Law of Independent Assortment
As well as two other rules:
•  Genes are passed unaltered from generation to
generation (except for rare mutations)
•  Expression of the genes in the offspring directly
influences their traits
Chapter 06
Lecture Outline
Some genes violate these rules, and deviate from the
expectations of simple Mendelian inheritance
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•  In this lecture we will discuss inheritance patterns that
do not conform to Mendelian predictions
o  Extranuclear inheritance
•  Genes not in nucleus
o  Mitochondria
o  Chloroplasts
•  These became part of the cell by endosymbiosis
o  Epigenetic inheritance and imprinting
•  Genes are altered in the offspring (ex: methylation)
o  Maternal effect
•  Gene expression in the mother determines traits of offspring
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Extranuclear Inheritance
Shell coiling in the water snail Lymnaea is due to
maternal effect inheritance
Chloroplast Genome
•  Extranuclear inheritance - Traits are inherited through
genes that are not in the nucleus, but are in other
organelles
o  Also known as cytoplasmic inheritance
•  The chromosome of both mitochondria and
chloroplasts is composed of a single circular
double-stranded DNA
•  Like bacteria, the chromosomes of mitochondria
and chloroplasts are found in nucleoids
•  The two most important examples are genes in
o  Mitochondria
o  Chloroplasts
o  The nucleoid may contain many copies of the one
chromosome
o  There may also be multiple nucleoids
nucleoid
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nucleoid
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6.2 Extranuclear Inheritance:
}  Extranuclear genes display non-Mendelian
inheritance, which has four characteristics:
Mitochondria
q 
q 
q 
General features of mitochondrial genomes
Predicting the outcome of crosses involving genetic
variation in mitochondrial genomes
How mutations in mitochondrial genes cause human
diseases
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◦  Typical Mendelian ratios do not occur, because meiosisbased segregation is not involved.
◦  Reciprocal crosses usually show uniparental inheritance. All
progeny have the phenotype of one parent, generally the
mother because the zygote receives nearly all of its
cytoplasm (including organelles) from the ovum.
◦  Extranuclear genes cannot be mapped to chromosomes in
the nucleus.
◦  If a nucleus with a different genotype is substituted, nonMendelian inheritance is unaffected.
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•  The main function of mitochondria
o  oxidative phosphorylation - produces ATP
o  ATP used as an energy source to drive cellular
reactions
Mitochondria Genome
•  Like chloroplasts, mitochondria contain a single
circular chromosome
•  The genetic material in mitochondria is referred to as
mtDNA
o  Contained within the nucleoid
o  Multiple copies of the single chromosome
•  Human mtDNA
o 17,000 bp
o Relatively few genes
o rRNA and tRNA genes
o 13 genes that function in oxidative
phosphorylation
•  Most mitochondrial proteins are encoded by genes
in the nucleus
o  Proteins made in cytoplasm but have a signal to
direct them to mitochondria
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Ribosomal RNA genes
Necessary for synthesis of
mitochondrial proteins
Transfer RNA genes
Mitochondrial Inheritance
NADH dehydrogenase subunit genes (7)
Cytochrome b gene
Cytochrome c oxidase subunit genes (3)
Function in oxidative phosphorylation
•  Like chloroplasts, mitochondria are usually – but not
always – inherited from the female parent via egg
cells
ATP synthase subunit genes (2)
Noncoding DNA
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tRNAPhe
tRNAPro
12S rRNA
tRNAVal
tRNAThr
tRNAGlu
16S rRNA
tRNALeu
tRNAIle
tRNAGln
tRNAMet
tRNALeu
tRNASer
tRNAHis
tRNATrp
tRNAAla
tRNAAsn
tRNAArg
tRNACys
tRNATyr
tRNAGly
tRNASer
tRNAAsp tRNALys
Figure 6.5
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Human Mitochondrial Diseases
•  Species with maternal inheritance may, on
occasion, exhibit paternal leakage
o  Mitochondria provided through the sperm
o  Example: In the mouse
•  Two mechanisms of mitochondrial disease:
1.  Transmitted from mother to offspring via the egg
• 
Follow a strict maternal inheritance pattern
2.  Mutations can occur in somatic cells during aging
•  1-4 paternal mitochondria are inherited for every 100,000
maternal mitochondria per generation of offspring
• 
Mitochondria are especially susceptible to DNA damage
from free radicals
•  Over 200 human mitochondrial diseases discovered
o  Typically chronic degenerative disorders affecting
cells that need high levels of ATP
•  Nerve and muscle cells
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•  Example: Leber’s hereditary optic neuropathy (LHON)
o  Affects the optic nerve
o  May lead to progressive loss of vision in one or both eyes
o  LHON is caused by mutations in several different mitochondrial
genes
•  Kearns-Sayre syndrome (OMIM 530000) produces
three types of neuromuscular defects:
o  Progressive paralysis of certain eye muscles.
o  Abnormal pigment accumulation on the retina, causing
chronic inflammation and degeneration of the retina.
o  Heart disease.
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Maternal Effect Inheritance
•  Maternal effect – Only the genotype of the mother
controls phenotype of offspring
o  Genotype of father has no effect
o  Genotype of offspring has no effect
o  Controlled by nuclear genes
6.5 Maternal Effect
q 
q 
q 
Definition of maternal effect
Predicting the outcome of crosses for genes that
exhibit a maternal effect pattern of inheritance
•  Phenomenon is due to the accumulation of gene
products that the mother provides to her developing
eggs
The molecular mechanism of maternal effect
inheritance
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•  Example: Coiling in the snail,
Lymnaea
•  The first example of a maternal
effect gene
•  Discovered in the 1920s by Arthur
Boycott
Reciprocal cross
Parental
generation
DD
•  Two different directions in which
F
the shell and internal organs can
twist
1
•  Right-handed (dextral) - more
common and dominant
•  Left-handed (sinistral)
x
dd
DD
generation
x
Dd
All dextral
Dd
All sinistral
F2 generation
•  Reciprocal crosses and crosses
of the of the F1s demonstrated a
non-Mendelian pattern of
F
inheritance
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•  Alfred Sturtevant proposed that coiling is controlled by a
maternal effect gene
•  Phenotypes controlled by the genotype of the mother
•  Not the genotype of father or offspring
x
dd
Males and females
1 DD : 2 Dd : 1 dd
All dextral
Cross to each other
generation
Males and females
•  Even though twist is controlled by a
nuclear gene
3 dextral : 1 sinistral
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Reciprocal cross
x
DD
dd
DD
F1 generation
–  Maturing animal oocytes are
surrounded by maternal cells
that provide them with nutrients
–  These nurse cells are diploid,
whereas the oocyte becomes
haploid
x
Dd
All dextral
Dd
All sinistral
F1 mothers are Dd
The dominant allele, D,
caused ALL the F2
offspring to be dextral
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•  The non-Mendelian
inheritance pattern of
maternal effect genes can be
explained by the process of
oogenesis
Parental
generation
x
dd
•  He proposed one gene with two alleles D and d
•  Dd mothers produce dextral offspring
even if the offspring are dd
•  dd mothers produce sinistral offspring
even if the offspring are Dd
Egg
Nurse cells
Dd
Dd
Dd
Dd
Dd
Dd
The nurse cells express
mRNA and/or protein
from genes of the
d allele (red) and the
D allele (green) and
transfer those products
to the egg.
Dd
F2 generation
1 DD : 2 Dd : 1 dd
All dextral
•  In the example
Males and females
Cross to each other
–  A female Dd is heterozygous
for the snail-coiling maternal
effect gene
–  The haploid oocyte received
just the d allele in meiosis
F3 generation
MalesFand
females
2 mothers
3 with the D
allele 1 with the d allele
3 dextral : 1 sinistral
Explains this 3:1 ratio
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•  Maternal effect genes encode RNA and proteins that play
important roles in the early steps of embryogenesis
–  Example - cell division, cleavage pattern, body axis
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Mitotic
spindle
•  Accumulation before fertilization allows development to
proceed very quickly after fertilization
Dextral
•  Therefore defective alleles in maternal effect genes tend to
have a dramatic effect on the phenotype of the individual
–  In Drosophila, geneticists have identified several dozen
maternal effect genes
•  Profound effects on the early stages of development
Sinistral
(c) An explanation of coiling direction at the cellular level
Figure 6.11c
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Extranuclear Inheritance
•  In many organisms, extranuclear genes inheritance
is maternal. This differs from maternal effect in two
ways:
o  Extranuclear inheritance is determined by genes in an
organelle, while maternal effect derives from nuclear
genes.
o  b.
Extranuclear genotype matches individual’s
phenotype, while in maternal effect the individual’s
phenotype results from its mother’s genes.
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