Download Cytoplasmic inheritance

Who’s your mama?
Cytoplasmic inheritance
• The transmission of cytoplasm differs
between sex cells
January 18
– Male contribution: Sperm or pollen transfer
little or no cytoplasm to the egg
– Female contribution: Egg contributes almost
all of the cytoplasm to the zygote
Organelle chromosomes
Organelle chromosomes
http://www.erin.utoronto.ca/~w3bio/bio207/index.htm
What is in the cytoplasm?
• What is in the cytoplasm that could contain
DNA?
– Mitochondria, chloroplasts
• Endosymbiotic hypothesis:
– Free living prokaryotes ancestors of
chloroplasts and mitochondria invaded plant
and animal cells but provide useful function
and so a symbiotic relationship developed
over time
Fungi
• The cells of fungi may
have many mitochondria
(but not chloroplasts)
• Neurospora crassa like
many fungi is haploid and
produces spores
• This fungus is a member
of the ascomycetes and
produces ascospores as
the products of meiosis
• A zygote inherits its organelles from the
cytoplasm of the egg: Maternal inheritance
• The pattern of inheritance is not
associated with meiosis or mitosis
because the organelles are in the
cytoplasm not the nucleus
• Organelles (Chloroplasts and
mitochondria) have circular chromosomes
Text p.104
Neurospora crassa
Neurospora “sex”
• Mating type is a simple
form of sex
• Mating types are
determined by two alleles
of a single gene: MAT-A
and MAT-a.
• Crosses between the two
haploid mating types
produces diploid
meiocytes then
ascospores
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Reciprocal crosses
Which parent is ”female”?
“Females”: give cytoplasm
• Reciprocal crosses of wild type Neurospora with
a mutant for growth called poky
• The fungi are also carrying either ad+ or ad- (ad
is a nuclear gene for ability to grow on adenine
deficient medium)
“Females”: give cytoplasm
• Mitochondria in the
first cross are from
the cytoplasm of the
parent that has a
poky phenotype
“Females”: give cytoplasm
• Mitochondria in the
second cross are
from the cytoplasm of
the female that has a
normal phenotype
• Mitochondria in the first cross are from the cytoplasm of
the parent that has a poky phenotype
• Mitochondria in the second cross are from the cytoplasm
of the female that has a normal phenotype
• Note the nuclear gene ad is a 1:1 ratio ad+ :ad- as
expected
Nuclear genes
Cytoplasmic or Extranuclear
• Non-Mendelian inheritance pattern
• The results of reciprocal crosses differs
depending on one parent in every
generation
• All progeny both male and female
resemble one of the parents (compare this
to sex-linkage of nuclear genes)
• Extranuclear/cytoplasmic genes cannot be
mapped to chromosomes
Chloroplasts
• Green plants have
chloroplasts in their
cells
• The chromosomes of
chloroplasts are
circular
Text p.55-57
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Results of crossing flowers
White
Green
Variegated
Homoplastic
Egg cell ♀
Phenotype of branch
bearing eggs (♀)
parent
Phenotype of branch
bearing pollen parent
(♂)
Phenotype of progeny
White
White
White
White
Green
White
White
Variegated
Green
White
Green
Green
Green
Green
Green
Variegated
Green
Pollen Zygote (2n)
cell ♂
White
Variegated
Green
Variegated, green,
white,
Variegated
Green
Variegated, green,
white,
Variegated
Variegated
Variegated, green,
white,
Text p.57
Cytoplasmic segregation
Cytoplasm of variegated plants
Variegated :
White
Green
Variegated
• Homoplastic: genetically identical organelles
• Heteroplastic: genetically mixed organelles
– Also known as cytohets or heteroplasmons
Text p.57
Cytoplasmic segregation
Human mitochondrial genes
• Always inherited
maternally
• Mutations in
mitochondrial genes can
give rise to disease if they
accumulate, by
cytoplasmic segregation
or random drift, to such
an extent that the affect
cell function.
Text p.105
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How is this disorder inherited?
A human mitochondrial disease
•
•
•
Cytoplasm male sterility
• Cytoplasmic male sterility in maize is a
mitochondrial trait, sterility results from
mitochondrial plasmids (circular DNA in
the mitochondria)
• The inheritance of mitochondrial alleles is
maternal
• But the expression depends on the
nuclear genotype
A child inherits its mitochondria from the cytoplasm of the egg:
Maternal inheritance
So the condition is always passed on through the mother never the
father
Occasional unaffected children of an affected mother probably
reflect the random assortment segregation in gamete-forming tissue.
Cytoplasmic male sterility
• Restorer of fertility R is a dominant nuclear gene
• R_ male fertility even when cells are homoplastic for
mitochondrial male sterility (s)
• Male sterility if rr AND homoplastic for s
• If rr and normal mitochondria (+) then plant is fertile
♀
RR
X
rr
s
+
+
Male sterile
Male fertile
s
Rr
Text ch.3
• Monoecy: male and
female flowers are
separated e.g.
Maize/corn
• Tassels have pollen
(♂)
• Ear shoots have the
egg (♀)
Cytoplasmic male sterility
• Since Restorer of fertility R is a dominant nuclear gene:
• ½ of the progeny will be Rr and male fertile even though
cells are homoplastic for mitochondrial male sterility (s)
• ½ of the progeny will be male sterile since they have rr
AND are homoplastic for s
♀
♂
+
s
s
Plants: monoecious
s
♂
s
s
s
rr
s
Male fertile
Male sterile
Rr
Male fertile
Plants: hermaphrodites
Plants: dioecious
• Many plant species of conifers and angiosperms
are hermaphrodite: having both male and female
organs
• Many plants are dioecious: individual
plants are either female or male
• Sex determination in the majority
dioecious plants is by sex chromosomes
• Not all dioecious plants have non-identical
chromosomes and those that do have the
XY system where the heterogametic sex is
male, producing anthers only.
s
s
s
s
Rr
X
s
s
s
½ Male fertile
rr
s
s
½ Male sterile
Dioecious plant
Text p.49
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Sex limited traits
• Nuclear autosomal genes that are
expressed only in one sex
– Secondary sex characters
– Milk production
– Breast development in humans
– Male and female plumage in birds
Male
Female
Text ch.3
Course Overview
Outline
Week
1
2
3
4
5
6
7
8
9
10
11
12
Topic
Course objectives and Introduction to genetics
Human Pedigrees
Patterns of Inheritance: sex-linkage
Chromosomal basis of inheritance
Changes in chromosome number
Gene Mapping
Gene to Phenotype
Modified Mendelian ratios
Model organisms and mutants
Genetics of Plant Development (Arabidopsis)
Genetics of Animal Development (Drosophila)
Behaviour Genetics/Quantitative genetics
Chapter
Ch. 1 & Ch. 2
Ch. 2
Ch. 2
Ch. 3
Ch. 15
Ch. 4 (Ch. 16)
Ch. 6
Ch. 6
Ch. 6 (Ch. 16)
Ch. 18
Ch. 18
Ch. 16 + papers
Evaluation
• Tutorial Quizzes/Assignments worth a total
of 15%
• 4 Term Tests worth a total of 50%:
– January 30
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