3. Maternal Effect
1. A phenotype of the offspring is determined not
by the genotype of the offspring but by the nuclear
genotype of the mother, with no influence by the
paternal nuclear genome.
2. mRNA or proteins deposited in the oocyte
before fertilization that direct early development of
the embryo.
3. Eg. shell coiling of the snail where D allele for
coiling to the right (dextral coiling) and the
recessive d allele for coiling to the left (sinistral
coiling).
4. 1:2:1
d/d snails have a coiling
phenotype specified not
by their own genotype,
but by the genotype of
their mother (D/d).
d/d progeny of the F2
d/d female snails. These
F3 snails are sinistral
because their
phenotype reflects the
F2 genotype of their
mother.
X
M F
D/- d/d
6. Basis for the coiling
A mother of genotype D/– deposits a gene product in oocyte
that specify a dextral coiling
D/- dextral coiling in progeny
A mother of genotype d/d codes for a product which is
nonfunctional, and this results in default sinistral coiling
d/d sinistral coiling in progeny
7. Maternal (organellar) Inheritance
1. DNA discovered outside nucleus
2. Organelle – chloroplast and mitochondira
3. Separate genome similar to prokaryote
8. Endosymbiotic Hypothesis
It is thought that chloroplasts and mitochondria
arose from ancient bacteria engulfed by
primitive eukaryotic cells.
The cells developed a symbiotic relationship that
gave the eukaryotic cells the ability to respire
aerobically (mitochondria) and capture light
energy (chloroplasts).
10. Characteristics of organellar inheritance:
1. Typical meiosis-based Mendelian segregation ratio is not found.
2. Results of reciprocal crosses are different.
3. Uniparental inheritance. Mostly maternal inheritance. the
zygote receives most of its cytoplasm from the female parent
and a negligible amount from the male parent.
4. Organellar genes cannot be mapped to the chromosomes in the
nucleus.
5. Organellar inheritance is not affected by substituting a nucleus
with a different genotype.
11. Maternal inheritance vs. Maternal effect
1. Phenotype is
determined by
organellar gene
1. Phenotype is
determined by
nuclear gene
2. Progeny’s
phenotype matches
its own genotype
derived from mother.
Therefore, the
progeny always have
the
maternal phenotype.
2. Progeny’s
phenotype matches
its mother’s nuclear
genotype and may
not match mother’s
phenotype.
12. Properties of organellar genome/DNA.
1. mtDNA and cpDNA are inherited independently
of nuclear DNA
2. Function of mitochondria and chloroplasts is
dependent on both organelle and nuclear DNA
3. Each cell contains as many as several hundred
chloroplasts and/or mitochondria.
13. Leaf Color in Four O’Clocks
Ovule Source
Pollen Source White Green Variegated
White White Green W, G, var
Green White Green W, G, var
Variegated White Green W, G, var
Inheritance is determined by the phenotype of the
ovule (egg).
14. Organelle Inheritance
Mutations in chloroplasts and mitochondria have
been useful in identifying organelle inheritance
patterns.
Inheritance of these mutations was determined by
the makeup of the organelle DNA.
It is important to note that an individual cell has
hundreds of chloroplasts and/or mitochondria,
which may not all be identical.
15. Mitochondrial Depletion Syndrome
Mitochondrial Depletion Syndrome is a maternally
inherited disease characterized by multiple
muscular and neural symptoms, with a wide
variety of severity in different individuals.
16. Mitochondrial Syndromes
The severity of the condition is dependent on
the number of disabled mitochondria present
in the egg.
An egg with a large number of disabled
mitochondria would result in a child with
severe abnormalities
An egg with only a few disabled mitochondria
would result in an individual only mildly
affected.
18. Infectious Heredity
An invading microorganism may exist in a
symbiotic relationship with its host organism.
The invader is then passed on in the maternal
egg cytoplasm (ooplasm) and confers the
beneficial phenotype to the offspring.
19. Infectious Heredity in Drosophila
CO2 sensitivity: Affected flies do not recover
normally from CO2 anesthesia. They become
permanently paralyzed and die.
The condition is due to a sesitivity to a virus,
sigma.
20. Infectious Heredity in Drosophila
Sex ratio: Affected flies produce predominantly
female offspring if reared at 21°C or lower.
The condition is transmitted only to
daughters, not to the small number of males
produced.
The responsible element is a protozoan. When
ooplasm from affected individuals or the
protozoan itself is injected into oocytes of
normal individuals, the temperature-sensitive,
altered sex ratio condition results.
