Genetic Maternal Effects and Cytoplasmic Gene (Extra-chromosomal) Inheritance

Question 1

What are Maternal effects?

Answer 1

They are modified phenotypic effects caused by the transmission of maternal products to the progeny through the egg, thus disguising the true genotype of the progeny

Question 2

What are the types of Maternal effects?

Answer 2

• Diminishing effects
• Non-diminishing effects

Question 3

What are Diminishing Effects?

Answer 3

Modified phenotype that results from maternal transmission of nuclear gene products to the progeny.
- temporary
- superseded in time by the phenotype based on the
progeny’s genotype
- maternally inherited gene products disintegrate and
replaced by gene products of the progeny

Question 4

What are Non-diminishing Effects?

Answer 4

Modified phenotype that results from maternal transmission of nuclear gene products to the progeny.
- lasts throughout the life of the individual and is hence
permanent or non-diminishing (lasts 1 generation) for
that individual (not for all subsequent generations)
- true genotype is revealed in next generation

Question 5

What are the diminishing effects in Ephestia kuehniella larvae (flour moth/ weeble)?

Answer 5

*Eye pigment in Ephestia kuehniella - diminishing effect

Wild type larva (Aa) - brown
Mutant larva (aa) - red
Male Female Female Male
Aa x aa Aa x aa
1 Aa : 1 aa 1 Aa : 1 aa
brown red larvae All brown larvae
*After some time
1 Aa : 1 aa
1 brown : 1 red

Question 6

What are the non-diminishing effects in Limnaea peregra (snail) shell?

Answer 6

*Coiling in the snail shell of Limnaea peregra

Male Female Female Male
DD x dd DD x dd
Dd (sinistral) Dd (dextral)
(All progenies are (Sinestral even though they
dextral) have dextral genotype)
Selfed Selfed
1 DD : 2 Dd : 1 dd 1DD : 2 Dd : 1 dd

All dextral (mother’s All dextral (mother’s
genotype) genotype)

*Note: Dextral- rightward coiling and Sinistral- leftward

Question 7

Why do non-diminishing effects occur in Limnaea peregra?

Answer 7

• Maternal effect transmitted through the egg initiates
  cleavage (cutting) of the zygote in a particular
  orientation.
• Once coiling is initiated in Limnaea peregra, even if the
  substance is replaced by true genotype, coiling cannot
  be reversed.
• Hence the term: non-diminishing effect.
• The true genotype is revealed in the next generation.

Question 8

What is Extra-nuclear Inheritance?

Answer 8

When the offspring inherits genes (eg organellar genes) that are not found in the nucleus/ chromosomes of the nucleus.

Not all of the eukaryote cell’s genes are located in the nucleus. Extra-nuclear genes are found on small circles of DNA in mitochondria and chloroplasts. These organelles reproduce themselves.
*Their cytoplasmic genes do not display Mendelian inheritance

Question 9

Where do these Extra-cellular genes usually come from? What organelles are usually passed on?

Answer 9

• Mitochondria
• Chloroplasts
  *You need to be able to identify all the parts

Question 10

What is the structure and organisation of organellar genomes?
Hint: Mitochondria and Chloroplasts

Answer 10

• DNA is naked, not complexed with proteins to form chromosomes
• DNA is a circular double helix (like in prokaryotes), not linear
• DNA molecules are much smaller (have few genes) compared to a nuclear chromosome
• DNA molecule found in multiple copies (640-5760 copies of each organellar gene per cell)

Question 11

What is the function of the Mitochondrial genome?

Answer 11

• Genes for respiration
• Genes for protein synthesis (transcription and translation)

Question 12

What is the function of the Chloroplast genome?

Answer 12

• Synthesis of chlorophyll and other plant pigments, translation (protein synthesis)
• Antibiotic resistance

Question 13

What is the Endosymbiotic Theory?

Question 14

How do the Cytoplasmic genomes behave?
Hint: that is different from nuclear genome

Answer 14

• No meiosis
• No recombination (all the DNA is uniform)
• Only source of variation is through mutation
• Random subset of organelles partitioned into each egg cell
• Male gametes, do not transmit organellar genome to offspring due to little cytoplasm
• Extra- nuclear inheritance is uniparental (maternal inheritance)
• Mendelian ratios are not obtained (due to no meiosis)
• Reciprocal differences since transmission is through maternal line
• Perpetual transmission is through maternal line (variation can only be obtained by mutation)

Question 15

What are the differences between Cytoplasmic genome and Nuclear genome?

Answer 15

Cytoplasmic Nuclear
- Small DNA molecule (16kb) - Large DNA molecules
- Naked DNA molecules - DNA molecules
complexed with proteins
to form chromosomes
- Circular DNA - Linear DNA
- Multiple copies of genome - Only 2 copies of each
(each organellar gene has gene in a diploid species
640-5760 copies)
- Possess few genes (100s) - 21,000 or more genes

Question 16

What are some examples of extra-nuclear inheritance?
Hint: 4 o’clock plant

Answer 16

4 o’clock plant (Mirabilis jalapa)
- Branches can be white, green or variegated (white streaks)
- Type of offspring depends on the branch from which the ovule was derived, not the pollen
- Leaf colour is dependent on the chloroplast contributed through the ovule system
- Sex inheritance -> white female + green male = all white offspring but white male + green female = all green offspring (branches) ie. whatever the female is = offspring

Question 17

What are some examples of extra-nuclear inheritance?
Hint: Chlamydomonas (unicellular alga)

Answer 17

Chlamydomonas (unicellular alga)
- Haploid, has 1 large chloroplast
- Mating results in a diploid that immediately undergoes meiosis
- The mating types are: Mt+ strain (female) and mt- strain (male)
- Str+ and str- strains (str= streptomycin, as antibiotic)
- Str resistance- mediated through the chloroplast