1. Organelle Genetics Maternal inheritance Non-Mendelian inheritance
Ratios do not fit those proposed by Mendel
Cytoplasmic inheritance
Nonchromosomal inheritance
Terms are not necessarily equivalent!
Plasmon = genetic elements of the cytoplasm

2. Examples of non-Mendelian inheritance:
Variegated-shoot phenotypes in four o’clocks (Correns, 1908)
Mixed chloroplasts
White/green
Mutant chloroplast
White
non-photosynthetic
Normal chloroplast
Green
photosynthetic

3. Maternal inheritance
The female phenotype in a cross is always expressed in its offspring
Experiments were performed by Correns on the four o'clock plant
Green, variegated (white and green) or white leaves
Normal flowers develop at different locations on the plant
Crosses were made among the flowers associated with:
Female Male Phenotype Progeny Phenotype Phenotype
Green Green, variegated or white Green
Variegated Green, variegated or white Variegated
White Green, variegated or white White
The progeny cross always exhibited the color of the leaf of the female
Trait expresses maternal inheritance
Most plants, including corn, Arabidopsis, wheat and tomato show strict maternal inheritance
of organellar (chloroplast and mitochondrial) DNA.

4. Maternal inheritance: with occasional paternal transmission

5. Biparental inheritance
Inheritance of variegation suggested both parents contribute, but ratios are non-Mendelian.
Pollen transmission of plastids found for several species
e.g. Pelargonium (geranium), Oenothera, Medicago, Phaseolus, Acacia
Some plants have regular paternal transmission of plastids through pollen.
Nuclear genes can affect “strength” of pollen transmission of organelles.
e.g. Oenothera, Pelargonium (geranium), Phaseolus, Hypericum

6. Origins of mitochondria & chloroplasts
Both mitochondria and chloroplasts are believed to be derived from endosymbiotic bacteria.
Endosymbiotic bacteria = free-living prokaryotes that invaded ancestral eukaryotic cells and established a mutually beneficial relationship.
Mitochondria - believed to be derived from a photosynthetic purple bacterium that entered a eukaryotic cell > billion years ago.
Chloroplasts - believed to be derived from a photosynthetic cyanobacterium.
Many required mitochondria and chloroplast proteins also are coded by nuclear genes.
numtDNA = nuclear mtDNA (mtDNA transposed to the nucleus)

7. Mitochondrial genome size and genic content
Size and gene content of mitochondrial genomes compared with a Proteobacterial (Rickettsia) genome
Circles and lines represent circular and linear genome shapes, respectively. For genomes >60 kbp, the DNA coding for genes with known function (red) is distinguished from that coding for unidentified ORFs and intergenic sequences (blue).
M. W. Gray, G. Burger, and B. F. Lang Science 1999: Mitochondrial Evolution

8. mtDNAs --- Overview
mtDNAs occur in (almost) all aerobic eukaryotic cells & generate energy for cells by oxidative phosphorylation (producing ATP).
Most mtDNA genomes are circular and supercoiled (linear mtDNAs occur in some protozoa and some fungi).
mtDNAs lack histone proteins.
Copy number is high, multiple genomes per mitochondria and many mitochondria per cell (can easily PCR).
Sizes of mtDNA varies widely.
Humans and other vertebrates ~17 kb
(all of mtDNA codes gene products)
Yeast ~80 kb
Plants ~100 kb to 2 Mb
(lots of non-coding mtDNA)

9. Human Mitochondrial Genomes
Human Nuclear Genome: About 30,000 genes on 23 chromosomes (3.3 billion base pairs/haploid cell)
Mitochondrial Genome contains 37 genes:
13 code for some of the proteins involved in oxidative respiration
22 tRNA genes
2 rRNA genes
16,569 base pairs, circular, very compact, filled with genes

10. Human Mitochondrial Diseases
Are maternally inherited: only offspring of affected mothers are affected
Show deficiency in mitochondrial function
Are caused by a mutation in a mitochondrial gene
Examples:
myoclonic epilepsy and ragged red fiber disease (MERRF)
Deafness, dementia, seizures
Point Mutation in a mitochondrial tRNA
Leber’s Hereditary Optic Neuropathy (LHON)
Sudden bilateral blindness
Point mutation in small subunit of NADH dehydrogenase
Kearns-Sayre Syndrome (KSS)
Symptoms in eyes, muscles, heart, brain
Deletion mutation in mtDNA

11. Why aren’t all mitochondrial mutations lethal? Heteroplasmy

12. 70% mutant mitochondria =severe symptoms?
Heteroplasmic Cells
Homoplasmic Cell
70% mutant mitochondria =severe symptoms?
30% mutant mitochondria =mild symptoms
HETEROPLASMY: Normal mitochondria with normal DNA vs Mitochondria with mutant DNA "Homoplasmic Cell. Healthy people have homoplasmic cells -- that is, each cell has normal mitochondrial DNA. People with mitochondrial DNA mutations have heteroplasmic cells. Each cell has a mixture of good and bad mitochondria.

14. Commonly Affected Systems in Mitochondrial Disorders

15. Maize mitochondrial genome“NB” genotype from the inbred B37 line
58 identified genes
33 known proteins
21 tRNAs (for 14 diff aa)*
3 rRNAs
*A tRNA is carried on
a 2 kb linear plasmid
569,630 nt
From Clifton et al.
2004, Plant Physiology

16. What is in the NB maize mitochondrial genome ?
Exons
Introns
rRNAs
ctDNA
tRNAs
???
ORFs
pseudo
Based on genome complexity (one copy of large repeats removed)
520 kb for maize NB
359 kb for rice
From Clifton et al.
2004, Plant Physiology

17. Half the Arabidopsis mitochondrial DNA sequences are both non-coding and novel
- In sugar beet, 55% of mt genome has no recognizable function or database homology
- Only 21% of sugar beet mtDNA is shared with Arabidopsis (Kubo et al. NAR 2000)
Marienfeld, Unseld and Brennicke,1999, TIPS 4: