Download BMS2042 Extranuclear Inheritance

BMS2042 Lecture 20 – Non-­‐Mendelian Inheritance Most genes follow a Mendelian pattern of inheritance, however some do not. Examples: 1. Extranuclear Inheritance à genes that are not in the cell nucleus 2. Maternal Effect (nuclear genes) 3. Imprinted Genes (nuclear genes) Extranuclear Inheritance also known as “cytoplasmic”, “extra-­‐nuclear” or “uni-­‐parental” inheritance Inheritance of genetic material not located within the nucleus 1. mitochondria – almost all eukaryotes 2. chloroplasts – plants and green algae Have their own chromosomes/genomes. Genome • circular chromosomes o smaller versions of bacterial chromosomes Genes encode products that function within the organelles Human Mitochondrial Genome à • many copies or organelle genome per cell o >1 chromosome per organelles o Many organelles per cell Origin Organelles share many properties with bacteria: • circular chromosomes
• no nucleus or histones
• same form of RNA polymerase to transcribe genes • divide by simple fission ∴endosymbiosis hypothesis arose: ⇒ mitochondria and chloroplasts derived from symbiotic bacteria that initially lived inside cells of ancestors of all eukaryotes (mitochondria), and ancestors of green algae plants (chloroplasts). ⇒ After initial symbiosis, a gradual transfer of some genes from organelle genome to the host nucleus occurred. o Now host and reduced endosymbiont(s) are mutually dependent. and BMS2042 In-­‐
Inheritance heritance • During somatic cell division: mitochondria and chloroplasts separate more or less evenly into daughter cells. • But during sexual reproduction: mostly transmitted through the egg (large, few, immotile), but not the sperm (small, many, motile). o So show strict maternal inheritance every generation. Reciprocal Extranuclear inheritance produces non-­‐Mendelian results in reciprocal crosses Cross Example: • pigmentation trait in 4 o'clock plants. • pigmentation of leaves entirely dependent on the mother: Maternal Inheritance • Phenotype of plant is due to a chloroplast gene o white phenotype due to a mutant allele that prevents green pigment formation.
o Chloroplasts inherited from mother ∴ does not follow Mendelian patterns of inheritance Human Diseases Ø Mitochondrial and Chloroplast DNA inheritance does not follow Mendelian patterns of inheritance Ø Pattern of extranuclear inheritance does not follow Mendelian patterns of inheritance varies between species. o maternal (most common) o bi-­‐parental o paternal (very rare) Some rare human diseases are caused by mitochondrial mutations. example: Leber's Hereditary Optic Neuropathy (LHON) ⇒ affects the optic nerve leading to a loss of vision ⇒ Point mutation in a mitochondrial gene encoding NADH dehydrogenase 4 subunit. BMS2042 Hetero-­‐
plasmy Other diseases: Most of the diseases are involved in energy production. Heteroplasmy • Usually all copies of organelle genome in one cell have the same DNA sequence. • But sometimes different mitochondria with slightly different DNA sequences co-­‐exist within the cytoplasm of a cell. o Presence of multiple types of mitochondria or chloroplasts within a cell is called heteroplasmy. • Variants can arise by mutationà Can produce variation in phenotype within organism. Due to this concept, for mitchondrial diseases, severity of disease is correlated with proportion of abnormal mitochondria. Example: As we can see, there is a “threshold” for phenotypic expression. The disease only manifests once this threshold has been exceeded. BMS2042 Example 2: Variegated Plants: plant leaf pigmentationà patches Most cells wil have both (some green, some whiteà and will appear green) however some will only have green and some will only have white . Reciprocol Cross So, in the ornamental industry,
if they wanted variegated
plants, they would use a female
with variegation instead of a
male as it shows maternal
inheritance