1. Alterations to Mendel Incomplete or partial dominance Codominance
Multiple alleles and Lethal alleles
Gene interactions & multiple genes
Epistasis and complementation
Effect of environment
Extranuclear inheritance
Sex-linked, sex-limited, & sex-influenced
Sex determination and Gene dosage
Polygenics

2. Complementation Several different genes may contribute to phenotype.
How can you tell whether a phenotype results from one gene or from several?
Create several mutants with the same phenotype.
Perform complementation tests: Cross different mutants. If mutations are in different genes, offspring will have one good copy of each gene, restoring WT phenotype: the mutants complement. If offspring show the mutant phenotype, mutation is in same gene.

3. Visual of complementation
In both genomes, D is not made due to malfunction of enzyme 1. Combining the genomes does not solve this. No complementation because defect is in same gene.
Genome-1: cell can’t make D because enzyme 1 isn’t made. Genome-2 cell can’t make D because enzyme 2 isn’t made. Combining the genomes, both enzyme 1 and 2 are now present, D is made. The genomes complement.

5. Complementation example
Example: bacteria unable to make the amino acid proline.
proB mutants: known mutations in the proB gene.
proA mutants: have mutations in the proA gene, etc.
In which proline gene does “pro-53” have a mutation? To find out, Create merozygotes. These are bacteria that have two copies of the genes of interest (bacteria normally have only one of each).
“Cross” pro-53 with each of the known mutants.

6. Complementation crosses
The unknown mutant (pro-53) is “crossed” with each of the other types of mutants. Does the cross result in a “cure” (the bacterium can now make proline) or is it still defective?
In this cross, DNA from a proA- mutant is added to the pro-53 bacterium.
The presence of this DNA is unable to rescue pro-53 (does not complement), because the mutation is in the SAME gene. The pro-53 must be a proA mutant.

7. The other crosses
pro-53 said to be in same complementation group as other pro-A mutants.
What happens when pro-53 is crossed with a proB mutant?
The added DNA cures pro-53 because now it has good copies of all 3 genes: a+ b+ c+
Do a cross with a proC mutant.

8. Environmental factors
Expression of genes is affected by genetic background (internal environment) but also by the external environment.
Penetrance vs. Expressivity
Penetrance refers to percentage of individuals in a population that show the trait to some extent.
6 fingers is a dominant trait, but…
Expressivity: the degree to which the trait is expressed within an individual.

9. External environment
Numerous examples in other organisms, e.g. sex determination in turtles and crocodiles.
Temperature effects
Conditional mutations, especially temperature sensitive mutations. Permissive vs. restrictive.
Himalayan rabbit, Siamese cat
Lethal genes in microbes
farpoint/saavik1.jpg

10. Internal environment Growth and development
Various human diseases, strike at various times of development. E.g. Huntington disease.
Puberty
Homeostasis
Internal fluctuations of many signals affect a wide variety of genes

11. Mitochondrial Inheritance
Endosymbiont hypothesis: mitochondria descended from endosymbiotic bacteria. Outer membrane separates organelle from cytoplasm. Inner membrane site of ATP synthesis. Inner and outer membranes analagous to Gram negative bacteria.
Mitochondria have their own DNA, ribosomes, tRNA (all bacteria-like) and replicate independently of cell cycle.
RES/Mito/mito.htm

12. Mitochondrial Inheritance-2
Mitochondria well integrated into cell
Some genes for structure & function of the mitochondrion are found in the nucleus.
Not a completely independent entity.
Mutations in Mt DNA cause genetic diseases which reflect function of mitochondria
Major function: ATP generation
Diseases feature breakdown of function in tissues requiring high levels of ATP
Muscle and nerve tissue disorders

13. Mitochondrial inheritance is maternal
During meiosis in females, cell division is asymmetric with most of the cytoplasm including mitochondria going into the eventual ovum.
Spermatozoa are DNA delivery devices with little cytoplasm.
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