2. Gene Masking--Epistasis
Epistasis occurs when one gene alters the phenotypic expression of another gene.
This example occurs in the coat color of mice.
Black, brown, albino

4. Polygenic Inheritance
The opposite of pleiotrophy (one gene, many characteristics) is polygenic inheritance which is the case where many genes act on a single characteristic.
For example: skin color is determined by at least 3 separately inherited genes. Variations of the genotype of these individuals produces all of the varieties of skin color we see.

6. Sex Determination
Whether or not a person is male or female is determined from their genotype: XX is female; XY is male.
In humans, the father determines the sex of the baby.
The chance of being a male or female is 50/50. Half of the sperm will inherit a Y, the other half will inherit the X.

7. Sex Determination and the Y Chromosome
The Y chromosome contains a region (SRY gene) which codes for proteins that induce the gonads to form testes.
In the absence of this protein, the gonads form ovaries.

8. Sex Determination and the X Chromosome
Inheriting an X chromosome from dad will give a female 2 X chromosomes.
Only one functions within the cell, the other is inactivated.
It becomes a Barr body.
The Barr body becomes reactivated in gametes so all of them have an active X chromosome when produced.

9. X Inactivation
This process is a totally random event and occurs independently in embryonic cells at the time of X inactivation.
Females consist of a mosaic of active X genes--those derived from the father and those derived from the mother.

10. X Inactivation
As the embryo continues to divide mitotically, then we now have groups of cells with active X chromosomes derived from the mom, and active X chromosomes derived from dad.
If a female is heterozygous for a sex linked trait, approximately 1/2 of the cells will express one gene, and 1/2 will express the other gene.

11. RNA Modification
The eukaryotic RNA transcript now gets modified before it enters the cytoplasm.
The 5’ end of the transcript gets modified before leaving the nucleus--a 5’ cap of nucleotides.
The 3’ end is also modified--numerous adenine nucleotides--called a poly-A tail.

12. Important Functions of the 5’ Cap and Poly-A Tail
They facilitate export of the mature mRNA from the nucleus.
They protect mRNA from degradation by hydrolytic enzymes.
They assist in the attachment of the ribosome to the 5’ end of the mRNA.

13. mRNA Modification
The mRNA is further processed after the ends have been modified--RNA splicing.
The initial transcript (~8000 bp) is reduced (to ~1200 on average).
The large, non-encoding regions of the DNA that get transcribed are spliced out.
Introns--intervening regions are removed.
Exons--expressed regions are kept.

14. mRNA Modification
Some untranslated regions of the exons are saved because they have important functions such as ribosome binding.
RNA splicing occurs via snRNP’s.
snRPs consist of RNA and protein and join together to form a spliceosome which interacts with the intron to clip it out and join the exons together.

16. tRNA Structure and Function
tRNA , like mRNA, is made in the nucleus and is used over and over again.
tRNA has an aa at one end and an anticodon at the other end. The anticodon acts to base pair with the complementary code on the mRNA molecule.

17. tRNA Structure and Function
As tRNA reads the mRNA transcript, it brings an aa to the ribosome and adds it to growing polypeptide.
The 2D shape is similar to a cloverleaf.

18. tRNA
tRNA is about 80 nucleotides long and full of complementary stretches of bases that H-bond to one another giving it a 3D shape like an “L.”

19. tRNA
From this, the anticodon loop comes out of one end and at the other end protrudes the 3’ end that carries the aa.

20. Frederick Griffith An experimental overview:
(S) smooth cells produce mucous capsules that protect the bacteria from an organism’s immune system--pathogenic.
(R) rough cells have no mucous capsule and are attacked by an organism’s immune system--non pathogenic.

21. Frederick Griffith, His Experiment
Mixed heat-killed pathogenic (S) bacteria with living non-pathogenic (R) bacteria, the non-pathogenic (R) bacteria began producing the mucous capsule and became pathogenic (S).
The new bacteria that arose from the bacteria were somehow transformed into pathogenic S. pneumonia.
Griffith called this process transformation.

22. Frederick Griffith, His Experiment

23. Griffith’s Transformation Experiment
Did not identify DNA as the transforming factor, but it set the stage for other experiments.

24. Oswaldt Avery
Avery worked for a long time trying to identify the transforming factor.
After isolating and purifying numerous macromolecules from the heat killed pathogenic bacteria he and his colleagues could only get DNA to work.
The prevailing beliefs about proteins vs. DNA continued to generate skepticism.

25. Nondisjunction
Normally, in meiosis, the chromosomes are distributed without fail and the numbers of chromosomes remains the same throughout the generations.
Occasionally, chromosomes don’t get separated properly in meiosis I or II.
Some gametes fail to receive a copy of a chromosome; others receive 2 copies.