1. Topic 3 Genetics
Genetics is the science of how inherited information is passed on from one generation to the next, using the DNA in genes.

2. 3.1 Genes
A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
Heritable means passed from parents to offspring, characteristic means a trait, like eye or hair color.
Humans have an estimated 21,000 genes, organized into chromosomes.

3. Where genes are located
A gene, for a particular trait, is always located at a specific spot called its locus (plural loci).
Scientists are mapping the loci of all the genes.
Scientists know that the gene for making the protein transducin is located on chromosome 1.
If you have the mutated version, you can’t see in color. There is only a single base difference.

4. Alleles
Variations or versions of a gene. Usually different by only a few base pairs.
While there are different alleles, they are always located at the some locus on everyone's DNA.
Example: Cystic Fibrosis is a inherited disease causes by a mutated gene on chromosome 7.
The normal allele produces the correct amount of mucus in your body, the mutated allele produced too much.

5. Base changes
Just one base change can create a mutation that causes disease.
Another example is a gene that determines the type of ear wax you have (cerumen). There are two versions (alleles) of this gene (chromosome 16), one causing dry ear wax and the other causing wax that is more fluid.
Wet earwax is more common in Europeans and Africans, dry more common in Asians

6. This gene also is involved in the production of breast milk with a possible link to breast cancer.
Different versions of genes, alleles, are created by mutation.
Sickle cell disease is a good example.

7. Mutations A random, rare change in genetic material.
A substitution mutation would involve a wrong base being substituted for the correct one, like in sickle cell disease.
Some mutations are good. There is a mutation that prevents people who have it from being able to become infected by the HIV virus.
Whether a mutation is good, bad or neutral depends on what they are AND the environment.

8. The gene that allows people to digest lactose is a mutation.
Believed to have originated in Europe, so most Europeans and North Americans are not lactose intolerant.
Gene therapy involves putting a good gene into someone who needs it. Dangerous, ethical issues.

9. Sickle cell disease Caused by a single base substitution mutation.
The gene is responsible for making the protein hemoglobin.
The mutation causes the hemoglobin protein to have a different shape, causing the red blood cells it is located in to also have a different shape.
Instead of glutamic acid, valine is added to the chain. Oxygen can’t be carried as efficiently, the cell shape causes clots.

10. Sickle cell disease
If the 2 alleles you have are both normal, you have normal hemoglobin and normal shaped red blood cells.
If the 2 alleles you have are both the bad ones, you have sickle cell disease.
If you have one of each, you are said to have the sickle cell trait and do show some signs of the disease but not nearly as severe.
Having one or two sickle cell alleles prevents malaria.

11. Genome
The complete set of a organism’s base sequences. In humans, about 3 billion bases.
Some organisms have been completely sequenced, fruit fly and E. coli.
How do geneticists figure out a genome?
Sanger Technique – DNA samples are chopped into pieces, and copies are made (PCR)
Primer added to start replication

12. Each piece is put into DNA polymerase and the 4 nucleotides so that replication can occur
Each time, a special tagged base is tested which, if added to the chain, will stop replication.
The pieces will all end with the tagged nucleotide and all the pieces are run through gel electrophoresis.

13. Human Genome Project
Began trying to sequence the human genome in 1990.
In 2003, they announced they had sequenced the entire human genome.
They are now working on what each gene does. As this happens, scientists know the locus of individual genes and what they do.
Before the Project, they knew where fewer than 100 genes were that caused disease, they now know 1400.

14. Information learned from the Human genome project can help scientists study evolutionary patterns and how we migrated over the past.
If we know where the genes are for making a certain protein, we can then use that gene in the lab to make molecules for people who can’t make them due to genetic abnormalities.
Carl Woese – Studying mutations in genes discovered Archaea
You can’t patent a gene

15. 3.2 Chromosomes
Because prokaryotes reproduce by binary fission, they have only one chromosome
Organisms that reproduce sexually will have two of every chromosome because the organism would get them from each parent.
Some prokaryotes and archaea have a plasmid, eukaryotes don’t.
Plasmids can replicate independently

16. Eukaryote Chromosomes
Chromosomes are made of DNA which is the genetic material of the cell.
The DNA is wrapped twice around 8 histone proteins (2 each of 4 diff types) forming an area called a nucleosome. Sometimes a ninth histone
The DNA is attracted to the histones because DNA has a negative charge and the histones have a positive charge.
The areas of nucleosomes won’t transcribe so the nucleosomes help to regulate the transcription process.

17. Multiple Chromosomes
Eukaryotes have more than one chromosome (usually), and there will always be two of each one.
Humans have 46 chromosomes, 2 sets of 23.
We get a set of 23 from each parent so we have 2 number one chromosomes, 2 number two chromosomes, etc.
These sets of two are called homologous chromosomes. The #1 from mom and the #1 from dad are homologous.

18. Homologous Chromosomes
Same genes in the same locus (loci), but not necessarily the same alleles.
Diploid is a term used to describe a cell that has 2 of each chromosome. Most cells in our body are diploid. (46 chromosomes)
Haploid is a term used to describe a cell that only has 1 of each chromosome, the sex cells or gametes. (23 chromosomes)
n represents the haploid number and 2n the diploid number

19. Chromosome number is a defining feature of a species
Some cells in a species can be different (n).
Some individuals can be born with less or more (non-disjunction)
Some cells don’t have any chromosomes, like red blood cells.

20. Karyogram/Karyotype
A karyotype is the specific number and appearance of the chromosomes of an individual.
A karyogram is a picture of the chromosomes.

21. Steps for making a karyogram
Cells are stained and put on a slide to look at under a microscope
A picture is taken of a cell in metaphase
Chromosomes are cut/pasted into order by size, largest to smallest, and position of centromere.
Exception is the 2 sex chromosomes which are shown last.

22. Sex Determination The sex chromosomes are X and Y.
The X is longer and contains many more genes
Unlike the other 22 pairs, the sex chromosomes can be very different.
XX = female (physically) - gametes will all be X
XY = male (physically) gametes will be X and Y
Male determines sex of child

23. Any chromosome that is NOT a sex chromosome is called an autosome.
Humans have 22 pairs of autosomes and one pair of sex chromosomes. (normally)
Traits described as autosomal would have their genes on one of the 22 pairs of autosomal chromosomes.
Traits described as sex linked would have their genes on one or both of the 2 sex chromosomes.

24. Autoradiography
X-rays use radioactivity to create an image of an object.
Autoradiography uses radioactivity being given off by an object to create an image.
Can be used to get images of DNA so its length can be measured.
Cairns’ technique uses radioactive material called radio markers. Radioactive form of Thymine using 3H
In 1962 used to prove bacteria DNA is a loop

25. 3.3 Meiosis
Gametes have only one set of chromosomes (n). That way, when an egg and a sperm combine, you get 46 (2n).
Meiosis is the process where a diploid cell (2n), creates 4 haploid gametes (n).
Mitosis is 46 (2n) nucleus creating two 46 (2n) nuclei.
Meiosis is 46 (2n) nucleus creating four 23 (n) daughter cells

26. DNA is replicated prior to meiosis, so each of the 46 chromosomes looks like an x , two sister chromatids connected by the centromeres.
The parent cell must divide twice during meiosis, the first division creating 2 cells that are haploid (n).
The second division seeing the 2 haploid each divide into 4 haploid.

28. Crossing Over
During the first division of meiosis, the homologous chromosomes pair up side by side.

29. Crossing over allows the mothers DNA to mix with the fathers DNA
This increases variation within the gametes
The new chromatids are called recombinant chromatids.

30. Random Orientation
During metaphase of the first division, the homologous chromosomes are going to line up, side by side in the middle of the cell.
How they line up is random.
This also increases variation within the gametes

31. Division I Prophase I Chromosomes coil and become visible
Nuclear membrane begins to break down
Spindle fibers form, nucleolus disappears
These steps are exactly the same as mitosis
Homologous chromosomes are attracted to each other.
Crossing over occurs

32. Division I
Metaphase I
Homologous chromosomes line up along the equator by random orientation.
Nuclear membrane is gone
Spindle fibers grow toward the chromosomes

33. Division I
Anaphase I
Spindle fibers attach to centromeres of chromosomes and pull them toward the poles.
Telophase I
Spindle fibers disintegrate.
Chromosomes uncoil, nuclear membranes form.
Many plants don’t have Telophase I
Cytokinesis occurs and created 2 haploid (n) cells

34. Division II Basically this is mitosis Prophase II
DNA condenses into visible chromosomes
Spindle fibers are produced, nucleolus disappears.
Metaphase II
Nuclear membrane gone
Individual chromosomes line up randomly on the equator
Spindle fibers attach to the centromeres

35. Division II Anaphase II
Centromeres split and sister chromatids move toward the poles.
In animals, cell membrane begins to pinch in, in plants, cell plates begin to form
Telophase
Chromosomes unwind, nuclear membranes form.

37. Fertilization and variation
Crossing over during Prophase I and random orientation during Metaphase I create variation among gametes.
Sexual reproduction also creates variation because of which of the different sperm cells fertilizes the egg cell.
The number of different gametes a human could produce is or 8,388,608. That doesn’t include crossing over.

38. Extra or missing Chromosomes
Sometimes organisms end up with less or more chromosomes than normal.
Example in humans is Down Syndrome, where a child is born with 47 chromosomes instead of 46 because they have an extra #21.
Caused by Non-disjunction, usually during Anaphase I.
Causes some eggs to have 2 #21 chromosomes and others to not have any.

39. Studies of families with children who have Down Syndrome (epidemiological studies) show that the age of the mother shows the highest correlation.
The older the mother is, especially after 30, the greater the odds.
A karyotype will show whether a child has a chromosome issue or not.

40. Obtaining Cells for Karyotyping unborn
Two different procedures
Amniocentesis uses a needle to extract amniotic fluid which will contain cells
Chorionic Villus Sampling obtains cells from the placenta’s finger-like projections called villi
In both cases, the fetal cells are grown in the lab and a karyotype is created.