Download Genetics

Chris Khan 2006
Genetics
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Domestic animals used to be wild creatures; they evolved.
Mendel observed pea plants, plants that can be short or tall. Mendel spent 8-10 years on this experiment and then tried again
while applying the Laws of Mathematics. Today, we use Mendel’s Laws.
DeVries experimented with fruit flies and found info similar to Mendel’s.
1st Law—Law of Dominance- in a cross between two pure, contrasting traits, only one of these will appear in the next
generation [Dominant trait]; the one that does not appear is the Recessive trait.
Mendel crossbred short and tall plants and the product was F1 tall; therefore, tall is DOMINANT and short is RECESSIVE.
But, tall plants carry short genes [Recessive genes]; if tall + tall = short, that is F2.
IF tall = “TT” and short = “tt” and those two individuals mate, 100% of the time will make a heterozygous Dominant gene of
tall that looks like “Tt”, so it carries the Recessive but is tall because the “T” is Dominant over the “t.”
IF “Tt” mates with “Tt,” then 75% of the offspring will be tall [25% homozygous Dominant and 50% heterozygous] and 25%
will be short [homozygous Recessive].
Stating homozygous and heterozygous or pure and hybrid is stating the genotype. But, stating “tall and red” or “short and
pink” states the phenotype.
In genetics, a test cross is used to determine whether an individual that displays a Dominant phenotype is homozygous or
heterozygous. The individual is crossed with a homozygous Recessive individual. If all offspring display the Dominant
phenotype, the individual in question is homozygous Dominant; if the offspring are split equally between the Dominant and
Recessive phenotype, the individual is heterozygous. An example of a test cross would be if you had a “TtYy” individual,
which was tall and yellow [heterozygous]. You would cross it with the homozygous Recessive of short and green [ttyy] and
you would make one side of the Punnett Square have TY, Ty, tY and ty and the other side [top] would have just a ty {because
that’s all you can get from a “ttyy”}. You would get TtYy, Ttyy, ttYy, and ttyy.
2nd Law—Law of Segregation- genes occur in pairs, which separate when gametes are formed; only one gene of each pair
goes to a gamete. {Disjunction}
3rd Law—Law of Independent Assortment- when di-hybrid individuals are crossed, the factor for each trait is distributed
independently of the factors for all other traits.
Let’s say you cross a homozygous Dominant tall and wrinkled plant with a homozygous Recessive short and smooth plant.
This is “TTWW x ttww.” The only gamete that can come from TTWW is “TW,” so we keep that and the only gamete that
can come from ttww is “tw,” so we keep that and they come together to make TtWw [the F1]. You mate two of these TtWw
individuals and when making the Punnett Square, you take all of the possibilities from these TtWw’s to make 16 squares.
The F2 would be TW, Tw, tW, and tw on one side of the Punnett Square and the same thing on the other side [top] of the
Punnett Square. When you do this Punnett Square, you will get a 9:3:3:1 ratio meaning 9/16 will have both Dominant
genes, 3/16 will have one Recessive and one Dominant gene, 3/16 will have the other Dominant and the other Recessive gene
and only 1/16 will have both Recessive genes.
Probability Rule: The chances of two events occurring simultaneously are the product of the chances probability. If you
were trying to find out the probability of getting a TtSSWW out of a TtSsWw breeding with another TtSsWw, you would do
Punnett Squares for each letter. First, Tt x Tt = TT, Tt, Tt, and tt; that is a 2/4 chance of getting the Tt you want. Then, Ss x
Ss makes SS, Ss, Ss, and SS; that is a ¼ chance of getting the SS you want. Lastly, Ww x Ww makes WW, Ww, Ww and
ww; that is a ¼ chance of getting the WW you want. Then, to find the probability, just multiply 2/4 x ¼ x ¼ to get 2/64.
Therefore, there is a 2 in 64 chance of getting a TtSSWW out of two TtSsWw’s.
4th Law—Law of Incomplete Dominance {Codominance expresses both alleles }- is a heterozygous condition in which
both alleles at a gene locus are partially expressed, often producing an intermediate phenotype (i.e. pink out of red and
white). If you did RR x ww, you would get four Rw’s. With the Law of Dominance, that would make all red species, but
with the Law of Incomplete Dominance, that would make all pink hybrid species. If you mated Rw with Rw, you would
get RR, Rw, Rw and ww; that is 25% red, 50% hybrid pink and 25% white.
Multiple Alleles—many possibilities (i.e. blood type).
Blood Types: A, B, AB, and O.
O is the Universal Donor; AB is the Universal Recipient.
The Red Blood Cell has receptor proteins such as A in A, B in B and so on.
Blood A has anti-B, B has anti-A, O has anti-A and anti-B and AB has no anti. Antibodies are found in plasma and fight off
stuff. The proteins in blood A have antibodies for type B, so transfusion to type A of type B blood will kill the type A
recipient or vice-versa.
Rh Factor—if you have it, you are Rh+ [Dominant] and if you don’t, you are Rh-.
IA = A gene; IB = B gene; i = O gene; IA IB = AB gene.
Blood cells MUST be compatible with the plasma of the recipient.
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In a cell, the chromosome is found on the locus. There are two genes per trait. Alleles are found on this locus and they
determine what you look/act like.
Albino—being albino means that one’s body does not make enough pigmentation [color] and one is very pale and has poor
eyesight. This is Recessive.
The blood of a mother and her baby do NOT mix.
Different types of genes: Multifactorial—many factors, genetic and no”n, contribute to the gene; Mendelian—follows
Mendel’s Laws; Polygenic- many genes contribute to it.
Karyotyping—to look at genes to see chromosomes to find out things such as results to a Chromosome Abnormality test.
Genes are linked when they have the same genes on the same chromosome.
Females [XX] have 22 autosomes [non-sex chromosome] and 2 homologous pairs; Males [XY] have 22 autosomes and 2
non-homologous pairs.
In humans, the Y decides gender, but in species such as fruit flies, the number of X’s determines gender. Bees determine
gender based on “N’s”. Reptiles determine based on temperature when born.
List of Sex-Linked Traits—baldness, red-green colorblindness, hemophilia. These are also Recessive {X-Linked}.
When using a Punnett Square to determine if diseases will be passed on or carried, make the XX in a female X hX, with h
being the disease.
A pedigree chart is a chart, which tells you all of the known phenotypes for an organism and its ancestors, most commonly
humans, dogs and horses.
Males have a disease if they have it on their X chromosome because there is no corresponding gene; so one defective gene is
enough. But females carry the disease if it is on one X chromosomes because they have another.
Sex-Linked traits go from male  daughter  son of the daughter.
Pleiotropism-- The control by a single gene of several distinct and seemingly unrelated phenotypic effects. [i.e. A cat is deaf
if it has white fur and blue eyes]
When a sperm fertilizes an egg cell without a chromosome, it becomes a 45-chromosome cell. This is “45XO,” which occurs
because of an abnormal number of chromosomes. This result is Turner’s Syndrome, while creates a sterile female who is
short and has some webbed skin.
When a sperm fertilizes an egg with an extra chromosome, it results in “47X[X or Y][X or Y],” which results in a Trisomy.
45YO cannot exist because a human NEEDS at least one X chromosome.
47XYY- Supermale- aggressive; tall and thin; has extra Y chromosome.
A syndrome is a collection of symptoms.
Aneuploidy-- The state of having a chromosome number that is not a multiple of the haploid number. Polyploid—cells or
organisms that contain more than two copies of each of their chromosomes.
A Barr Body is the condensed, inactive X-chromosome found in the nuclei of somatic cells of most female mammals.
Chromosome Abnormality Symptoms
Down Syndrome—Trisomy 21—extra #21 chromosome; characterized by “47, 21.” Results in mental deficiency,
hypotonia, open mouth, short head, wide fingers.
Edwards Syndrome—Trisomy 18; characterized by “47, X[X or Y]; results in: small size, single umbilical artery, bigger
skull, small lower jaw, hypertonicity; overlapping fingers; rocker bottom feet; mental deficiency.
Turner’s Syndrome—Monosomy X; characterized by “45, XO”; results in small stature, webbed posterior neck, low
posterior hairline, hyperconvex, streak ovaries, cardiac abnormalities.
Klinefelter Syndrome— characterized by “47, XXY”; results in hypogonadism, intertility, long limbs, sex chromatin
positive.
Human Genetic Disorders
Cystic Fibrosis—Autosomal Recessive; mucus forms in lungs and interferes with breathing and digestion. (Europeans)
Tay Sachs—Autosomal Recessive; enzyme deficiency; degenerative disorder of the nervous system. (Eastern European
Jews)
Thalassemia—Autosomal Recessive; anemia due to a defective synthesis of hemoglobin. (Greek and Mediterranean)
Phenylketonuria [PKU]—Autosomal Recessive; enzyme deficiency, the PHE will not become TYR; controlled by diet;
build up of phenyalanine causes brain damage. (ALL)
Huntington’s Disease—Autosomal Dominant; late onset; progressive physical and mental deterioration. (ALL) {One
becomes a “vegetable” –Mrs. Walsh)
Sickle Cell Anemia—Autosomal Recessive; abnormal hemoglobin; if you have the heterozygous, you are immune to
malaria. (Blacks)
Hemophilia—sex-linked Recessive; enzyme deficiency; poor clotting. (ALL)
When talking about evolution, we look at the population. A population must be same species and must live in the same
location.
Gene Pool—genes in a given population.
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Hardy-Weinberg Equilibrium—once a gene is in the population, the percent with it will remain constant IF: 1) there is a
large population, 2) no migration, 3) no natural selection, 4) no mutations and 5) must be random mating.
P  frequency of Dominant gene. Q  frequency of Recessive gene.
Use these formulas to find percent of homozygous and heterozygous Dominants and Recessives in a population with basic
algebra: [BRING CALCULATOR]
o p2 + 2pq + q2 = 1
o p+q=1
MUTATIONS contribute to evolution. Mutations can be lethal [kill babies before birth or after birth] or not. Some types of
mutations are: deletion, inversion, translocation, and duplication. These are point mutations.
Gene Therapy is used to change genes; Genetic Engineering inserts a gene into a cell’s active part but it needs plasmids
[DNA in a prokaryotic {no chromosomes} cell] and a restriction enzyme.
Amniocentesis—checks for diseases by drawing fluid from amniotic fluid by taking a CVS, or a chorionic villus sampling.
Blunt-End mutations in a cell’s DNA sequence cuts the sequence right down the middle like while Sticky End mutations cut
the sequence kind of like an L shape.
Genes can be Structural Genes, Regulated Genes or Operators. All three of these things when together make up an
Operon.
Structural Genes synthesize proteins into enzymes. Each “SG” makes enzymes and they do this job as a chain.
The Operator tells the “RG” what to do based on the end product [of ATP]. If there are too many ADP’s, then the Operator
tells the RG to tell the SG’s to make more phosphates to make more ATP.
Promoters turn on and Repressors turn off [stuff].
Cytoplasmic Inheritance-- inheritance of traits through DNA that is not connected with the chromosomes but rather to DNA
from organelles in the cell.
Mitochondrial DNA-- is DNA, which is not located in the nucleus of the cell but in the mitochondria. They are from one’s
mother and are inherited from the cytoplasm of the egg cell.
Genomic imprinting is the phenomenon whereby a small subset of all the genes in our genome is expressed according to
their parent of origin. Some imprinted genes are expressed from a maternally inherited chromosome and silenced on the
paternal chromosome; while other imprinted genes show the opposite expression pattern and are only expressed from a
paternally inherited chromosome. Contrary to expectation, 'imprints' can act as a silencer or an activator for imprinted genes.
Hormones influence the growth of a baby in the uterus.
Transposon-- a segment of DNA that is capable of independently replicating itself and inserting the copy into a new position
within the same or another chromosome or plasmid.
Two genes can exchange DNA with each other over a __________ [don’t ask].
Evolution by Changes in the Gene Pool:
o Mutations—{see above somewhere}
o Migration—move from one place to another
o Gene Flow—transfer of genes from one population to another.
o Genetic Drift—random fluctuations in the frequency of the appearance of a gene in a small isolated population,
presumably owing to chance rather than natural selection.
o Non-Random Mating—self-explanatory.
When trying to match up blood, one extracts DNA [though there is a limited amount] which is barely enough to test and they
will use PCR [polymerase chain reaction] to make “copies” of the DNA from the insufficient amount of blood.
Rather than changing water heat when using PCR to suit different enzymes, scientists now use heat-resistant enzymes.
RFLP-- intraspecies variations in the length of DNA fragments generated by the action of restriction enzymes and caused by
mutations that alter the sites at which these enzymes act, changing the length, number, or production of fragments.
Agarose-- a polysaccharide obtained from agar that is the most widely used medium for gel electrophoresis procedures.
Electrophoresis—a method of separating substances, especially proteins, and analyzing molecular structure based on the rate
of movement of each component in a colloidal suspension while under the influence of an electric field.
Ethidium bromide-- is an intercalacting agent commonly used as a nucleic acid stain in molecular biology laboratories for
techniques such as agarose gel electrophoresis. When exposed to ultraviolet light, it will fluoresce with a red-orange color,
intensifying almost 20-fold after binding to DNA. Because it binds to DNA, ethidium bromide is a very strong mutagen, and
may possibly be a carcinogen or teratogen.
Probe-- substance, such as DNA, that is radioactively labeled or otherwise marked and used to detect or identify another
substance in a sample.
Restriction Enzyme-- any of a group of enzymes that catalyze the cleavage of DNA at specific sites to produce discrete
fragments, used especially in genetic engineering.