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AP Biology: Mendelian Genetics and Chi Square
Campbell 6th Ed.
MENDELIAN GENETICS
Gregor Mendel 1st to quantify “crossing experiments”
MENDEL’S EXPERIMENTS
Mendelian Experimental Design
1. use pure strains (via self fertilization)
2. use cross fertilization
3. quantitative data (ratio is key)
Results
1. F1 noticed no blending of traits, hence confirmation of dominant / recessive idea (ratio 100% dominant trait
shown)
2. F2 noticed some recessive traits re-emerged showed a phenotypic ratio of 3:1 (actually genotypic ratio of 1:2:1
since recessive trait is present but masked)
“Rules” of Genetics (summary of pg. 249-250 #1-4)
Parents transmit genes, genes combine to form a trait (like purple or white), a chromosome is full of genes each found at at
least one loci (location of the gene) that can be seen on a gene map (fig 15.7). Each version of this gene is called an allele
(like “W” or “w”). Alleles can be dominant or recessive…based on the relationship of the alleles, they are said to be
homozygous or heterozygous. Each allele does not destroy the other when “masking,” yet both alleles are not always
expressed.
-lettering system (less common trait is the letter)
-defn. genotype / phenotype.
Experimental Genetics Principles:
How can I tell homo. dom. vs. a het.?…..Test cross
How? Why do it that way?
1st Law of Heredity (Law of Segregation)- alleles don’t blend (dominant/recessive), 1 allele is passed on in sex cells,
probability determines what the sex cell will get
2nd Law of Heredity (Law of Independent Assortment)- genes sort independently in meiosis meaning the allele for one
trait does not influence the inheritance of an allele of another trait (technically this is true, exceptions exist however due to
gene linkage)
OTHER GENETIC OCCURANCES
(all above, the dominant- recessive thing, is considered “Mendelian Inheritance”) the following are exceptions:
1. multiple alleles- that is, multiple alleles controlling one trait (like ABO blood type)
2. epistasis-in the end, few phenotypes are controlled by only one gene…biochemical cascades, genes producing products
that effect other genes each is interdependent on the other…
brief defn. = many genes controlling one phenotype or trait
3. pleiotropy- an allele may be involved in one or more phenotypes (agouti gene in mice: coat color, obesity, certain
tumors)
4. incomplete dominance- blending of traits
5. codominance- both traits expressed simultaneously
6. environmental effects (arctic fox…color related enzyme)
7. sex linked traits
8. polygenic- similar to epistasis, but they are independent of each other and have an additive effect…it’s not all or none
which is very likely in epistasis
Alan Cotten, Fossil Ridge High School, Keller ISD. – Mendelian Genetics & Chi Square
PUNNETS, ETC.
Monohybrid and dyhibrid crosses…got it…what if I’m looking at 3 or 4 traits??
-rules of multiplying fractions
-rules of adding fractions
Example: S = tall, Y = green, H = hairless
How many SsYyHh would probability predict in the F1 generation if the Parent generation is:
SsYYHh x SsYyHH?
SsYYhh x ssyyHh?
How many Tall, green, hairless folks would you expect to see in each case?
CHI SQUARE
What’s the use?
Example: If I were to flip a coin 100 times. How many heads and how many tails should I get? Obviously, 50 heads : 50
tails
Suppose that I got 51 heads and 49 tails….did I just disprove probability? What about 55:45? What about 60:40? At
what point do we conclude that our coin is likely abnormal or that some other aspect of our experiment is askew? CHI
SQUARE will tell us!
Getting Started
Null Hypothesis vs. Alternative Hypothesis
Null: no statistically significant difference between expected and observed results
Alternative: explanation (hypothesis) of why the results are so different from what’s
expected
Chi Square Equation
 2   ( Fo  Fe ) F
e
2
where Fe = Frequency expected and Fo = Frequency observed
Using the results of our  equation we can compare to the
certainty that our data is legitimate.
2
2
chart we can determine with a specified amount of
P values measure from 0 - 1. P = 1 is perfectly expected data (like 50 heads and 50 tails) meaning that we accept our null
hypothesis. P = 0 is completely unexpected data (like 100 heads and 0 tails OR 0 heads and 100 tails) and we should
create an alternative hypothesis. Most data will fall somewhere in between. In “life science circles”, it is accepted that a P
value of 0.05 or higher is considered acceptable.
One last rule when looking at  chart:
-Degrees of freedom = possibilities - 1. Example: In our coin example we can get heads or tails, so we have 2 possibilities
– 1 = 1 degree of freedom. If we were looking at a 6 sided die, we would have 6 - 1 = 5 degrees of freedom.
2
SAMPLE PROBLEM:
#1 Suppose that you cross Aa x Aa (A = Yellow, a= albino) to create the F1 generation. You get the following results: 75
yellow and 13 albino. Write your null hypothesis…what phenotypic ratio would we expect in the F1 generation? Run a
 2 analysis showing all work and determine if the results support your null hypothesis.
very last fraction calculation!)
Alan Cotten, Fossil Ridge High School, Keller ISD. – Mendelian Genetics & Chi Square
(can use calculator only for the
HUMAN CHROMOSOMES & CHROMOSOME ABNORMALITIES
RECESSIVE INHERITANCE:
Sickle Cell Anemia- “anemias” in general denote poor oxygen carrying capability in blood…many causes…low Fe 2+, low
red blood cell count, or cells that are not shaped properly (sickled cells) due to a single mutation of hemoglobin….SS has
normal cells (can catch malaria), Ss (mostly normal cells (can’t catch malaria), ss (higher death rate from anemia)…hence
the trait is selected for (anti malaria) and against (anemia) in high malaria areas and among descendents of these countries
Cystic Fibrosis- 1 in 20 white people are carriers!! Found on chromosome 7,
-Bad Cl- channel
-getting close to curing possibly…have inserted corrected gene into a virus, infected a mouse with the virus, the
DNA was taken up by the cells and started producing the healthy Cl- channel
-still have quite a way to go, but maybe… soon curable?
Tay Sachs- 1/300,000 general US births, 1/28 of Jewish births,…cause is lack of specific lipid digesting enzyme in
lysosomes found in brain tissue…hence buildup of these fats in the lysosomes until they burst!!
Using Recombination to make gene maps…linked genes are close by and don’t cross over as frequently…
Example: Gene A crosses over with D at a frequency of 30%, C crosses over with A at a frequency of 45%, B crosses
over with A at a frequency of 20%, C crosses over with B at a frequency of 25%, D crosses over with B at a frequency of
50%. Map these genes!
DOMINANT INHERITANCE:
Huntington’s Disease- CAG repeats!
What happens when chromosomes are not properly separated during meiosis?….if it leads to a child with an extra
chromosome (ie. 2 chromosomes in meiosis …hence 3 chromosomes instead of a pair in somatic cells) is called primary
nondisjunction. Resulting in a trisomy
Down Syndrome- (nondisjunction) 3 chromosome #21 (see karyotype fig 15.14)
-1 in every 750 kids effected!!
-likelihood of downs increases with mom’s age (egg shelf life good to about age 30, by age 40-45 she has a 1 in 16 chance
of downs baby)….why doesn’t dad’s age matter??
-oddly, genes on Chromosome 21 causing Downs = related to cancer, alzheimer’s (dementia, senility)
SEX CHROMOSOMES:
In sex chromosome nondisjunction, the X chromosome is most important…Y chromosome has few functional genes (it
must be this way or all females would be messed up!…)
Barr Bodies (fig. 15.10)… “proof” is in calico cats…orange or black is on X chromosome
(XXX) - sterile female (2 Barr Bodies)..otherwise is normal
(XXY) Kleinfelter’s syndrome- sex chromosome nondisjunction….sterile male with feminine characteristics
(XO ) Turner’s syndrome- that’s just a single X , webbed features, no changes really in puberty, so she’s sterile
(XYY) Jacob’s syndrome- once thought linked to criminal behavior (very controversial…most say no good evidence)
Hemophilia- since a biochem. cascade occurs, any missing protein will keep blood from clotting. Some of these proteins
are found on X chromosomes, some not…therefore some (most actually) forms of hemophilia are sex linked. A female
has two chances of getting a “good” X chromosome, a male only 1 chance
Muscular Dystrophy (Duchenne’s)- sex linked to X chr. 1 in every 3500 males!
TYPES OF CHROMOSOMAL ALTERATIONS (fig 15.13)
-Deletion
-Duplication
-Inversion
-Translocation
Alan Cotten, Fossil Ridge High School, Keller ISD. – Mendelian Genetics & Chi Square