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Ch. 16
Part 2: Genetics, monohybrid vs.
Dihybrid crosses, Chi Square
•
Genes
– Section of DNA that codes for a specific
protein
•
Alleles
– Dominant: allele that has an affect in a
heterozygous organisms
– Recessive: allele that does not have an
affect in heterozygous organism
– Symbols:
•
•
Use letters where capital letter looks
different than lowercase letter
Genotype
– Homozygous (organism has TWO identical
alleles for a gene…dominant or recessive)
– Heterozygous
•
Phenotype
– Observable characteristics of an
organism; determined by genotype
•
F1 generation
– Offspring resulting from cross between
organism with a HOMOZYGOUS Dominant
parent and a HOMOZYGOUS recessive
parent
•
F2 generation
– Offspring resulting from cross between
two F1 generation (heterozygous
organism)
• Co-dominant
– Both alleles express their characteristic
at the same time
• Multiple Alleles
– When a gene has more than two
versions (alleles)
• Ex. Blood groups
– Caused by genetic mutation: base
substitution, deletion, insertion, etc.
• Sex Inheritance
– Alleles found on sex chromosomes (pair
23)
– Sex-linked gene  gene found on X
chromosome and not matched on Y
chromosome
• Use X chromosome as gene symbol with
superscript
• Mother can be carrier
• Male either has allele or does it have allele
• Ex. FACTOR VIII
– Gene that codes for production of protein
needed for blood clotting
» Dominant allele  produces protein (H)
» Recessive allele  does NOT produce
protein (h)
Genetic Diagrams
• Standard way of
showing the genotypes
of offspring that might
be expected from two
parents
– Monohybrid Crosses
• Diagrams showing
inheritance of ONE gene
• Test Cross
– Cross organism showing
dominant phenotype
with organism that is
homozygous recessive
– Phenotypes of offspring
will guide you to
determine the
phenotype of Dominant
parent (homozygous or
heterozygous)
Dihybrid Crosses
Diagrams that show inheritance of TWO genes at
once
– Independent assortment
• Two ways in which 2 pairs of chromosomes can line up along
equator during METAPHASE 1
– Many cells going through meiosis  chromosomes in half will line up
one way and the other half will line up the other way
– Predict gametes formed from heterozygous cells appear in equal
numbers):
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Interaction between LOCI
• Same Locus Interactions
– Dominant alleles
– Dominant & recessive
alleles
– Multiple Alleles
• Different Loci Interactions
– Genes at one loci affect
inheritance of genes at
another loci
Autosomal Linkage
• When two or
more gene loci
are on the same
chromosome
• Do not sort
independently
• Inherited
together
(usually)
Crossing Over
• Prophase 1 of meiosis 1
• A bivalent (pair of homologous
chromosomes) joined by
CHIASMATA
– Chromatids of bivalent can break
and reconnect to another NONSISTER CHROMATID
• Leads to exchange of gene loci
between maternal and paternal
chromosome
• Cross-over value
– Percentages of offspring that belong
to recombinant class
– Measure of the distance apart of
two gene loci on chromosome
– Smaller cross-over value = closer
gene loci are
– Chance of crossing over DIRECTLY
related to distance apart
• Farther apart = higher chance of
crossing over
Chi-Squared Test
• Statistical test that allows
comparison of observed
results with expected results
to determine if there is a
significant difference between
the two
• Ratios from genetic diagrams
represent PROBALITLITIES of
phenotypes
– Do NOT always come out to
that ratio
• Expected phenotypic ratio vs.
actual phenotypic ratio
Stage 1 of Chi-squared Test
• Complete dihybrid cross for
expected results
• Record expected results on
table
• Observe and record your
actual observed results onto
table
• Calculate difference between
each set of results
– Squaring gets rid of negative
signs (irrelevant)
• Divide each square difference
by expected value
• Add up all answers for x2
value
•
•
•
•
Finding expected value
with ratio:
(using 9:3:3:1)
Take ratio (9/16) and
multiply by total
number of organisms
actually produced
Take ratio (3/16) and
multiply by total
number of organisms
actually produced
Take ratio (3/16) and
multiply by total
number of organisms
actually produced
Take ratio (1/16) and
multiply by total
number of organisms
actually produced
•
Stage 2 of Chi-Squared Test
Degrees of Freedom of Results
– Takes into account the number of
comparisons made
– Calculation of degrees of freedom:
•
•
Degrees of Freedom = # of classes of data – 1
• For dihybrid cross, we have 4
possible resulting phenotypes
(classes of data)
• Degrees of freedom = 4-1 = 3
Look at table of X2 probabilities
– Probabilities in table are the probabilities
that the differences between our
respected and observed results are due
to chance
– Probability of 0.05 is CRITICAL in
biological investigations
• If X2 value represents probability of
0.05 or larger = differences b/t
observed and expected results are
due to chance
• When x2 value is LESS than critical
value for that specific degree of
freedom, it is GOOD!
– NOT SIGNIFICANT DIFFERENCE
• If X2 value represents probability
smaller than 0.05 = differences b/t
observed and expected results are
SIGNIFICANT, reconsider
assumptions about cross
Probability 0.1
means 1 in 10
chance
Critical value tells us how often we
expect to see difference between
observed and expected result
0.05 means 5%
Hypothesis vs. Null Hypothesis
• Null hypothesis is always what is expected to be.
– Looking at a dihybrid cross, our null hypothesis would be our expected results.
– “There is NO significant difference between the observed and expected
frequencies”
• Hypothesis: The loss of my homework is due to an alien abduction.
• Null Hypothesis: The loss of my homework has nothing to do with an alien
abduction.
• When X2 is LESS than critical value, null hypothesis is accepted (GOOD!)
• When X2 is MORE than critical value, null hypothesis is NOT accepted (it is
REJECTED)
– Something other than chance causing observed results
We want Chi-squared value to be LESS than critical value!