A. Rules of probability
1. Rule of multiplication
a. how do we determine the chance that two or more
independent events will occur together in a specific
combination?
b. compute probability for each event then multiply
independent probabililities to obtain the overall probability
2. Rule of addition
a. a. used to find the probabilility of an event that can occur
in two or more different ways is the sum of the
separate probabilities of those ways
Example: TtRr X TtRr
• The probability of getting a tall offspring
is ¾.
• The probability of getting a red offspring
is ¾.
• The probability of getting a tall red
offspring is
¾ x ¾ = 9/16
Comment
• Use the Product Rule to calculate the
results of complex crosses rather than
work out the Punnett Squares.
• Ex: TtrrGG X TtRrgg
• What is the probability that you would get
a Tall plant with Wrinkled (dom) Green
seeds?
Solution
What is the probability that you would get a
Tall plant with Wrinkled (dom) Green
seeds?
“T’s” = Tt X Tt =
“R’s” = rr X Rr =
“G’s” = GG x gg =
Product is:
3. Rules can be combined
a. Trihybrid cross
PpYyRr x Ppyyrr
What is the probability that offspring will exhibit
recessive phenotypes for at least two of the 3
traits?
Possibilities:
ppyyRr,
ppYyrr,
Ppyyrr,
PPyyrr,
ppyyrr,
B. Extending Mendelian Genetics
1. Incomplete Dominance
2. Codominance
3. Multiple alleles
4. Pleiotropy – “pleion” (Greek for “more”)
a. one gene affecting many phenotypes
5. Epistasis – (Greek for “standing upon”)
a. a gene at one locus alters phenotype of a gene at a second locus
b. Mice coat color and pigment deposit (color) or not (albino)
6. Polygenic Inheritance
a. an additive effect of 2 or more genes on a single phenotypic trait
(converse to pleiotropy)
b. skin pigmentation (3 separately inherited genes)
Comment
• Rh blood factor is a separate factor from
the ABO blood group.
• Rh+ = dominant
• Rh- = recessive
• A+ blood = dihybrid trait
Skin Pigmentation
Polygenic Inheritance
Quantitative character
Summary for chapter 14
• Know the Mendelian crosses and their
patterns.
• Be able to work simple genetic problems
(practice).
• Watch genetic vocabulary.
• Be able to read pedigree charts.
• Understand “carriers” and be able to work
problems with these.
• Know Huntington disease and sickle cell
Chapter 15 Highlights
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Sex linked traits
Duchenne Muscular dystrophy
Hemophilia
Color blindness
Seen more in males…why?
X inactivation in Females
• One of the X chromosomes is inactivated in each cell of
a female during embryonic development.
• Inactive X is condensed and called a “Barr body”…lies
along the inside of the nuclear envelope.
• Occurs randomly and independently (females consist of
a “mosaic” of 2 types of cells)
• Example: Tortoiseshell cat and Calico cats
• How is the X chromosome inactivated”
– Modification of the DNA (attachment of a methyl group to
nucleotides) chapter 18
– XIST gene
Mapping the distance btw genes using
recombination data
• Recombination frequencies depend on the
distance between genes on a
chromosome
– Can create a “linkage map” from this data
– Distances btw genes are “map units”
– A map unit is 1% of recombination frequency
• Genes are linked if they are on the same
chromosome
How you tell
if genes are
linked
A Test Cross to
determine
linkage and if
linkage, what is
the percentage
of recombination
so you can
“map” the
genes.
They
don’t look
like the
parents
1. Abnormal # of Chromosomes: Nondisjunction
a. members of a chromosome pair fail to separate
b. Meiosis I
c. Meiosis II
d. Monosomic vs. Trisomic
e. Abnormal # of sex chromosomes (“aneuploidy”)
f. polyploidy (triploidy 3n or tetraploidy 4n)
2. Alterations in chromosome structure
a. Deletion
c. Inversion
b. Duplication
d. Translocation
Know the following disorders and
causes for the disorders:
• down syndrome
• Klinefelter syndrome
•Turner syndrome
• cri du chat
Genomic Imprinting
• The differential expression of genetic material depending
on whether it is inherited from the male or female parent
• Occurs during meiosis and results in the silencing of one
allele of certain genes.
• Example: mouse gene Igf2…only the paternal gene is
expressed (it had methyl groups attached to cytosine
nucleotides) exception to the rule that methylated DNA is silenced (not
expressed)
• Most imprinted genes are critical for embryonic
development. (mouse experiment)…normal
development requires embryonic cells have exactly one
copy of certain genes.
Organelle genes
• Cytoplasmic genes = genes outside of the
nucleus
• Mitochondria, chloroplasts, other plant
plastids
• Contain small circular piece of DNA
• Distributed to offspring from the maternal
parent (egg)
• Mitochondrial diseases (Myopathy, Leber’s
hereditary optic neuropathy)