Chapter 14
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In the 1800s the popular inheritance
theory was “blending”--offspring were a
mixture of their parents
◦ this suggests that organisms will
become uniform over time (we know this
isn’t true)
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Mendel had a “particulate” theory (genes)
◦ this was observed through his
observations of pea plants
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He carefully planned all his breeding experiments,
taking careful notes on the results.
his experiments started with true-breeding
varieties
◦ then followed the offspring for 2 generations.
 (P, F1, and F2)
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Through thousands of crosses, Mendel’s observations
led to 2 fundamental principles of heredity.
◦ Law of Segregation
 two alleles separate during gamete formation
(meiosis) and end up in different gametes
 dominant and recessive alleles
 two heterozygous parents crossed always have a
phenotypic ratio of 3:1 (Punnett Squares)
◦ Law of Independent Assortment
 each pair of alleles segregates independently of
each other pair of alleles during meiosis
 the chance of inheriting one trait from either parent
is separate from all other traits
 for typical Medelian inheritance only
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Certain patterns of inheritance are more complex than those
discovered by Mendel (either controlled by one gene or 2+
genes)
When trait is controlled by a single gene...
◦ Complete Dominance--classic Mendelian patterns (strictly
dominant or recessive)
◦ Incomplete dominance--neither allele is completely dominant
(blending in heterozygous phenotype)
 flower color
◦ Codominance--two alleles shown independently in
heterozygous phenotype
 animal coloration
Codominance
Incomplete
Dominance
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multiple alleles-when a gene for a specific trait
has more than two alleles. Results in multiple
phenotypes.
◦ This usually works in combination with incomplete
or codominance
 Human ABO blood groups
 Rabbit Fur Color
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pleiotropy--when a gene has multiple
phenotypic effects.
◦ Single gene affects multiple things in an
organism.
◦ Most genetic diseases present this way
 Cystic fibrosis and Sickle Cell anemia
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Lethal Genes: a gene that leads to the death
of the organism when inherited in
homozygous genotype (either dominant or
recessive)
◦ Dwarfism in humans (dominant allele)
◦ Manx cats (recessive)
◦ Yellow coat color in mice (dominant)
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When a trait is determined by two or more
genes...
◦ epistasis-the phenotype at one locus alters the
gene at a second locus
 Interaction of two genes to control a single
phenotype, does not have an additive effect
 Might mask another gene, or cause a completely
new phenotype
 Labrador Retrievers and coat color
 2 genes: E (pigment) e (no pigment) ; B (black), b
(brown)
◦ polygenic inheritance--an additive effect of two or
more genes on a single phenotypic character
 Many genes working together to determine a
particular trait
 skin color, height, weight, hair color, eye color in
humans
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When inheritance depends on
chromosomes...
◦ sex-linked traits--specific
traits are carried on the X or Y
chromosome.
 results in some traits affecting
boys more often than girls
 X-linked traits: carried
on X chromosome
 females carriers;
males have trait or not
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Colorblindness, baldness, sickle-cell anemia,
hemophilia, Duchenne muscular dystrophy all
are examples of sex-linked traits.
If a normal-sighted woman whose father was
colorblind marries a colorblind man, what
percentage of their sons will be colorblind?
Daughters?
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Chromosome Number
◦ During meiosis chromosomes can fail to split
evenly (nondisjunction)
 Aneuploidy
◦ Results in severe phenotypic changes in an
individual
◦ Diagnosed via karyotype
◦ Down Syndrome (trisomy 21)
◦ Klinefelters Sydrome (XXY)
◦ Turner Syndrome (X)
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Chromosome Structure
◦ Sometimes parts of chromosomes are altered
during cell division or altered due to environment
 Deletion: missing piece
 Duplication: extra piece
 Inversion: attach upside down in homologous pair, or
within chromosme
 Translocation: piece joins non-homologous
chromosome
 Cri du chat: deletion chromosome 5
 Leukemia: translocation (chromosome 9 attaches to
22) “Philadelphia Chromosome”
 Fragile X: duplication (repeat at end of X)
Good Morning AP Bio!
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Today we are going to discuss our last type of
inheritance pattern (linked genes)…then
practice solving some of those problems.
Reminder: Test corrections are due tomorrow!
You will have time tomorrow to work through
and finish your genetics practice problems
packet (due Monday)
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Remember: crossing over occurs during
meiosis, when chromosomes trade alleles
◦ Produces “recombinant chromosomes”
Some genes are located very closely on a
chromosome, and are usually inherited
together.
They are called “linked genes”
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linked genes: genes located near each
other on the same chromosome are often
inherited together
◦ genes do not assort independently, so
ratio of offspring varies depending on
location of genes
 result in genetic recombination
(offspring with traits different from
parents)
 This lack of independent assortment
indicates the genes are on the same
chromosome.
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Thomas Morgan and his grad student first
discovered linked genes in drosophila (fruit
flies).
When crossing a heterozygous wild-type fly
(b+b vg+vg) to a black body, vestigial wings
(b vg) he discovered allele frequencies that
didn’t match the prediction
◦ 83% parental types, 17% recombinant types
◦ Identified that crossing over had occurred.
http://www.bozemanscience.com/geneticrecombination-gene-mapping/
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The recombination frequency (%) is the same
as the map units (distance) between genes on
a chromosome
◦ Less than 50% recombination = same chromosome
 We can use this information to map genes
 Smaller number = closer together
◦ Greater than 50% recombination = different
chromosome
 Not able to map
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The crossover frequency (recombination)
between genes E and F is 6%, between E and G
is 10% and between F and G is 4%.
Determine the sequence of genes on the
chromosome.
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Environmental
Influence
◦ Nature vs.
nurture
◦ Expression of
traits determined
by environmental
influences
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Nonnuclear Inheritance (mitochondria and
chloroplasts)
◦ These organelles have their own DNA that
replicates separately from nuclear DNA
◦ Follows non-mendelian inheritance
◦ All your mitochondrial DNA (mDNA) is from your
mom!
 “mitochondrial diseases”—result from mutations in
mDNA
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Genomic Imprinting
◦ Phenotype depends on if allele is inherited from
mom or dad (autosomal)
 Allele from either parent is “silenced” by the
presence of other allele
 Example of epigenetics
◦ Affects very few genes, not common
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Chimera
◦ Single organism
composed of
genetically distinct
traits
 Two genomes, one
organism!
 Results from multiple
fertilized eggs fusing
during development
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Used to visually trace traits within human
families (helps identify inheritance patterns)
◦ Circle= female
◦ Square = male