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Genetics Part 3
Modes of Inheritance
Remember…
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Humans have 23 pairs of chromosomes
Two types of chromosomes:
 Autosomes: all chromosomes
except sex chromosomes
 Sex Chromosomes: X and Y
Inheritance can be categorized according
to the type of chromosome from which the
trait comes.
(You don’t have to write this down).
In humans (and fruit flies),
= Female
=
Male
It is the presence of a Y chromosome that
determines if an individual will be male or not.
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Autosomal Dominant: trait
controlled by a dominant allele
located on one of the autosomes
Examples:
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Huntington’s Disease (brain disorder that causes mental illness))
Dwarfism
Polydactyly (extra fingers or toes)
A person with an autosomal dominant disorder can
have a HH or Hh genotype
Just because a trait is dominant does NOT mean it is
more common in a population!
Examples of people from a variety of human races who exhibit the
albino trait.
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Autosomal Recessive: trait controlled by a recessive
allele on one of the autosomes – Examples:
Albinism (no pigmentation)
 Cystic Fibrosis (excessive mucus production
in lungs)
 Sickle cell anemia (red blood cells are shaped like a
“C” instead of round)
A person with an autosomal recessive disorder must have a
homozygous recessive genotype (hh).
Carrier: a heterozygous person who does not have the disorder, but
carries the recessive allele so that it can be passed on to future
generations (Hh)
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(You don’t have to write this down).
Sex chromosomes
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Differ in appearance
Only small parts are homologous
(carrying same genes)
Sex-linked genes
–
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Only 20 genes on Y
–
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Found only on one of the sex
chromosomes
Related to male characteristics.
1500 genes on X
–
Related to a variety of traits (few are
gender-related).
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Sex Linkage: inheritance from a gene
located on one of the sex chromosomes
Y-Linked: inheritance from a gene on the Y
chromosome – only affects males
 The Y chromosome has very few genes
that cause an observable phenotype so
there is not a lot of y-linkage.
–
Ex: hypertrichosis pinnae (hairy ears)
Write this down!
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X-Linked: inheritance from a gene on the X
chromosome
Most X-linked disorders that we know of are Xlinked recessive – Examples:
 Colorblindness
 Hemophilia
 Muscular Dystrophy
 Pattern Baldness
Muscular
dystrophy
Male baldness: XbY
Female baldness: XbXb
Why is it more
common for
males to be
affected by an Xlinked disorder
than a female?
ARE YOU COLORBLIND?
http://www.toledo-bend.com/colorblind/Ishihara.html
http://www.toledo-bend.com/colorblind/Ishihara.html
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© Archimedes' Lab
How they are written:
Sex-Linked Recessive: Females who
are XaXa and males who are XaY will
express/show the recessive
disease/trait being studied.
 A female who is XAXa is a carrier for
it, but will not express it.
 Males have only one X and cannot be
carriers.
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Genetics Part 4
Scientists observe how traits are inherited by studying phenotypes
of a certain species from generation to generation
One tool scientists have developed to study inheritance is a
pedigree.
Pedigree: A family tree diagram that shows
how a trait is inherited over several
generations.
Draw all of the symbols and their
meanings!
Interpreting a Pedigree Diagram
1) Determine if the pedigree chart shows an
autosomal or X-linked disease.
 If most of the males in the pedigree are
affected, then the disorder is X-linked
 If it is a 50/50 ratio between men and
women the disorder is autosomal.
Interpreting a Pedigree Chart
2) Determine whether the disorder is
dominant or recessive.
 If the disorder is dominant, one of the
parents must have the disorder.
 If the disorder is recessive, neither
parent has to have the disorder because
they can be heterozygous.
Examples: Pedigree #1
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Is it Autosomal or X-linked?
Answer:
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Autosomal
Examples: Pedigree #1
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Is the trait Dominant or Recessive?
Answer:
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Dominant
Examples: Pedigree #2
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Is the trait Autosomal or X-linked? Dominant
or Recessive?
Answer:
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Autosomal Recessive
Examples: Pedigree #3
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Is the trait Autosomal or X-linked? Dominant
or Recessive?
Answer:
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X-linked Recessive