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What are other patterns of inheritance?
How are genetic diseases inherited?
(Exceptions to Single-trait Mendelian
Inheritance)
Codominant Traits, Sex-Linked Traits,
& Polygenic Traits
Dominant vs. Recessive Genotypes & Phenotypes
Example of Autosomal Dominant
“Disorders” - Huntington’s disease
Examples of Autosomal Recessive
“Disorders”
• Cystic fibrosis
• Tay Sachs
• Sickle-cell Anemia
How do recessive alleles get passed on? Genetic
Carriers
• Genetic Carrier: an individual that “carries” the
recessive allele, but does not show the trait
Sickle Cell Anemia – Example of Autosomal Recessive
AND Codominant Trait in Humans
• Condition in which there
aren’t enough healthy
red blood cells to carry
enough oxygen
throughout body
• B/C of disorientated
shape Sickle cells can get
stuck into small blood
vessels & block blood
flow
• No current cure for
condition
Diagnosis, Symptoms, & Treatment
• http://www.mayoclinic.org/diseasesconditions/sickle-cellanemia/basics/definition/con-20019348
• http://learn.genetics.utah.edu/content/disord
ers/singlegene/sicklecell/
Sickle Cell is an Autosomal Recessive Disease
Sickle Cell Anemia & Malaria
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Sickle Cell Anemia and Malaria
In the United States, about 1 in 500 African-Americans develops sickle cell anemia. In Africa, about 1 in 100
individuals develops the disease. Why is the frequency of a potentially fatal disease so much higher in Africa?
The answer is related to another potentially fatal disease, malaria. Malaria is characterized by chills and fever,
vomiting, and severe headaches. Anemia and death may result. Malaria is caused by a protozoan parasite
(Plasmodium) that is transmitted to humans by the Anopheles mosquito. When malarial parasites invade the
bloodstream, the red cells that contain defective hemoglobin become sickled and die, trapping the parasites
inside them and reducing infection.
Compared to AS heterozygotes, people with the AA genotype (normal hemoglobin) have a greater risk of
dying from malaria. Death of AA homozygotes results in removal of A alleles from the gene pool. Individuals
with the AS genotype do not develop sickle cell anemia and have less chance of contracting malaria. They are
able to survive and reproduce in malaria-infected regions. Therefore, BOTH the A and S alleles of these
people remain in the population. SS homozygotes have sickle cell anemia, which usually results in early death.
In this way, S alleles are removed from the gene pool.
In a region where malaria is prevalent, the S allele confers a survival advantage on people who have one copy
of the allele, and the otherwise harmful S allele is therefore maintained in the population at a relatively high
frequency. This phenomenon will be examined in the Allele Frequencies and Sickle Cell Anemia Lab, which
relates the change in allele frequency in a population to evolution.
The frequency of the S allele in malaria-infected regions of Africa is 16%. The sickle cell allele is also
widespread in the Mediterranean and other areas where malaria is or used to be a major threat to life. In
contrast, the S allele frequency is only 4% in the United States, where malaria has been virtually eliminated.
Malaria was once common in the United States, but effective mosquito control caused the number of cases
to drop. Recently, however, there has been an increase in the number of malarial cases because of increased
travel, immigration, and resistance to medication. In Southern California there was a 1986 outbreak of nearly
30 cases of malaria transmitted by local mosquitos!
http://chroma.gs.washington.edu/outreach/genetics/sickle/sickle-back.html
What are pleiotropic traits? Why is sicklecell anemia considered by be a pleiotropic
trait?
• Pleiotropic Trait: trait controlled by ONE gene, but can
have multiple effects on an organism
• Ex: Sickle cell caused by mutation in a single gene, but
expression of the allele can have multiple effects on
different organ systems including the skeletal system,
the heart and lungs, and spleen and kidneys.
• Ex: Albinism is caused by mutation in a single gene that
controls melanin. Individuals with this trait do not
produce melanin, which can effect vision in addition to
skin/hair color.
•While some diseases are single gene
(Mendelian trait/disease) that are inherited
on a “dominant vs. recessive” basis, most
genetic diseases contradict Mendel’s
inheritance predictions.
• http://learn.genetics.utah.edu/content/disorders/
•http://www.kumc.edu/AMAMSS/Study/table_of_genetic_disorders.htm
What does it mean for a trait to be
codominant?
• Alleles for a
codominant trait
BOTH affect the
phenotype when
an individual is
heterozygous for
that trait. (AKA:
one allele is NOT
dominant to the
other)
• Results in more
than 2 phenotypes
Predicting Distribution of Codominant
Alleles using Punnett Squares (Testcrosses)
Other examples of codominance
What about traits that have more than just two
alleles? (Multiple alleles)
• Alleles are generated
through mutations
• Alleles differ by just one or
a few bases
• Individuals can only
possess 2 alleles
• Multiple alleles increase
the number (variation) of
different phenotypes in a
given population.
• Multiple alleles can be
dominant, recessive, or
codominant.
Multiple Alleles & Codominance in Human Blood Type
Trait
• http://learn.genetics.utah.edu/content/inheritance/blood/
How does polygenic inheritance contribute to variation in
continuous traits?
How does polygenic inheritance
determine you eye color?
• Read Handouts with Articles about Eye Color
Determination
Pathways for Genotype → Phenotype
Although there are
thousands of traits
that follow simple
Mendelian genetics,
it is actually rather
uncommon for a
trait to be
determined by only
a single gene.
More commonly,
multiple genes
interact with each
other and also with
the environment to
produce very
complex outcomes.
What
numbers
do you
see in
each
circle?
Compare and Contrast X & Y Chromosomes
Pedigree for Hemophilia in Royal Family
How can we use pedigree charts to show AND
make inferences about the inheritance of traits?
• Read Page 181-182
• What do circles
represent?
• What do squares
represent?
• What do roman
numerals
represent?
• What does color
tell you?
Genetic Counseling – How is risk for disease calculated?
Just a few of the human genetic diseases and disorders inherited in
this simple fashion include cystic fibrosis, Huntington's disease, Tay
Sachs disease and achondroplasia (a type of dwarfism). For a
fascinating peek at a vast encyclopedia of human genetic disorders,
check out this link to the site Online Mendelian Inheritance in Man.
Try typing one of the above disorders into the site's search function.
You will get masses of information about the disease's clinical
features and inheritance. There is also a Gene Map features on the
left sidebar that shows the gene's locus (location) on its chromosome.
This is highly technical, but demonstrates the vast quantity of data
collected and compiled by geneticists to date.
• http://omim.org/ Online Mendelian Inheritance in Man®- An
Online Catalog of Human Genes and Genetic Disorders -Updated
16 October 2015
• http://learn.genetics.utah.edu/content/history/geneticrisk/
Review of Definitions