Transcript Example

Sometimes,
things don’t always
follow Mendel’s Laws
I. Incomplete Dominance
• The production of a phenotype that is
INTERMEDIATE to that of the two
homozygous parents.
• Neither allele seems DOMINANT or
recessive—both traits are partially
expressed.
Examples
• Four o’clock flowers—A red flower
crossed with a white flower makes a
PINK flower (not red or white, as in
normal dominance/recessiveness)
• With incomplete dominance, a cross
between organisms with two different
phenotypes produces offspring with a third
phenotype that is a blending of the parental
traits.
• I remember Incomplete Dominance in the
form of an example like so:
• RED Flower x WHITE Flower ---> PINK
Flower
Another Similar Example…
Cattle Color: “Red” cattle and
“white” cattle are both dominant
and when crossed—both colors are
partially expressed as in “roan”.
• With codominance, a cross between
organisms with two different phenotypes
produces offspring with a third phenotype
in which both of the parental traits appear
together.
• I remember codominance in the form of an
example like so:
• red x white ---> red & white spotted
This can occur in other animals as well…
Snapdragon Example
Incomplete Dominance in Humans & Plants
• Mendel did not observe this because it
does not occur in peas. It was discovered
when crossing other organisms (the four
o’clock flowers)
• In a Punnett Square or genotype, the
dominant parents are expressed with 2
capital letters (one for each dominant
allele). (The letters are different)
II. Multiple Alleles
• Sometimes, there are more than two
forms a gene can take.
• Example: There are three possible
alleles for blood type (A, B, and O
alleles). Any person still only can
inherit two, one from mom and one
from dad.
Examples
**Blood Type is a special example
because it both demonstrates
Incomplete/Co-dominance
AND
multiple alleles.
Examples
• Blood Type AB
o Blood Types A, B are both dominant
alleles
o The O allele is recessive
o If a person gets both alleles, then
both are expressed and the person
has AB blood type.
Examples
o If a person has blood type A, then either they
inherited two A alleles (AA) or an A and an O
(AO).
o If a person has blood type B, then either they
inherited two B alleles (BB) or a B and an O (BO).
o If a person has blood type O, what would be their
genotype?
OO
o AB blood type would be
AB
o Remember the difference
between genotype and phenotype!
Multiple Alleles & Co Dominance in Blood Type
III. Multiple Genes
• More than one gene can control a
trait. This is called POLYGENIC
INHERITANCE
• The effect of each allele by itself is
small, but the combination of alleles
can produce a wide variety.
• Example: Eye color
– One allele may control pigment production and how this
one interacts with the color gene will vary in individuals.
That is why there is such a variety in eye colors.
•Traits controlled in this way show
much VARIETY in a population.
Other examples of traits controlled by
polygenic inheritance are:
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Fingerprints
height
weight
body build
shape of eyes/ears/lips
There are other traits in other organisms
that are controlled by mulitple genes, too.
Sometimes mutations in genes can cause an unwanted
trait. There are several diseases that can be caused
by mutations in the alleles controlling a particular
trait. Not all mutations produce harmful results.
Recessive Genetic Disorders
• Person must get TWO copies of the
recessive gene to get the disease.
Recessive Genetic Disorders
• Sickle Cell Anemia--red blood cells
are sickle-shaped instead of discshaped
– Can’t carry enough oxygen
– Don’t move through blood vessels easily
– Treated by transfusions of normal cells
or drugs to increase oxygen capacity
Cells are sickle-shaped instead of disc-shaped.
• Cystic Fibrosis--thick mucus instead
of thin fluid in lungs and intestines
– Most common genetic disorder among
Caucasians
– 1 in 20 white people carry a recessive
allele for the disorder
– Four babies born in U.S. with this
disease everyday
Cystic Fibrosis affects the lungs!
People who have it tend not to live very long lives.
Sex Determination
• Sex or gender in many organisms is
determined by X and Y chromosomes
• These chromosomes differ from one
another in shape and size. The Y
chromosome is very short.
• Two X chromosomes will produce a
female and one X and one Y will
produce a male.
Is it a boy or
a girl?
A girl !
23rd Chromosome Pair
Sex-linked Disorders
• Some genetic disorders are “linked” to the
X and Y chromosome and are therefore
called “sex-linked”
• An allele inherited on a sex chromosome is
called a sex-linked gene
• Traits controlled by genes on the X
chromosome show up most often in males
(they inherit the trait from their mothers)
• Males are not carriers for sex-linked
traits…(Females are!)
• Some examples: Colorblindness and
hemophilia
Color Blindness “tests”
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The test below is simpler.
The individual with normal color vision will see a 5 revealed in the dot pattern.
An individual with Red/Green (the most common) color blindness will see
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a 2 revealed in the dots.
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Pedigrees
• A tool used to trace a trait through a
family is called a pedigree
• In pedigrees, a square represents a
male and a female is represented by a
circle.
Pedigrees
• If the square or circle is filled in or
darkened, the person has the trait or
condition.
• If it is half colored or half darkened,
the person is a carrier (has one allele
for the trait, but doesn’t show it)
Sample Pedigree:
Shows how the cystic fibrosis gene (recessive) got passed in this family
Genetic Engineering
• Chemical or biological methods to
change the DNA sequence that
Genetic Engineering Clip
makes up a gene
• Already used to treat diabetes,
improve crop production for
Manipulating Genes
different plants
Pedigree Chart
• A visual tool for following a trait through
several generations in a family.
• Males are represented by a square
• Females are represented by a circle
• Completely filled shape-trait is visible in
the organism
• Half colored shape-carrier of the trait but
not visible
• Empty-the trait is neither visible or a carrier