Transcript Real World Genetics - South Kingstown High School

Mendel’s Genetics
• Mendel’s research provided
the groundwork for our
understanding of
inheritance of traits.
Mendel’s Conclusions
• Mendel's four conclusions – although there are exceptions,
his conclusions apply to plants, animals and humans! Many
of our deadly diseases are found to be carried on one
recessive allele.
• First Conclusion: biological inheritance is determined by
factors that are passed form one generation to the next.
Today we know these factors as genes.
• Second Conclusion: Principle of dominance – Some alleles
(forms of a gene) are dominant and some are recessive. An
organism with a dominant allele for a particular form of a
trait will always have that form.
Mendel’s Conclusions
• Third Conclusion: Mendel's Law of Segregation = Each
pea plant has 2 alleles (different forms of a gene) for
each trait. One allele comes from each parent. This
means, that during the formation of each gamete (egg
and sperm), the two alleles must separate or segregate
from each other. Each gamete only carries a single
gene (allele).
• Fourth Conclusion: Principle of Independent
Assortment - genes for different traits can separate
INDEPENDENTLY during the formation of gametes.
Real World Genetics
• MOST traits are not inherited in such a simple
manner as shown by Mendel. For example:
– Many traits are controlled by more than one gene
– Many alleles are neither completely recessive nor
dominant
• We now know a gene is actually a small
section of a DNA molecule.
Exceptions to Mendel
•
•
•
•
Incomplete Dominance
Codominance
Multiple Alleles for a Trait
Multiple Genes for a Trait
Jackalope =
cross between
a jack rabbit
and an
antelope
Exceptions to Mendel
• Incomplete dominance – neither allele is
dominant.
– For example: Red + White sometimes makes …
Incomplete Dominance
• What CONCEPT does red + white = pink appear
Blending
to follow? __________________
• How does this differ from blending?
• Genetics of four-o-clock flowers:
– R = red allele
– W = white allele
RR = red flower, WW = white flower, RW = pink flower
Incomplete Dominance
• Draw a Punnett Square showing a cross between a red
flowering four o’clock and a white flowering four o’clock.
• What are the phenotypes and genotypes of the offspring?
Incomplete Dominance
• Phenotype: All of the
offspring are pink
• Genotype: All of the
offspring are RW
• Draw a Punnett Square
showing a cross between
two pink flowering four
o’clock.
• What are the phenotypes
and genotypes of the
offspring?
Incomplete Dominance
• Is this blending?
• Why or why not? R
• Have the alleles
(R and W) been
“blended away?”
• Now show a cross W
between a pink
and white flower.
R
RR
RW
W
RW
WW
Incomplete Dominance
• Each allele in the example provides
instructions to a cell for making an enzyme.
• The R allele produces an enzyme necessary for
the production of red pigment.
• The W allele produces an enzyme necessary
for the production of white pigment.
• Both alleles are active in incomplete
dominance.
Exceptions to Mendel
• Codominance -neither allele is dominant or
recessive.
• Example:
– variety of cattle with all red hair = RR
– variety of cattle with all white hair = WW
• A cross of a red cow with a white bull
produces offspring (RW) that have both red
and white hair
Codominance
• Other examples:
+
=
Codominance
• Other examples:
+
Exceptions to Mendel
• Multiple alleles: More than two alleles exist
for a trait--however, each individual still only
has two alleles.
Exceptions to Mendel
Example: Hair Color
1. Some alleles, such as brown-ebony, produce enzymes with very high
activity, which in turn produce a large quantity of the brown pigment.
2. Some alleles, such as brown-Swedish-blonde, produce enzymes with very
little activity, which in turn produce only a small amount of the brown
pigment.
3. All alleles fall somewhere on a continuum, from no activity to very high
activity--which geneticists refer to as an allelic series.
4. In hair color, alleles with more activity are dominant to alleles with less
activity. More activity —› more pigment —› hair color phenotype.
• http://www.indiana.edu/~oso/lessons/Genetics/RealColors.html
Multiple Alleles for a Trait
• Example: Variations of a tabby cat
Multiple Alleles for a Trait
• Example: Human Blood
Groups
– Before the 1900’s people
thought all blood was
the same. Fatalities
resulted from
• Transfusions of animal
blood into humans!
• Transfusions of some
human blood to other
humans
Multiple Alleles for a Trait
• It was then discovered there are different
types of red blood cells.
• Human Blood Groups – Three alleles: A, B, O
Multiple Alleles – Blood Types
• What are all of the possible genotypes using
these three alleles? (Hint: Remember each
person only has two alleles for a trait!!)
AA
AO
BB
BO
AB
OO
Multiple Alleles – Blood Types
Genotypes
• AA
• AO
• BB
• BO
• AB
• OO
Phenotypes
Blood Type A
Blood Type A
Blood Type B
Blood Type B
Blood Type AB
Blood Type O
Multiple Alleles – Blood Types
What is the difference
between the
phenotypes?
•Type A = contains one
type of antigen
•Type B = contains a
different type of antigen
•Type AB = contains
both types of antigens
•Type O = does not
contain any antigens
Multiple Alleles – Blood Types
–Antigen: A foreign substance that triggers an
immune response (production of antibodies)
–Antibodies: part of the body’s defense for fighting
off infection
–Example: If a person has Type A blood, the antigen
on his blood cells is NOT foreign to him.  the
body does not produce antibodies.
–Example: If Type B blood is transfused into a
person with Type A blood or Type O blood, the
body will attack the new blood cells as though they
are an infection!
Multiple Alleles – Blood Types
• Which blood type shows codominance?
AB
_________
• Which blood type is the universal donor?
_________
O
• Which blood type is the universal recipient?
_________
AB
Multiple Alleles – Blood Types
• Blood types in the United States are more or
less common based on race and ethnicity.
• However, Type O is most common among all
groups and Type AB is least common.
Rh Factor
• Research conducted on rhesus monkeys.
• Rh is a protein either found in the blood Rh+
or absent Rh-.
• The Rh factor follows dominant/recessive
patterns of inheritance.
Rh Factor
• Mother-fetus incompatibility occurs when
the mother is Rh- (dd) and her fetus is Rh+
(DD or Dd).
• Maternal antibodies can cross the placenta
and destroy fetal red blood cells. The risk
increases with each pregnancy.
• Treatment: If you are Rh-, you will be given a
shot of Rh immunoglobulin during pregnancy.
It prevents the production of Rh antibodies.
Rh Factor
• Rh- : Found in 15% Europeans; 7% African
Americans
• Set up two Punnett Squares showing a cross
between a father who is Rh+ and a mother
who is Rh-.
• What is the possibility of the Rh- mother
producing a fetus that is Rh+?
Multiple Alleles – Blood Types
• Complete Reading on Blood Types
• Discuss articles with your group
• Answer questions
Exceptions to Mendel
• Multiple genes for a single trait:
– Example: height. Height is made up of the length
of different sections of your body added together.
Each section is controlled by a different gene.
– Example: Eye color in humans. Controlled by
interactions of at least 3 different genes.
Multiple Genes for a Single Trait
• Skin color:
– Scientists now believe that 3 genes control skin color.
– You get 3 alleles from your mother and 3 alleles from your
father for 6 alleles.
– If all 6 of the alleles are for dark skin, you will have the
darkest possible skin.
– If you have 5 dark alleles and one light, you will have very
dark skin.
– If you have all 6 light alleles then you will have the lightest
skin possible.
– http://www.dep.anl.gov/
Exceptions to Mendel
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