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Genetics
Gregor Mendel: "father of genetics"
Blending Theory of Inheritance - offspring of two
parents "blend" the traits of both parents
Particulate Theory of Inheritance - traits are
inherited as "particles", offspring receive a
"particle" from each parent.
Evidence for Particulate Theory of Inheritance: A plant
with purple flowers is crossed with another plant that
has purple flowers. Some of the offspring have white
flowers (wow!). Mendel set out to discover how this
could happen.
Some stuff on Mendel
* parents were farmers
* he became ordained as a priest
* studied science and mathemathics at the
University of Vienna
Mendel's Experiments
Mendel chose pea plants as his experimental subjects,
mainly because they were easy to cross and showed a
variety of contrasting traits (purple vs white flowers,
tall vs short stems, round vs wrinkled seeds)
1. Mendel chose true-breeding lines of each plant/trait
he studied (true breeding lines always produced
offspring of the same type)
2. He crossed a true breeding plant with a plant of the
opposite trait (purple x white). He called this the
Parental (P) generation.
3. He recorded data on the offspring of this cross
(First Filial, F1)
4. He self pollinated the F1 offspring
5. He recorded data on the offspring of the second
generation, calling it the Second Filial generation
(F2)
Analysis:
* The F1 generation always displayed one trait (he
later called this the dominant trait)
* The F1 generation must have within it the trait
from the original parents - the white trait
* The F2 generation displayed the hidden trait, 1/4
of the F2 generation had it (he later called this
hidden trait the recessive trait)
* Each individual has two "factors" that determine
what external appearance the offspring will have. (We
now call these factors genes or alleles)
Mendel established three principles (or Laws) from his
research
1. The Principle of Dominance and Recessiveness - one
trait is masked or covered up by another trait
2. Principle of Segregation - the two factors (alleles)
for a trait separate during gamete formation
3. Principle of Independent Assortment - factors of a
trait separate independently of one another during
gamete formation; another way to look at this is,
whether a flower is purple has nothing to do with the
length of the plants stems - each trait is
independently inherited
Modern Genetics
Mendel's factors are now called ALLELES. For every
trait a person have, two alleles determine how that
trait is expressed.
We use letters to denote alleles, since every gene has
two alleles, all genes can be represented by a pair of
letters.
PP = purple, Pp = purple, pp = white
Homozogyous: when the
individual is said to
Letters designating a
capital or lowercase,
AA, bb, EE, dd
alleles are the same, the
be homozygous, or true breeding.
homozgyous individual could be
as long as they are the same. Ex.
Heterozygous: when the alleles are different, in this
case the DOMINANT allele is expressed. Ex. Pp, Aa
Monohybrid cross = a cross involving one pair of
contrasting traits. Ex. Pp x Pp
Punnet Square: used to determine the PROBABILITY of
having a certain type of offspring given the alleles of
the parents
Genotype: letters used to denote alleles (BB, Pp..etc)
Phenotype: what an organism looks like (brown,
purple..)
How to Solve a Punnet Square
1. Determine the genotypes (letters) of the parents. Bb
x Bb
2. Set up the punnet square with one parent on each
side.
3. Fill out the Punnet square middle
4. Analyze the number of offspring of each type.
In pea plants, round seeds are dominant to wrinkled.
The genotypes and phenotypes are:
RR = round
Rr = round
rr = wrinkle
If you get stuck make a "key". Sometimes the problems
won't give you obvious information. Example: In
radishes, a bent root is a dominant trait, though some
roots are straight (which is recessive). If a straight
rooted plant is crossed with a heterozyous bent root
plant, how many of the offspring will have straight
roots?
Say what? First of all, this problem doesn't
make it easy. Start by assigning genotypes and
phenotypes. Pick the letter of the dominant trait. B
for bent
BB = bent
Bb = bent
bb = straight Now use the key to figure out your
parents. In this case you have a straight root plant
(bb) crossed with a heterozyous bent plant (Bb). Once
you've figured that out, the cross is simple!
If a heteroyzous round seed is crossed with itself (Rr
x Rr) a punnett square can help you figure out the
ratios of the offspring.
Incomplete Dominance & Codominance
There is no dominant or recessive, the heterozygous
condition results in a "blending" of the two traits.
Example: Snapdragons can be red, white, or pink
(heterozygous)
You cross a heterozygous pink flowering plant (Rr) with
another heterozygous pink flowering plant (Rr).
Diagram a punnett square that shows this cross and the
possible offspring.
Dihybrid Crosses: Crosses that involve 2 traits.
For these crosses your punnet square needs to be
4x4
In any case where the parents are heterozygous for
both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio.
If you cross other combinations, you will need to
do a square. Try RrYy x rryy. Click to see solution.
Sex Linked Traits
The genes for these traits are on the X chromosome,
because boys only receive one X chromosome they are
more likely to inherit disorders passed to them from
their mother who would be a carrier.
Hemophilia and Colorblindness are sex linked
traits, the punnet square below shows how a woman who
is a carrier passes the trait to her son, but not her
daughters.
Muliple Allele Traits
Traits that are controlled by more than two
alleles. Blood type in humans is controlled by three
alleles: A, B, and O
Phenotype
Genotype
A
IaIa
B
IbIb or Ib i
AB
IaIb
O
ii
Iai
Examples of Blood type crosses
Blood Transfusions
Blood can only be transferred to a body of a person
who's immune system will "recognize" the blood. A and B
are antigens on the blood that will be recognized. If
the antigen is unfamiliar to the body, your body will
attack and destroy the transfused blood as if it were a
hostile invader (which can cause death).
O is like a blank, it has no antigens. O is called
the universal donor because a person can receive a
transfusion from O blood without having an immune
response
AB is the universal
AB blood has both the A
body, A and B blood can
well as O) and the body
attack.
acceptor, because a person with
and B antigens already in the
be transfused to the person (as
will recognize it and not
Polygenic Traits
Traits controlled by many genes: hair color, height
weight, intelligence
Sex Influenced Traits
Traits are influenced by the environment. Pattern
baldness affects men because testosterone activates the
genes.
Environmentally Influenced Traits
Siamese cats have dark ears and feet due to the
temperature. Height in humans is influenced by the
environment (diet)