Classical (Mendelian)
Genetics
Gregor Mendel
Vocabulary
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Genetics: The scientific study of heredity
Allele: Alternate forms of a gene/factor.
Genotype: combination of alleles an organism has.
Phenotype: How an organism appears.
Dominant: An allele which is expressed (masks the
other).
• Recessive: An allele which is present but remains
unexpressed (masked)
• Homozygous: Both alleles for a trait are the same.
• Heterozygous: The organism's alleles for a trait are
different.
History
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Principles of genetics were developed in the
mid 19th century by Gregor Mendel an
Austrian Monk
Developed these principles without ANY
scientific equipment - only his mind.
Experimented with pea plants, by crossing
various strains and observing the
characteristics of their offspring.
Studied the following characteristics:
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Pea color (Green, yellow)
Pea shape (round, wrinkled)
Flower color (purple, white)
Plant height (tall, short)
Made the following observations (example
given is pea shape)
When he crossed a round pea and wrinkled
pea, the offspring (F1 gen.) always had
round peas.
When he crossed these F1 plants, however,
he would get offspring which produced
round and wrinkled peas in a 3:1 ratio.
Laws of Inheretance
• Law of Segregation: When gametes
(sperm egg etc…) are formed each
gamete will receive one allele or the other.
• Law of independent assortment: Two or
more alleles will separate independently of
each other when gametes are formed
Punnett Squares
• Genetic problems can be easily solved
using a tool called a punnett square.
– Tool for calculating genetic probabilities
A punnett square
Monohybrid cross
(cross with only 1 trait)
• Problem:
• Using this is a several step process, look
at the following example
– Tallness (T) is dominant over shortness (t) in
pea plants. A Homozygous tall plant (TT) is
crossed with a short plant (tt). What is the
genotypic makeup of the offspring? The
phenotypic makeup ?
Punnet process
1. Determine alleles of
each parent, these
are given as TT, and tt
respectively.
2. Take each possible
allele of each parent,
separate them, and
place each allele
either along the top,
or along the side of
the punnett square.
Punnett process continued
• Lastly, write the letter
for each allele across
each column or down
each row. The
resultant mix is the
genotype for the
offspring. In this case,
each offspring has a Tt
(heterozygous tall)
genotype, and simply a
"Tall" phenotype.
Punnett process continued
• Lets take this a step
further and cross these
F1 offspring (Tt) to see
what genotypes and
phenotypes we get.
• Since each parent can
contribute a T and a t to
the offspring, the
punnett square should
look like this….
Punnett process continued
• Here we have some more
interesting results: First
we now have 3
genotypes (TT, Tt, & tt) in
a 1:2:1 genotypic ratio.
We now have 2 different
phenotypes (Tall & short)
in a 3:1 Phenotypic
ratio. This is the
common outcome from
such crosses.
Dihybrid crosses
• Dihybrid crosses are made when phenotypes
and genotypes composed of 2 independent
alleles are analyzed.
• Process is very similar to monohybrid crosses.
• Example:
– 2 traits are being analyzed
– Plant height (Tt) with tall being dominant to short,
– Flower color (Ww) with Purple flowers being dominant
to white.
Dihybrid cross example
• The cross with a pure-breeding (homozygous)
Tall,Purple plant with a pure-breeding Short,
white plant should look like this.
F1 generation
Dihybrid cross example continued
• Take the offspring and cross them since they are
donating alleles for 2 traits, each parent in the f1
generation can give 4 possible combination of alleles.
TW, Tw, tW, or tw. The cross should look like this. (The
mathematical “foil” method can often be used here)
F2 Generation
Dihybrid cross example continued
• Note that there is a 9:3:3:1
phenotypic ratio. 9/16
showing both dominant traits,
3/16 & 3/16 showing one of the
recessive traits, and 1/16
showing both recessive traits.
• Also note that this also
indicates that these alleles are
separating independently of
each other. This is evidence of
Mendel's Law of independent
assortment
Chromosomes and Classical Genetics
• Walter Sutton in 1902 proposed that
chromosomes were the physical carriers of
Mendel's alleles
• Problems arose however regarding the following
question:
• Why are the number of alleles which undergo
independent assortment greater than the
number of chromosomes of an organism?
• This was explained understanding of 2
additional factors; Sex Linkage and crossing
over
Sex Linkage
• All chromosomes are homologous except on
sex chromosomes.
• Sex chromosomes are either X or Y.
• If an organism is XX, it is a female, if XY it is
male.
• If a recessive allele exists on the X
chromosome. It will not have a corresponding
allele on the Y chromosome, and will therefore
always be expressed
Sex linkage example
• Recessive gene for white eye
color located on the Xw
chromosome of Drosophila.
• All Males which receive this
gene during fertilization (50%)
will express this.
• If a female receives the Xw
chromosome. It will usually
not be expressed since she
carries an X chromosome with
the normal gene
Human Sex Linkage
• Hemophilia:
– Disorder of the blood
where clotting does not
occur properly due to a
faulty protein.
– Occurs on the X
chromosome, and is
recessive.
• Thus a vast majority of
those affected are males.
– First known person known
to carry the disorder was
Queen Victoria of England.
Thus all those affected are
related to European royalty.
Hemophilia and Royalty
Other Factors: Multiple Alleles
• Phenotypes are controlled by more than 1 allele. Eg. Blood types
are regulated by 3 separate genes.
• ABO Blood typing
– Humans have multiple types of surface antigens on RBC's
– The nature of these surface proteins determines a person's
Blood Type.
– There are 3 alleles which determine blood type IA, IB, or IO. This
is referred to as having multiple alleles
– Human blood types are designated as A, B or O.
• Type A denotes having the A surface antigen, and is denoted by IA
• Type B denotes having the B surface antigen, and is denoted by IB
• Type O denotes having neither A or B surface antigen, and is
denoted by IO
– There are several blood type combinations possible
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A
B
AB (Universal recipient)
O (Universal donor)
Blood & Immunity
• A person can receive blood only when the donor's blood type does
not contain any surface antigen the recipient does not. This is
because the recipient has antibodies which will attack any foreign
surface protein.
• Thus, Type AB can accept any blood types because it will not attack
A or B surface antigens. However, a type AB person could only
donate blood to another AB person. They are known as Universal
Recipients.
• Also, Type O persons are Universal donors because they have NO
surface antigens that recipients' immune systems can attack. Type
O persons can ONLY receive blood from other type O persons.
• There is another blood type factor known as Rh.
• People are either Rh+ or Rh- based on a basic dominant/recessive
mechanism.
• Not usually a problem except with pregnancy.
• It is possible that an Rh- mother can carry an Rh+ fetus and develop
antibodies which will attack & destroy the fetal blood
• This usually occurs with 2nd or 3rd pregnancies, and is detectable
and treatable.
Other Factors: Incomplete Dominance
• Some alleles for
a gene are not
completely
dominant over
the others. This
results in
partially masked
phenotypes
which are
intermediate to
the two
extremes.
Other Factors: Continuous Variation
• Many traits
may have a
wide range of
continuous
values. Eg.
Human height
can vary
considerably.
There are not
just "tall" or
"short" humans
Other Factors
• Gene interaction:
– Many biological pathways are governed by multiple
enzymes, involving multiple steps. If any one of these
steps are altered. The end product of the pathway
may be disrupted.
• Environmental effects:
– Sometimes genes will not be fully expressed owing to
external factors. Example: Human height may not be
fully expressed if individuals experience poor
nutrition.
The
Average
American
Phenotype