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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
The Importance of the Environment
The environmental influences the expression of the genotype so the
phenotype is altered.
Hydrangea flowers of the same genetic variety range in color from blueviolet to pink, depending on the acidity of the soil.
Multifactorial; many factors, both
genetic and environmental,
collectively influence phenotype in
examples such as skin tanning
Chromosome Theory of Inheritance
Improved microscopy techniques, understand cell processes and genetic
studies converged during the late 1800’s and early 1900’s.
It was discovered that Mendelian inheritance has its physical basis in the
behavior of chromosomes during sexual life cycles.
Walter S. Sutton
Theodor Boveri
Hugo de Vries
Pedigree analysis reveals Mendelian patterns in human inheritance
In these family trees, squares symbolize males and circles represent
females. A horizontal line connecting a male and female (--) indicates a
mating, with offspring listed below in their order of birth, from left to right.
Shaded symbols stand for individuals with the trait being traced.
Disorders Inherited as Recessive Traits
Over a thousand human genetic disorders are known to have Mendelian
inheritance patterns. Each of these disorders is inherited as a dominant or
recessive trait controlled by a single gene. Most human genetic disorders are
recessive.
A particular form of deafness is
inherited as a recessive trait.
Many human disorders follow
Mendelian patterns of inheritance
Cystic fibrosis, which strikes one
out of every 2,500 whites of
European descent but is much rarer
in other groups. One out of 25
whites (4% ) is a carrier.
The normal allele for this gene
codes for a membrane protein that
functions in chloride ion transport
between certain cells and the
extracellular fluid. These chloride
channels are defective or absent.
The result is an abnormally high
concentration of extracellular
chloride, which causes the mucus
that coats certain cells to become
thicker and stickier than normal.
Tay-Sachs disease is caused by a dysfunctional enzyme that fails to break
down brain lipids of a certain class. Is proportionately high incidence of TaySachs disease among Ashkenazic Jews, Jewish people whose ancestors
lived in central Europe
Sickle-cell disease, which affects one out of 400 African Americans.
Sickle-cell disease is caused by the substitution of a single amino acid in
the hemoglobin protein of red blood cells
Dominantly Inherited Disorders
Achondroplasia, a form of dwarfism with an incidence of one case among
every 10,000 people. Heterozygous individuals have the dwarf phenotype.
Huntington’s disease, a degenerative disease of the nervous system, is
caused by a lethal dominant allele that has no obvious phenotypic effect
until the individual is about 35 to 45 years old.
Sex-Linked Disorders in Humans
Duchenne muscular dystrophy, affects about one out of every 3,500 males
born in the United States. People with Duchenne muscular dystrophy rarely
live past their early 20s. The disease is characterized by a progressive
weakening of the muscles and loss of coordination. Researchers have traced
the disorder to the absence of a key muscle protein called dystrophin and
have tracked the gene for this protein to a specific locus on the X
chromosome.
Posture changes during
progression of Duchenne
muscular dystrophy.
Hemophilia is a sex-linked recessive trait defined by the absence of one or
more of the proteins required for blood clotting.
Color Blindness In Humans: An X-Linked Trait
Numbers That You Should See If You Are In One Of The Following
Four Categories: [Some Letter Choices Show No Visible Numbers]
Sex-Linked Traits:
1. Normal Color Vision:
A: 29, B: 45, C: --, D: 26
2. Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -3. Red Color-blind:
A: 70, B: --, C: 5, D: 6
4. Green Color-Blind:
A: 70, B: --, C: 5, D: 2
Pattern Baldness In Humans: A Sex Influenced Trait
Baldness is an autosomal trait and is apparently influenced by sex hormones
after people reach 30 years of age or older.
In men the gene is dominant, while in women it is recessive. A man needs
only one allele (B) for the baldness trait to be expressed, while a bald woman
must be homozygous for the trait (BB).
What are the probabilities for the children for a bald man and
woman with no history of baldness in the family?