Unit Three
“Cell Proliferation and
Genetics”
“Foundations of Genetics”
Gregor Mendel and the Pea
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Gregor Mendel, a Catholic monk, was the first to
focus on using Quantitative observations (numbers)
to better understand the mechanisms of heredity
Prior to his initial research in the mid 1800’s there
were others before him who carried out similar
crosses
Because Mendel used numbers and kept very good
records, his results were more reliable and less likely
to be refuted
http://www.catholicnewsagency.com/news/googlerecalls-catholic-priest-who-was-father-of-moderngenetics/
Gregor Mendel
Mendel’s Experimental System
1. Mendel selected seven pairs of lines that
differed in easily distinguishable traits (smooth
versus wrinkled, green versus yellow etc.)
 2. He knew from the anecdotal findings of
previous researchers, he would be able to
quantify his results
 3. Pea plants are small, easy to grow, produce
many offspring, and mature quickly
 4. Mendel carried out the fertilization process in
a controlled environment (stop uncontrolled
variables from negatively affecting results)
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Mendel’s Experimental Design
1. Mendel began by letting each variety selffertilize for several generations; this ensured
“True-Breeding”…producing same expression of
trait repeatedly; these lines were called the “P
Generation” (P = Parental)
 2. Mendel then conducted his experiment: crossed
two pea varieties exhibiting different expression of
a trait: green versus yellow pea; the offspring that
resulted were called “F1 Generation” (First Filial)
 3. Lastly, Mendel allowed the plants produced in
the previous cross to self-fertilize; the offspring
that resulted were called “F2 Generation” (Second
Filial)
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Mendel’s Observations
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For each pair of contrasting expressions of a
trait that Mendel crossed, he observed the
same result: an expression of a trait
disappeared in the First Filial generation, but
then reappeared in the Second Filial
generation
In the case of flower color, when he crossed
purple and white flowers, all the First Filial
plants he observed were purple….Mendel
called this “Dominance”, and he called the
disappearance of the white color, “Recessive”
Mendel’s Observations
In the Second Filial generation, he observed there
were some white flowers along with the predominant
color of purple
 The white color (Recessive) physically disappeared
from the First Filial generation, and then reappeared
in the Second Filial generation
 Mendel determined the Recessive white flower color
did not really disappear, but rather, was not expressed
in the First Filial generation
 Mendel was beginning to realize the difference
between “Phenotype” and “Genotype”
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Phenotype
Genotype
3:1
1:2:1
Second Filial Generation
Mendel’s Hypotheses….Theory
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Mendel proposed a series of hypotheses
that later came to be known as the
“Theory of Heredity” (now Laws)
– Hypothesis 1: Parents do not transmit traits
directly but rather transmit coded information
that generates the expression of traits
– Hypothesis 2: Each parent contains two
copies of the trait, both can either be the
same expression of the trait in question or
differing expressions
Mendel’s Hypotheses….Theory
Hypothesis 3: Alternate expressions of a trait are
called “Alleles”; “Phenotype” is the physical
expression of the trait, “Genotype” is the allelic
composition present which is not always the
same as what is physically expressed
Hypothesis 4: The two alleles that an individual
possesses do not affect each other/change each
other
Hypothesis 5: The presence of an allele does not
mean that allele will necessarily be expressed
(remember Phenotype versus Genotype)
Analyzing Mendel’s Results
An individual receives one allele for each
trait from each parent as a result of
Meiosis
 Dominant alleles are designated with
capital letters, and recessive alleles are
designated with lowercase letters
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– Example:
Purple flower – “A”
White flower – “a”
Analyzing Mendel’s Results
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AA = Homozygous Dominant – Purple
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Aa = Heterozygous – Purple
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aa = Homozygous Recessive - White
Punnett Squares
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In a Punnett Square, the possible gametes of
one individual are listed along the horizontal
side of the square, while the gametes of the
other individual are listed along the vertical
side
Two types of crosses are completed using
Punnett Squares:
– Monohybrid Cross – one trait focus
– Dihybrid Cross – two trait focus
Monohybrid Cross
Dihybrid Cross
Testcross
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To determine the genotype of individuals,
a “Testcross” is conducted:
Mendel’s Laws
First Law = Segregation = two alleles of a
trait separate from each other during the
formation of gametes, so that half of the
gametes will carry one copy and half will
carry the other copy
 Second Law = Independent Assortment –
genes located on different chromosomes
are inherited independently of one
another
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Mendel’s Laws
Facts about DNA & Traits
DNA makes RNA makes Protein
 Genes influence the phenotype (physical
expression of trait) by specifying the types of
proteins found in the body, which greatly
influences how well the body functions
 Mutations (changes) alter the DNA, which in
turn, alters the expression of traits
 Mutations cause Evolution to occur, which
then leads to Natural Selection, then finally,
Adaptations
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The Exceptions to Mendelian
Inheritance
Polygenic Inheritance – expression of trait
is determined by several genes (segments
of DNA), therefore, there can be many
variations of said trait; many genes affect
one trait
 Pleiotropic Effects – an individual allele
(one section of DNA on one chromosome)
has more than one effect on phenotype
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The Exceptions to Mendelian
Inheritance
Incomplete Dominance – alleles are
neither dominant nor recessive and as a
result, produce a heterozygote that is
intermediate between both parents;
example is a red flower and white flower
produce a pink flower progeny
 Codominance – often in heterozygotes
there is not a dominant allele; instead the
effects of both alleles are expressed;
example is blood type: A, B, O
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Chromosomes are the vehicles of
Mendelian Inheritance
Sex Linkage – a trait determined by a
gene on the sex chromosome; recall the
sex chromosomes are the last two, and for
humans that means they are the 45th and
46th
 A commonly used example that
demonstrates sex linkage is “Hemophilia”
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Hemophilia Pedigree
Recessive Chromosomal
Disorders
Often, disorders/diseases manifest
themselves only when a person is
homozygous recessive
 A commonly used example is Sickle-cell
Anemia
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Dominant Chromosomal
Disorders
Disorders/diseases that only manifest
themselves when someone is homozygous
dominant or heterozygous
 A commonly used example is Huntington’s
Disease
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Genetic Counseling and Therapy
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The older a person is, the higher the chances
his/her children could have a genetic disorder
Also, a propensity toward a certain
disease/disorder can “run in the family”
For the above two reasons, it is recommended
that couples who are preparing to have children
get genetic counseling
Once a woman is pregnant, ultrasounds and
amniocenteses can detect genetic disorders
DNA Screening with IVF patients….ethical
questions!?!