Introduction to Genetics
Genetics: The scientific study
of heredity
Heredity – the passing of traits
from parents to offspring
Inheritance
• You get your genes from your parents
• In meiosis, half of the chromosomes in a
pair come from the Dad, half come from
the Mom
• What we know today is based on the work
of Gregor Mendel – Austrian monk
Key terms to know
• Allele – each form of a gene for a certain
trait (R or r)
• Gene – sequence of DNA that codes for a
protein a thus determines a trait
• Genotype – combination of alleles for a
given trait ( RR or Rr or rr)
• Phenotype – Appearance of trait ( round
seeds or wrinkled seeds
Terms continued
• Homozygous - when you have 2 or the same alleles for
a given trait (RR or rr)
• Heterozygous – when you have 2 different alleles for a
trait (Rr)
• Incomplete dominance – blending of alleles to produce a
different phenotype (red and white produce pink)
• Codominance – Both alleles are expressed equally
(produces combinations of each- red and white produce
red spotted white)
• Multiple alleles – a set of 3 or more different alleles
controlling a trait ( eye color, skin color)
Painting of Mendel
Gregor Mendel
• Born in 1822 in Czech Republic
• Became a priest and studied math and
science at the University of Vienna
• Worked for next 14 years in the monastery
as head of monastery garden and taught
at the high school
Mendel studied seven/eight
different pea plant traits…
• Trait – a specific characteristic that
varies from one individual to another
(ex. Seed color, height, hair color)
• Mendel’s studied traits had two contrasting
characters or “alleles” -- different forms
of a gene
Section 11-1
Figure
11-3 Mendel’s even F1 Crosses
on Pea Plants
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Flower
Position
Axial
Tall
Yellow
Terminal
Short
Green
Axial
*Flower color – purple (P) vs. white (p)
Seed coat color and flower color are often put
in for one another – thus, the EIGHT traits!!!
Go to
Section:
Plant
Height
Tall
A genetic cross
Alleles, alternative versions of a gene
Mendel’s test crosses….
• Testcross: experimental cross between an
individual with the dominant phenotype for a
given trait (genotype unknown, though) and
another individual with the recessive
phenotype (homozygous recessive)
• P1 generation – parents that Mendel crosspollinated
• F1 generation – offspring of P1 that were allowed
to self-pollinate
• F2 generation – offspring of F1 generation
Mendel tracked heritable characters for three generations
Mendel’s Conclusions….
• Biological inheritance is determined by
“factors” that are passed from one generation
to a next – today, called genes
• Law of Dominance: where there are two or more
forms of a gene for a single trait, some alleles are
dominant and other alleles are recessive
• Law of Segregation: alleles segregate (separate)
from each other during the process of meiosis (gamete
formation)
• Principle of independent assortment: genes for
different traits can segregate independently during the
formation of gametes
Concept Map
Section 11-3
Gregor
Mendel
concluded
that
experimented
with
Pea
plants
“Factors”
determine
traits
Some alleles
are dominant,
and some alleles
are recessive
which is
called the
Law of
Dominance
Go to
Section:
This one follows from the law of segregation –
all alleles are not permanently associated with
one another….
Alleles are
separated during
gamete formation
which is
called the
Law of
Segregation
Principle of Independent
Assortment
Punnet Squares
• Used to predict the possible gene
combinations for a a cross
• Traits are represented by letters
– Lower case letters = recessive traits
– Upper case letters = dominate traits
How to do punnett squares
1. determine the genotypes of the parent
organisms
2. write down your "cross" (mating)
3. draw a p-square
4. "split" the letters of the genotype for each
parent & put them "outside" the p-square
5. determine the possible genotypes of the
offspring by filling in the p-square
6. summarize results (genotypes &
phenotypes of offspring)
7. bask in the glow of your accomplishment !
Beyond Mendel
• Walther Flemming – German biologist
who stained cells with dye and saw tiny,
threadlike structures in the nucleus 
CHROMOSOMES!!!
• 1902 Walter Sutton – American biologist
who supports idea that “factors” are
located on chromosomes
• 1909
Wilhelm Johannsen –
Danish biologist who coined the term
“gene” to define the physical units of
heredity
• GENE: segment of DNA molecules that
carries the instructions for producing a
specific trait
• Besides straight dominant and recessive genes, two
other possibilities for combinations were proven:
Codominance: when 2 alleles work together and BOTH
are expressed without one masking the other
Multiple Alleles: when more than two possibilities for a
trait are present.
•
Example: Blood type
There are 3 alleles for blood type -- A, B, O
Possible combinations:
AA, AO
-- Type A blood
BB, BO
-- Type B blood
AB
-- Type AB blood
OO
-- Type O blood
•
Here, A and B are dominant over O, but if A and B
are present together, neither dominates!!! This is
codominance – they share the power of expression.
• Incomplete Dominance: when BOTH
alleles in an individual affect the
appearance of a trait and you get a brand
new color that was not found in the original
parents. Both traits are written in capitals
and have different letters because BOTH
control the appearance.
•
Example: flower color in snapdragons
Pure red (RR)
X
Pure white (WW)
Humans are difficult to study…
Why?
1. # of human genes is extremely large (each cell
has ~100,000 different genes)
2. Humans cannot be easily controlled by an
investigator
3. Time span between generations is long
4. Only a small # of offspring are produced by
each set of parents
5. Environment has a HUGE effect on a person’s
development…
Have developed ways to approach
the difficulties…
• Pedigree analysis – family history for a
particular trait
• Study of Genetic diseases
• Twin studies – Nature vs. nurture
• Population Sampling
• Genetic Technology
Pedigree analysis