What is Genetics?
Genetics is the scientific study of
heredity
What is a Trait?
 A trait is a specific characteristic that varies
from one individual to another.
 Examples: Brown hair, blue eyes, tall, curly
What is an Allele?
 Alleles are the different
possibilities for a given
trait.
 Every trait has at least two
alleles (one from the
mother and one from the
father)
 Example: Eye color –
Brown, blue, green, hazel
Examples of Alleles:
A = Brown Eyes
a = Blue Eyes
B = Green Eyes
b = Hazel Eyes
What are Genes?
 Genes are the
sequence of DNA
that codes for a
protein and thus
determines a
trait.
Gregor Mendel
 Father of Genetics
 1st important studies of
heredity
 Identified specific traits in the garden pea
and studied them from one generation to
another
Mendel’s
Conclusions
1. Law of Segregation – Two alleles for each
trait separate when gametes form; Parents
pass only one allele for each trait to each
offspring
2.Law of Independent Assortment – Genes
for different traits are inherited
independently of each other
Dominant vs. Recessive
 Dominant - Masks the other trait; the trait that
shows if present
 Represented by a capital letter
R
 Recessive – An organism with a recessive allele
for a particular trait will only exhibit that trait
when the dominant allele is not present; Will
only show if both alleles are present
 Represented by a lower case letter
r
Dominant & Recessive Practice
T – straight hair
t - curly hair
 TT - Represent offspring with straight hair
 Tt - Represent offspring with straight hair
 tt - Represents offspring with curly hair
Genotype vs. Phenotype
 Genotype – The genetic makeup of an organism;
The gene (or allele) combination an organism has.
 Example: Tt, ss, GG, Ww
 Phenotype – The physical characteristics of an
organism; The way an
organism looks
 Example: Curly hair,
straight hair, blue eyes,
tall, green
Homozygous vs. Heterozygous
 Homozygous – Term used to
refer to an organism that has two
identical alleles for a particular
trait (TT or tt)
 Heterozygous - Term used to
refer to an organism that has two
different alleles for the same trait
(Tt)
RR
rr
Rr
Punnett Squares
 Punnett Square – Diagram showing the
gene combinations that might result from a
genetic cross
 Used to calculate the
probability of inheriting
a particular trait
 Probability – The chance
that a given event will
occur
Punnett Square
Parent
Parent
Offspring
How to Complete a Punnett Square
Y-Yellow
y-white
Genotype:
1:2:1
(YY:Yy:yy)
Phenotype:
3 Yellow
1 White
You Try It Now!
 Give the genotype and phenotype for the following
cross: TT x tt (T = Tall and t = Short)
TT x tt
Step One: Set Up Punnett Square (put one parent on the top
and the other along the side)
T
t
t
T
TT x tt
Step Two: Complete the Punnett Square
T
T
t
Tt
Tt
t
Tt
Tt
TT x tt
Step Three: Write the genotype and phenotype
T
T
t
Tt
Tt
t
Tt
Tt
Remember: Each box is 25%
Genotype:
4 - Tt
Phenotype:
100% Tall
You Try It Now!
 Give the genotype and phenotype for the following
cross: Tt x tt
Tt x tt
Step One: Set Up Punnett Square (put one parent on the top
and the other along the side)
T
t
t
t
Tt x tt
Step Two: Complete the Punnett Square
T
t
t
Tt
tt
t
Tt
tt
Tt x tt
Step Two: Complete the Punnett Square
T
t
t
Tt
tt
t
Tt
tt
Remember: Each box is 25%
Genotype:
Tt - 2 (50%)
tt - 2 (50%)
Phenotype:
50% Tall
50% Short
Some Terminology
 P1 – Original parents
 F1 – First generation
 F2 – Second generation
 P1 X P1 = F1
 F 1 X F 1 = F2
Incomplete Dominance
 Incomplete Dominance - Situation in
which one allele is not completely dominant
over another.
 Example – Red and
white flowers are
crossed and pink
flowers are produced.
Codominance
 Codominance - Situation in which both
alleles of a gene contribute to the phenotype of
the organism.
 Example – A solid white cow is crossed with a solid
brown cow and the resulting offspring are spotted
brown and white (called roan).

+
Multiple Alleles
 Multiple Alleles- Three or more alleles of
the same gene.
 Even though three or more alleles exist for a
particular trait, an individual can only have
two alleles - one from the mother and one
from the father.
Examples of Multiple Alleles
1. Coat color in rabbits is determined by a
single gene that has at least four different
alleles. Different combinations of alleles
result in the four colors you see here.
Examples of Multiple Alleles
2. Blood Type – 3 alleles
exist (IA, IB, and i),
which results in four
different possible blood
types
3. Hair Color – Too many
alleles exist to count

There are over 20
different shades of
hair color.
Multiple Alleles
 There Are Always Multiple Alleles!
 Genetic inheritance is often presented with
straightforward examples involving only two alleles
with clear-cut dominance. This makes inheritance
patterns easy to see.
 But very few traits actually only have two alleles with
clear-cut dominance. As we learn more about
genetics, we have found that there are often hundreds
of alleles for any particular gene.

We probably know this already - as we look around at other
people, we see infinite variation.
Polygenic Trait
 Polygenic Trait - Trait
controlled by two or more
genes.
 Polygenic traits often show a
wide range of phenotypes.
 Example: The wide range of
skin color in humans comes
about partly because more
than four different genes
probably control this trait.