3. What is a Trait?
A trait is a specific characteristic that varies
from one individual to another.
Examples: Brown hair, blue eyes, tall, curly
4. 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
5. What are Genes?
Genes are the
sequence of DNA
that codes for a
protein and thus
determines a
trait.
6. 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
7. 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
8. Dominant vs. Recessive
Dominant - Masks the other trait; the trait that
shows if present
Represented by a capital letter
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
r
9. Dominant & Recessive Practice
TT - Represent offspring with straight hair
Tt - Represent offspring with straight hair
tt - Represents offspring with curly hair
T – straight hair
t - curly hair
10. 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
11. 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
12. 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
16. You Try It Now!
Give the genotype and phenotype for the following
cross: TT x tt (T = Tall and t = Short)
17. 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
18. TT x tt
Step Two: Complete the Punnett Square
T T
t
t
Tt Tt
Tt Tt
19. TT x tt
Step Three: Write the genotype and phenotype
T T
t
t
Tt Tt
Tt Tt
Genotype:
4 - Tt
Phenotype:
100% Tall
Remember: Each box is 25%
20. You Try It Now!
Give the genotype and phenotype for the following
cross: Tt x tt
21. 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
22. Tt x tt
Step Two: Complete the Punnett Square
T t
t
t
Tt tt
Tt tt
23. Tt x tt
Step Two: Complete the Punnett Square
T t
t
t
Tt tt
Tt tt
Genotype:
Tt - 2 (50%)
tt - 2 (50%)
Phenotype:
50% Tall
50% Short
Remember: Each box is 25%
24. Some Terminology
P1 – Original parents
F1 – First generation
F2 – Second generation
P1 X P1 = F1
F1 X F1 = F2
25. 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.
26. 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).
+
27. 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.
28. 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.
29. 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.
30. 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.
31. 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.
