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.
