Genotype and Phenotype: Punnett Square

Contributed by:
Sharp Tutor
This presentation will give an overview of Genetics and a concern upon Genotype and Phenotype. The Punnett square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach in 1905. The diagram is used by biologists to determine the probability of an offspring having a particular genotype.
2. What is Genetics?
Genetics is the scientific
study of heredity
3. What is a Trait?
A trait is a specific characteristic
that varies from one individual to
Examples: Brown hair, blue eyes, tall,
4. What is an Allele?
Alleles are the
different possibilities
for a given trait.
Every trait has at least
two alleles (one from Examples of Alleles:
the mother and one A = Brown Eyes
a = Blue Eyes
from the father) B = Green Eyes
Example: Eye color – b = Hazel Eyes
Brown, blue, green,
5. What are Genes?
Genes are the
sequence of
DNA that
codes for a
protein and
determines a
6. Gregor Mendel
Father of Genetics
1st important studies of
Identified specific traits in the
garden pea and studied them from
one generation to another
7. Mendel’s
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
Represented by a lower case letter r
9. Dominant & Recessive Practice
T – straight hair
t - curly hair
TT - Represent offspring with straight
Tt - Represent offspring with straight
tt - Represents offspring with 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 RR
has two identical alleles for
a particular trait (TT or tt) rr
Heterozygous - Term used
to refer to an organism that
has two different alleles for
the same trait (Tt)
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
13. Punnett Square
Parent Offspring
14. How to Complete a Punnett Square
15. Y-Yellow
3 Yellow
1 White
16. You Try It Now!
Give the genotype and phenotype for the
following cross: TT x tt (T = Tall and t =
17. TT x tt
Step One: Set Up Punnett Square (put one parent
on the top and the other along the side)
18. TT x tt
Step Two: Complete the Punnett Square
t Tt Tt
t Tt Tt
19. TT x tt
Step Three: Write the genotype and phenotype
t Tt Tt 4 - Tt
t Tt 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
22. Tt x tt
Step Two: Complete the Punnett Square
T t
t Tt tt
t Tt tt
23. Tt x tt
Step Two: Complete the Punnett Square
T t
Tt - 2 (50%)
t Tt tt tt - 2 (50%)
t Tt tt Phenotype:
50% Tall
50% Short
Remember: Each box is 25%
24. Some Terminology
P1 – Original parents
F1 – First generation
F2 – Second
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 - 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
3. Hair Color – Too
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
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.