Transcript Punnett Squares

By Ms. Duko
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.
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
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
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)
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 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