Download Constructing a Punnett square

Review
What is an allele? Each person inherits two
alleles for each gene, one allele from each
parent.
What is a gene? The basic unit of heredity.
Define phenotype. The physical appearance
or visible traits displayed by offspring.
Define genotype. The genetic makeup or
allele combination present in offspring.
Punnett Square
What is it?
A graphical grid showing the potential
offspring allele combinations from two
parents.
Punnett Squares are grids that show the
potential offspring of two parents.
P
A
R
E
N
T
T
T
t
Tt
Tt
t
Tt
Tt
OFFSPRING
PARENT
KEY:
T = Tall
t = short
Background
Professor Reginald Punnett, (1875-1967) a
British geneticist created the Punnett square, a
tool in genetics which is still used by biologists
today to predict the probability of possible
genotypes of offspring.
Constructing a Punnett square
TT x tt
Begin by drawing a
grid.
Next, you put the
genotype (allele
combination) of one
parent across the top
and that of the other
parent down the left
side.
1st parent: T T
2nd parent: t t
Note: Capital letters denote dominance
T
T
t
t
KEY:
T = Tall
t = short
Constructing a Punnett square
TT x tt
Next, all you have to
do is fill in the boxes
with the potential
offspring
combinations.
BUT HOW???
Like a multiplication
table, combine each
column-head letter
and row letter into the
empty squares.
1
3
t
4
t
2
T
T
1&3
2&3
Tt
Tt
1&4
2&4
Tt
Tt
KEY:
T = Tall
t = short
Interpreting a Punnett Square
What does it tell us?
TT x tt
1st parent: TT
Tall
Phenotype?_________
2nd parent: tt
Short
Phenotype?_________
Tt
Offspring: ____
All Tall
Phenotype?_______
T
T
t
Tt
Tt
t
Tt
Tt
TT x tt
One parent dominant homozygous,
other parent recessive homozygous.
KEY:
T = Tall
t = short
New Terms
Homozygous: having 2 identical alleles for a trait
Heterozygous: having 2 different alleles for a trait
Dominant: the term dominant gene refers to the allele
that causes a phenotype that is seen
Recessive:
a gene that does not produce its effect
when it occurs with a dominant gene, but
produces its effect only when there are
two copies of it