Transcript Dominant and Recessive using Punnett Square

Using Punnett Squares
Dominant & Recessive
Punnett Squares
It is a special chart, or
grid system, named after
its inventor Reginald
Crundall Punnett.
(devised in the early
1900’s)
Punnett Squares
Geneticists often
use Punnett
squares to depict
genetic crosses and
to determine the
possible gamete
combinations of the
offspring.
Punnett Squares
Punnett square: a
diagram used to
visualize genetic
crosses (a chart that
shows all the possible
combinations of
alleles that can result
from a genetic cross)
Punnett Squares
How to work Dominant
and Recessive Genetic
Problems with Punnett
Squares
Show a cross between a tall and
short pea plant
Step One
Write down the set of contrasting traits.
Assign the capitol letter to the dominant
trait and the lower case letter to the
recessive trait.
Tall T
Short t
Show a cross between a tall and
short pea plant
Step Two
Write down possible
genotypes and phenotypes.
TT
Tall
Tt
Tall
tt
Short
Show a cross between a tall and
short pea plant
Step Three
Fill in the genotypes of the
parents on the punnett
square.
Punnett Squares
On the top- the
possible gametes
of the female are
listed.
Down the left sidethe possible
gametes of the
male are listed
t
T
T
t
Show a cross between a tall and
short pea plant
Step Four
Fill in the genetic
possibilities of the
F1
t
T
T
t
Show a cross between a tall and
short pea plant
Step Four
Take first allele
from the male and
first allele from the
female and place in
the first square.
t
T Tt
T
t
Show a cross between a tall and
short pea plant
Step Four
Take first allel
from the male and
second allele from
the female and
place in the second
square.
t
T
T
t
Tt
Show a cross between a tall and
short pea plant
Step Four
Take second allele
from the male and
first allele from the
female and place in
the third square.
t
T
T Tt
t
Show a cross between a tall and
short pea plant
Step Four
Take second allele
from the male and
second allele from
the female and
place in the fourth
square.
t
t
T
T
Tt
Show a cross between a tall and
short pea plant
Step Four
Fill in the genetic
possibilities of the
F1
t
t
T Tt
Tt
T Tt
Tt
Show a cross between a tall and
short pea plant
Step Five
Write down the genotypic ratio
and the phenotypic ratio.
Purebred dominant : Hybrid : Purebred recessive
Tall : Short
Show a cross between a tall and
short pea plant
Step Five
Write down the
genotypic ratio
and the
phenotypic ratio.
t
t
T Tt Tt
T Tt Tt
0:4:0
4:0
Punnett Squares
The gametes are
then combined in
each of the boxes
within the square
to give the possible
gamete
combinations of
the offspring.
F
f
F
FF F f
f
Ff
f f
1:2:1
3:1
F1
Punnett Squares
Besides showing
possible gene pairings,
a Punnett square gives
the probability of each
pairing.
It shows how often, on
the average, a given
pairing will occur.
F
f
F
FF F f
f
Ff
f f
1:2:1
3:1
F1
Punnett Squares
Punnett squares
cannot tell which
traits a specific
offspring will
exhibit but rather
the chance, or odds,
of having certain
traits
1:2:1
3:1
Punnett Squares
The number of boxes
in a Punnett square
does not stand for the
number of offspring an
organism will produce.
The boxes represent
the genetic possibilities
of the offspring.
Punnett Squares
A small # of offspringmay not have the exact
ratio of gene pairings.
1:2:1
3:1