T
t
T
Tt
t
Punnett Squares
SINGLE TRAIT PUNNETT
SQUARE
¼
Genetics
• Genetics is the study of
genes and inheritance
• Inheritance is how traits
are passed on from
generation to generation.
• Chromosomes are made
up of genes, which are
made up of DNA.
Gregor Mendel
• Austrian monk
• Experimented with pea
plants
• He discovered that traits
were carried from one
generation to the next.
TERMS TO KNOW
ALLELES
DIFFERENT FORMS OF A
TRAIT THAT A GENE MAY
HAVE
HOMOZYGOUS
AN ORGANISM WITH
TWO ALLELES THAT ARE
THE SAME
HETEROZYGOUS
AN ORGANISM WITH
TWO DIFFERENT
ALLELES FOR A TRAIT
T,t
TT, tt
Tt, Gg
TERMS TO KNOW
HYBRID
ONE OF EACH TRAIT
(SAME AS
HETEROZYGOUS)
DOMINANT
A TRAIT THAT
DOMINATES THE OTHER
FORM OF THE TRAIT
THE TRAIT THAT IS
VISIBLE
RECESSIVE
THE TRAIT BEING
DOMINATED OR
COVERED UP BY THE
DOMINATE TRAIT
THE TRAIT IS NOT
VISIBLE
Tt, Gg
REPRESENTED BY AN
UPPERCASE LETTER
T G
OR
REPRESENTED BY A
LOWER CASE LETTER
t g
or
TERMS TO KNOW
PHENOTYPE
THE PHYSICAL
APPEARANCE OF AN
ORGANISM
(WHAT IT LOOKS LIKE)
TALL, SHORT,
GREEN,
WRINKLED
GENOTYPE
THE GENE ORDER OF AN
ORGANISM
(WHAT ITS GENES LOOK
LIKE)
TT, GG, Tt, gg
Gg, tt
RATIO
THE RELATIONSHIP IN
NUMBERS BETWEEN
TWO OR MORE THINGS
(SAME AS MATH)
3:1, 2:2, 1:2:1
Mendel’s Experiment
He chose to work with
pea plants because
– Readily available
– Reproduced quickly
– Traits were distinct
Mendel’s Experiment
Mendel crossed true breeding plants that had
two distinct and contrasting traits, like purple
and white flowers. After the first cross, the
plants self-fertilized.
Mendel’s Experiment
P generation is
parental generation.
F1 generation is the
first generation.
F2 generation is the
second generation.
Mendel’s Experiment
He studied these seven traits
Mendel’s Phenotypic Conclusions
In the F1 generation,
all plants had the
same trait.
In the F2 generation,
there was always a
3:1 ratio of a single
trait.
Mendel’s Genotypic Conclusions
In the F2 generation,
The genotypic ratio
is different.
Remember P is
dominant so the
plants will be purple
even with Pp.
TOOLS TO KNOW
A PUNNET SQUARE
IS A TOOL USED TO
PREDICT THE
POSSIBLE
GENOTYPES FOR
THE OFFSPRING OF
TWO KNOWN
PARENTS.
PARENT’S GENES
PARENT’S GENES
HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT
SQUARE
THE PARENTS’ ALLELES
GO ON THE OUTSIDE OF
THE SQUARE
BB X bb
b
b
B
B
HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT
SQUARE
THE PARENTS’ ALLELES
GO ON THE OUTSIDE OF
THE SQUARE
b
UPPERCASE
LETTERS SHOULD
GO FIRST
b
B
B b
B b
B
B b
B b
HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT
SQUARE
WHAT DO THE
RESULTS SHOW?
B
B IS THE
DOMINANT
ALLELE FOR
BLACK
b IS THE
RECESSIVE
ALLELE FOR
BROWN
B
b
Bb
Bb
RESULTS:
Bb Bb Bb Bb
b
PHENOTYPIC:
Bb
Bb
100% BLACK
4:0 RATIO, BLACK
TO BROWN
GENOTYPIC:
100% Bb
4:0 ALL Bb
HOW TO USE A PUNNETT SQUARE
LET’S LOOK AT
ANOTHER
PUNNETT
SQUARE AND
PREDICT THE
OUTCOME
T
T
t
T T
Tt
RESULTS:
TT, Tt, Tt, tt
T IS THE
PHENOTYPIC:
DOMINANT
ALLELE FOR
TALLNESS
75% TALL
25% SHORT
t IS THE
RECESSIVE
ALLELE FOR
SHORTNESS
t
T t
t t
3 TO 1 RATIO: TALL
TO SHORT
GENOTYPIC:
1TT: 2Tt: 1tt
1:2:1 RATIO
25 %TT,
50% Tt, 25% tt
Probability and Punnett Squares