Download Punnett Square Notes

Punnett Square Notes
A Punnett Square is a grid for organizing genetic
information. It is VERY similar to how you
predict probability.
A punnett square indicates the PROBABILITY of
what the offspring from two individuals will look
like.
So how do we determine what are the
possibilities of the offspring between two hybrid
tall pea plants?
First: What is the genotype (alleles) for a hybrid
tall pea plant?
Tt
Second: What alleles can one parent possibly
give to their offspring?
T or t
Third: What alleles can the other parent give to
their offspring?
T or t
• Place each allele at the top (male) or side
(female) of the punnett square.
T
T
t
t
• Fill in the punnett square by reading the
alleles down & across.
T
t
T TT T t
t T t tt
• The squares represent the possible genotypes
of the offspring from these two individuals. It
does NOT mean that there are four offspring.
T
t
T TT T t
t T t tt
• The square has just given you a probability of getting
traitsTT
Tt
tt
T
t
T TT T t
t T t tt
• Probability of having a tt offspring
(phenotype = short) = 25% or 1 out of 4
• Probability of having a Tt offspring
(phenotype = tall) = 50% or 2 out of 4
• Probability of having a TT offspring
(phenotype = tall) = 25% or 1 out of 4
• What are the offspring possibilities between a
hybrid widow peaked dad and a non-widow
peaked mom ?
• Genotype for mom=
ww
• Genotype for dad =
Ww
• What are the offspring possibilities between a
hybrid widow peaked dad and a non-widow
peaked mom ?
W w
w W w ww
w Ww w w
Probability of Ww offspring (Widow’s Peak) =
2 out of 4 or 50%
Probability of WW offspring (Widow’s Peak) =
0 out of 4 or 0%
Probability of ww offspring (no Widow’s Peak) =
2 out of 4 or 50 %
Monohybrid Punnett Squares
• A monohybrid cross is exploring the
possibilities of only one trait. For example,
what will the offspring look like when a hybrid
green and yellow podded plant are crossed.
You are ONLY looking at one trait- pod color.
• Green X Yellow
• Green alleles = G or g
• Yellow alleles = g
G
g
g
G g
g g
g
Gg
g g
Dihybrid Punnett Squares
• A dihybrid cross is exploring the possibilities of
TWO traits. For example, what will the
offspring look like when a hybrid yellow,
wrinkled pea plant is crossed with a green,
hybrid round pea plant. You are looking at
two traits- pea color and pea texture.
• Yellow, Wrinkled = Yyrr
• Green, Round = yyRr
Dihybrid Punnett Squares
• For a dihybrid cross, you must think of all of
the possible combination of alleles.
• For example, a yellow wrinkled plant (Yyrr)
can offer the possible allele combinations:
Yr, yr.
What are the possible allele combinations for a
green, smooth wrinkled plant (yyRr)?
yR, yr
Dihybrid Punnett Squares
• What are the possible allele combinations for
a yellow, smooth plant (YyRr)?
YR, Yr, yR, yr
• What are the possible allele combinations for
a green, wrinkled plant (yyrr)?
yr
Yellow, wrinkled (Yyrr) x Green, Round (yyRr)
Yr
Y r
yr
y R Yy Rr Y y Rr y y R r
y r Yy rr Y y r r y y r r
R
y
y
Rr
R
Y
y
Y
r
y
R
r
y
yr
y yR r
y yr r
y yR r
r
r
r
y
r
r
y
y
r
y
Y
Y
y
y
r
r
y r
Test Cross
• A test cross is used to determine the genotype
of an unknown organism. For example, you
have a tall pea plant but you do not known if it
is homozygous (TT) or heterozygous (Tt). To
determine this, you test cross the plant with a
homozygous recessive plant- or a SHORT
plant. If the unknown is Tt, you will have
some short offspring, if it is TT, you will have
all tall offspring.
Tall plant (TT or Tt) x short plant (tt)