Download LECTURE #28: Punnett Squares

Lecture #28
Punnett
Squares
Honors Biology
Ms. Gaynor
What is “Probability”?



Why did Mendel get his results?
What is the probability that
Mendel would get his results
EACH time?
PROBABILITY = likelihood
(chance) that an event will
happen
2 Probability Rules
Multiplication Rule
 What are the chances of _?_ AND
_?_ happening at the same time?
 MULTIPLY probability of EACH
event happening TOGETHER
Addition Rule
 What are the chances of _?_ OR
_?_ happening at same time?
 ADD the probability of EACH event
happening TOGETHER
Probability Example Problem
Ms. Gaynor has 4 sisters. What
is the probability that this will
happen (her mother having 5
girls in a row)?
 ½ x ½ x ½ x ½ x ½ =??


0.03125 or 3.125 % chance
What is a Punnett Square?
a
diagram (box) used to
predict the probabilities of
the possible outcomes
for offspring that will result
from a cross between 2
parents
 SHOWS
EXPECTED RESULTS
(not necessarily actual!!!!!)
Monohybrid Cross
a cross between 2 individuals that
looks at 1 trait
 There are 4 different types


Ex: freckles OR dimples
Dihybrid Cross

a cross between 2 individuals that
looks at the possibilities of inheriting
2 DIFFERENT traits at one time

Ex: Freckles AND dimples
Monohybrid Punnett Square
Mom’s genotype (Hh) x Dad’s genotype (hh)
Tall
#2
Short
Mom’s allele #1
Mom’s allele
H
Dad’s
Allele #1
Dad’s
Allele #2
h
h
h
Hh
hh
Hh
hh
Genotypic Ratio vs. Phenotypic
Ratio
 Genotypic
Ratio (Probability or
Frequency) = the GENOTYPE
probabilities (expected results) of
offspring
Ex: 50 % AA and 50 % Aa
 Phenotypic Ratio (Probability or
Frequency) = the PHENOTYPE
probabilities (expected results) of
offspring
Ex: 100% Tall
Probabilities can be expressed as
a:
 DECIMAL
Ex:
0.75 and 0.25
 FRACTION
Ex: ¾ and ¼
 PRECENTAGE
Ex: 75% and 25%
 RATIO
Ex: 3: 1
Punnett Square
Genotypic ratio = 50 % Hh, 50% hh
Phenotypic ratio = 50 % Tall, 50% short
Hh
hh
Hh
hh
COMPLETE
DOMINANCE:
4 different types of
monohybrid crosses
#1: Homozygous Dominant x
Homozygous Recessive
Mom’s genotype (HH) x Dad’s genotype (hh)
Tall
Short
H
H
h
Hh
Hh
h
Hh
Hh
Genotypic ratio = 100% Hh (4:0)
Phenotypic ratio = 100% Tall (4:0)
#2: Homozygous Recessive x
Homozygous Recessive
Mom’s genotype (hh) x Dad’s genotype (hh)
short
Short
h
h
h
hh
hh
h
hh
hh
Genotypic ratio = 100% hh (4:0)
Phenotypic ratio = 100% short (4:0)
#3: Homozygous Dominant x
Heterozygous
Mom’s genotype (HH) x Dad’s genotype (Hh)
Tall
Tall
H
H
H
HH
HH
h
Hh
Hh
Genotypic ratio = 50% HH, 50% Hh (2:2)
Phenotypic ratio = 100% Tall (4:0)
#4: Heterozygous x
Heterozygous
Mom’s genotype (Hh) x Dad’s genotype (Hh)
Tall
Tall
H
h
H
HH
Hh
h
Hh
hh
Genotypic ratio = 25% HH, 50% Hh, 25% hh (1:2:1)
Phenotypic ratio = 75% Tall, 25% short (3:1)
3
Phenotype
Genotype
Purple
PP
Purple
(homozygous)
Pp
(heterozygous)
Pp
Purple
1
Figure 14.6
1
2
(heterozygous)
pp
White
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
The Testcross
 Allows
us to determine the genotype
of an organism with the dominant
phenotype, but unknown
genotype

Genotype is not immediately obvious
 Cross:
 dominant
phenotype individual x
homozygous recessive for same
trait (H _ ?_ x hh)
Test Cross
Mom’s genotype (H?) x Dad’s genotype (hh)
Tall
Short
H
?
h
h
HH
?h
Hh
?h
Genetypic ratio = ?
Phenotypic ratio =?
IF THERE ARE ANY SHORT OFFSPRING, THEN WE
KNOW THE UNKNOWN GENOTYPE HAS TO BE
Hh
Mendel’s 2nd Law:
law of independent assortment
that the genes are
inherited independently
of other genes
 Occurs during Metaphase I
 Mendel assumed traits occur
on different chromosomes!
 says
DIHYBRIDS
Review…
Mendel’s 1st Law: law of segregation says
that the each gamete gets 1 allele
PER GENE
Mendel’s 2nd Law: law of independent
assortment says that the genes are
inherited independently of other
genes
Dihybrid Crosses

crosses involving crossing 2
DIFFERENT traits at one time
 Example: Mate 2 parents and
look at the probability of
seeing 2 traits, such as:
eye color AND hair color
freckles AND dimples
How do You Do Dihybrid
Crosses?
2 ways
1. one BIG Punnett Square
(16 boxes instead of 4)
2. Make 2 separate Punnett
Squares
1 square for EACH trait
Then use PROBABILITY
RULES and MULTIPLY
One BIG Punnett Square
1. Find the possible gametes using
F.O.I.L method (ONLY 1 “letter” per gene)
AaBb
x
AABb
AB
AB
Ab
Ab
aB
AB
ab
Ab
2. Place the gametes in the Punnett Square
& fill in boxes
AB
Ab
AB
Ab
AB
Ab
aB
ab
AABB
AABb
AaBB
AaBb
AAbB
AAbb
AabB
Aabb
AABB
AABb
AaBB
AaBb
AAbB
AAbb
AabB
Aabb
Genotypic Ratio
AAbB = 8/16 or 1/2
AaBB = 2/16 or 1/8
Aabb = 2/16 or 1/8
AABB = 2/16 or 1/8
AAbb = 2/16 or 1/8
Phenotypic Ratio
A= freckles, a= no freckles
B = Dimples, b= no dimples
AAbB = 8/16 (50%) freckles, dimples
AaBB = 2/16 (12.5%) freckles, dimples
Aabb = 2/16 (12.5%) freckles, no dimples
AABB= 2/16 (12.5%) freckles, dimples
AAbb = 2/16 (12.5%) freckles, no dimples
So…12/16 (75%) freckles AND dimples
4/16 (25%) freckles AND no dimples
Now…the easier way!

Write out a key for EACH trait





AA, Aa= Freckles, aa= No freckles
BB, Bb= Dimples, bb= No dimples
Figure out the genotypes of EACH parent and
put them together
Draw 2 Punnett squares & figure out the
FRACTION ratio of possible genotype and
phenotypes
 Draw punnett square #1 for trait #1
 Draw punnett square #2 for trait #2
Figure out genotype/phenotype possibilities
of offspring by multiplying fractions
AaBb x aabb
AA, Aa= Tall aa= short
b
B
b
BB, Bb= Black bb= white
b
Bb
Bb
bb
bb
½ Bb
½ bb
½ Black ½ white
So… babies could look:
25% Black & Tall (½ * ½)
25% Black & short (½ * ½)
a
a
A
Aa
Aa
a
aa
aa
½ Aa
½ Tall
½ aa
½ short
25% white & tall (½ * ½)
25% white & short (½ * ½)
Probability Rules and
Dihybrid Crosses
A
dihybrid or other
multicharacter cross
 Is equivalent to two or more
independent monohybrid
crosses occurring
simultaneously (AT
THE SAME TIME)