Unit 4 – Lecture 6
Dihybrid Cross
 a 4x4 representation of crossing TWO traits
 monohybrid = 1 trait w/ 2 alleles
= 1 allele gamete each for 2x2 box
 dihybrid = 2 traits w/ 2 alleles EACH = 4 alleles
= 2 allele gametes each for 4x4 box
[one allele for each trait in each gamete]
Solving a Dihybrid
 Determine the genotypes of each parent
 Given to you, or you have to solve for them
 Given: Parent #1 AABB
 Given: Parent #2 aabb
Solving a Dihybrid
 Create your dihybrid cross [4x4]
Solving a Dihybrid
 Determine Gametes:
 pair first allele of first trait with
first allele of second trait
AABB
 first gamete = AB
Solving a Dihybrid
 Determine Gametes:
 pair first allele of first trait with
second allele of second trait
AABB
 second gamete = AB
Solving a Dihybrid
 Determine Gametes:
 pair second allele of first trait with
first allele of second trait
AABB
 third gamete = AB
Solving a Dihybrid
 Determine Gametes:
 pair second allele of first trait with
second allele of second trait
AABB
 fourth gamete = AB
Solving a Dihybrid
 Determine Gametes:
 pair first allele of first trait with
first allele of second trait
 aabb
 first gamete = ab
Solving a Dihybrid
 Determine Gametes:
 pair first allele of first trait with
second allele of second trait
 aabb
 second gamete = ab
Solving a Dihybrid
 Determine Gametes:
 pair second allele of first trait with
first allele of second trait
 aabb
 third gamete = ab
Solving a Dihybrid
 Determine Gametes:
 pair second allele of first trait with
second allele of second trait
 aabb
 fourth gamete = ab
Solving a Dihybrid
 Label cross w/ parent gametes above / to left
AB
ab
ab
ab
ab
AB
AB
AB
Solving a Dihybrid
 Fill in boxes:
 fill DOWN each column w/ whatever’s there
AB
AB
AB
AB
ab
A B
A B
A B
A B
ab
A B
A B
A B
A B
ab
A B
A B
A B
A B
ab
A B
A B
A B
A B
Solving a Dihybrid
 Fill in boxes:
 fill ACROSS each column with whatever’s there
AB
AB
AB
AB
ab AaBb
A B AaBb
A B AaBb
A B AaBb
A B
ab AaBb
A B AaBb
A B AaBb
A B AaBb
A B
ab AaBb
A B AaBb
A B AaBb
A B AaBb
A B
ab AaBb
A B AaBb
A B AaBb
A B AaBb
A B
Solving a Dihybrid
 Determine Genotype Ratios
 record all genotypes you have
 count and record number of each genotype
AB
AB
AB
AB
ab AaBb AaBb AaBb AaBb
 AaBb: 16/16
[or 100%]
ab AaBb AaBb AaBb AaBb
ab AaBb AaBb AaBb AaBb
ab AaBb AaBb AaBb AaBb
Solving a Dihybrid
 Determine Phenotype Ratios
 count how many of each phenotype you have
 record # of phenotypes represented
AB
AB
AB
AB
ab AaBb AaBb AaBb AaBb
 purple, round: 16/16
[or 100%]
ab AaBb AaBb AaBb AaBb
ab AaBb AaBb AaBb AaBb
ab AaBb AaBb AaBb AaBb
Solving a Dihybrid #2
 Determine the genotypes of each parent
 Given to you, or you have to solve for them
 Given – both parents are heterozygous for both traits.
 Given: Parent #1 QqRr
 Given: Parent #2 QqRr
Solving a Dihybrid
 Create your dihybrid cross [4x4]
Solving a Dihybrid
 Determine Gametes:
 pair first allele of first trait with
first allele of second trait
QqRr
 first gamete = QR
Solving a Dihybrid
 Determine Gametes:
 pair first allele of first trait with
second allele of second trait
QqRr
 second gamete = Qr
Solving a Dihybrid
 Determine Gametes:
 pair second allele of first trait with
first allele of second trait
QqRr
 third gamete = qR
Solving a Dihybrid
 Determine Gametes:
 pair second allele of first trait with
second allele of second trait
QqRr
 fourth gamete = qr
Solving a Dihybrid
 Label cross w/ parent gametes above / to left
QR
QR
Qr
qR
qr
Qr
qR
qr
Solving a Dihybrid
 Fill in boxes:
 fill DOWN each column w/ whatever’s there
QR
Qr
qR
qr
QR
Q R
Q
r
qR
q r
Qr
Q R
Q
r
qR
q r
qR
Q R
Q
r
qR
q r
qr
Q R
Q
r
qR
q r
Solving a Dihybrid
 Fill in boxes:
 fill ACROSS each column with whatever’s there
QR
Qr
qR
qr
QR
QQRR
Q R QQRr
Q r QqRR
qR QqRr
q r
Qr
QQRr
Q R Q
QQrrr QqRr
qR Qqrr
q r
qR
QqRR
Q R Q
QqRrr qqRR
qR qqRr
q r
qr
QqRr
Q R Q
Qqrrr qqRr
qR
qqrr
q r
Solving a Dihybrid
 Determine Genotype Ratios
 record all genotypes you have
 count and record number of each genotype
QR
 [not going to make
QR
Qr
qR
qr
QQRR QQRr QqRR QqRr
you do it for this one..
Qr
just know how to]
QQRr QQrr QqRr Qqrr
qR
QqRR QqRr qqRR qqRr
qr
QqRr Qqrr
qqRr
qqrr
Solving a Dihybrid
 Determine Phenotype Ratios
 count how many of each phenotype you have
 record # of phenotypes represented
QR
 9/16 - purple, round
 3/16 - purple, wrinkled
 3/16 - white, round
 1/16 - white, wrinkled
Qr
qR
qr
QR
QQRR QQRr QqRR QqRr
Qr
QQRr QQrr QqRr Qqrr
qR
QqRR QqRr qqRR qqRr
qr
QqRr Qqrr
qqRr
qqrr