Download Advanced Punnet Squares Pages 183-184, 244 Test Cross: Used to

Advanced Punnet Squares Pages 183-184, 244
Test Cross: Used to determine if the dominant phenotype is homozygous or heterozygous.

To perform the test cross you breed the dominant phenotype with the recessive phenotype.
 The recessive phenotype must by homozygous
Ex: Black Bunny (BB or Bb) x White Bunny (homozygous recessive)

B
B
b
Bb
Bb
b
Bb
Bb
B
b
b
Bb
bb
b
Bb
bb
If test cross produces a homozygous recessive offspring, we conclude the parent was
heterozygous.
Incomplete Dominance: The allele doesn’t completely mask the recessive allele and the phenotypes
mix.
Ex: Red & White flowers.
 Red allele: R
White allele: r
 RR= Red flower
rr= White flower
Rr= Pink Flower
R
r
Genotype Ratio:
1:2:1
R
RR
Red
Rr
Pink
Phenotype Ratio:
1:2:1
r
Rr
Pink
rr
White
Co- Dominance: Alleles are expressed when present

Heterozygous shows both traits at the same time
Ex: Sickle cell anemia
 Normal Red Blood Cells: N
Sickle Red Blood Cells: S
 NN= All normal cells
NS= Half normal cells SS= Only sickle cells
N
S
Genotype Ratio:
1:2:1
N
NN
Normal
NS
Both
Phenotype Ratio:
1:2:1
S
NS
Both
SS
Sickle
Sex-Linked Traits: diseases/genes found only on the X sex chromosomes.
Gender is determined by the sex chromosomes (#23 for humans)
 XX= female
XY =male
 Recessive Sex linked traits are more common in males than females because if the male receives
a recessive gene then there is no other gene present to mask it.
Ex: Colorblindness

Normal Color Vision Gene: XR
Deficient Color Vision Gene: Xr

NN= All normal cells
SS= Only sickle cells
NS= Half normal cells
XR
Xr
XR
XR XR
XR Xr
Y
XR Y
Xr Y
OFFSRING
XR
Xr
XR
Normal
Female
Carrier
Female
Y
Normal
Male
Color Blind
Male
Genotype
Phenotype
XR XR
Female : Normal Color Vision
XR Xr
Female : Normal Color Vision but a carrier of the
recessive gene
Female: Color Blind
Xr Xr
Very Rare: Father was colorblind and mother was carrier
XR
Y
Xr Y
Male: Normal Color Vision
Male: Color Blind
All his daughters will be carriers