9-2: Genetic Crosses
Help to predict the genetic
makeup + appearance of
offspring
Genotype & Phenotype

Genotype – genetic makeup of an
organism
– Consists of alleles inherited from parents
– Ex. PP or Pp = purple flower; pp = white
flower

Phenotype – the appearance of an
organism as a result of its genotype
– Ex. PP or Pp = purple flower; pp = white
flower
More words to know…

Homozygous – both alleles in a pair are
alike
– Also known as a PUREBRED
– Homozygous dominant =
PP
– Homozygous recessive = pp

Heterozygous – alleles are different
– Also known as a HYBRID
– Heterozygous =
Pp
Probability

The likelihood that a specific event will
occur
– May be expressed as a decimal, percentage,
fraction, or ratio
– Equation:
 # times an event is expected to happen
# opportunities for it to happen
Probability (cont)

Examples:
– Mendel observed that yellow seed color in F2
appeared 6,022 times and green seed color
appeared 2,001 times
 Total number of seeds = 8,023
– Probability of dominant trait appearing
 6,022/8,023 = .75, 75%, ¾, or 3:1
– Probability of recessive trait appearing
 2,001/8,023 = .25, 25%, ¼, or 1:3
Probability (cont)

What does probability tell us about the
offspring of 2 heterozygous individuals?
– There are 3 out of 4 chances the offspring will have
the dominant trait
– There is 1 out of 4 chances the offspring will have the
recessive trait
 *The results of probability are more likely to occur
if you have more trials*
Probability (cont)
Ex. Flipping a coin
 What is the probability
a coin will land on
heads?

– 50%

What is the probability
a coin will land on
tails?
– 50%
Monohybrid Crosses

A cross between individuals involving one
pair of contrasting traits is called a
monohybrid cross
– There are 4 possible outcomes (22)

Results are created with the use of
Punnett Square
– A diagram that helps biologist predict the
probability that certain traits will be inherited
by offspring
Example 1: Homozygous (Dominant) x
Homozygous (recessive)
 P – purple flower; p – white flower

Cross = PP x pp
p
p
P
Pp
Pp
Pp
Pp
Genotypes = 4 Pp
 Phenotypes = 4 purple flowers

P
Example 2a: Homozygous (Dominant) x
Heterozygous
B – black coat; b – brown coat
 Cross = BB x Bb

BB
Bb
BB
Bb
Genotypes = 2 BB, 2Bb
 Phenotypes = 4 black coat

Example 2b: Homozygous (recessive) x
Heterozygous
B – black coat; b – brown coat
 Cross = bb x Bb
B

b
b
Bb
bb
b
Bb
bb
Genotypes = 2 bb, 2 Bb
 Phenotypes = 2 brown coat, 2 black coat

Example 3: Heterozygous x
Heterozygous
B – black coat; b – brown coat
 Cross = Bb x Bb
B

B
b
BB
Bb
Bb
bb
Genotypes = 1 BB, 2Bb, 1bb
 Phenotypes = 3 black, 1 brown

b
Gentoypic vs. Phenotypic Ratio

Genotypic ratio for the last cross
–1BB : 2Bb : 1bb

Phenotypic ratio for the last cross
–3 black :1 brown
Example #4: Testcross

A testcross is used to
find an unknown
genotype
– Can help determine
genotype of any
individual whose
phenotype is dominant
– Cross individual w/
homozygous recessive
– Examples:
Complete vs. Incomplete
Dominance

Complete dominance
– When one allele is completely dominant over
the other
 Ex.

Bb
Incomplete dominance
– No dominant or recessive allele; a
heterozygous individual has an intermediate
phenotype
 Ex.
Bb
Example #5: Incomplete Dominance

Japanese 4 o’clock flowers (snapdragons)
– R = red
– r = white

Cross: RR x rr
Rr
Rr
Rr
Rr
Genotypes: 4 Rr
 Phenotypes: 4 pink flowers

Cross 2 heterozygous flowers


Cross: Rr x Rr
RR
Rr
Rr
rr
What is the probable genotypic ratio?
–1 RR : 2 Rr : 1 rr

What is the probable phenotypic ratio?
–1 Red : 2 Pink : 1 white
CODOMINANCE

When both alleles are dominant and
influence the phenotype of the individual
– Ex. Blood type
B
B
A
AB
AB
A
AB
AB
Dihybrid Crosses

A dihybrid cross involves crossing 2
contrasting traits
– There are 16 possible outcomes (24)
– Punnett square:
Example #1: Homozygous Dominant x
Homozygous Recessive
R = round seeds
 r = wrinkled seeds

Y = yellow seed color
 y = green seed color

Cross = RRYY x rryy
 How do we determine the alleles given off
by each parent?

– The “FOIL” method!!!
FOIL (First-Outer-Inner-Last)

Parent 1
 Alleles
RRYY
: RY,RY,RY,RY
FOIL (First-Outer-Inner-Last)

Parent 2
 Alleles:
rryy
ry,ry,ry,ry
ry
ry
ry
ry
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RY

Genotypes: 16 RrYy
 Phenotypes: 16 round , yellow
Heterozygous x Heterozygous
Cross: RrYy x RrYy
 Alleles from each parent:

RY, Ry, rY, ry
– Parent 2 = RY, Ry, rY, ry
– Parent 1 =
RrYy x RrYy
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
RY
Ry
Results of the cross
 Genotypes –
– 1 RRYY, 2RRYy, 1 RRyy, 2 RrYY, 4 RrYy,
2 Rryy, 1 rrYY, 2 rrYy, 1 rryy
 Phenoytpes –
– 9 round, yellow; 3 round, green;
3 wrinkled, yellow; 1 wrinkled, green
Multiple Alleles + Polygenic Traits

Multiple Alleles – 3 or
more alleles
– Blood types
 A, B, O

Polygenic – 2 or more
genes
– Height
– Weight
– Eye color*
– Intelligence
– Skin color