Genetic Crosses
Homozygous vs. Heterozygous
 Recall:
 Alleles received by offspring may either be
heterozygous or homozygous
 Homozyous = two alleles are the same
 Heterozygous = two alleles are different
Phenotype & Genotype
 Genotype: the genetic makeup of an organism
 Phenotype: the physical characteristics of an
organism (what the organism looks like)
Probability
 Probability: the likelihood that a specific event will occur
 Can be expressed as a decimal, percentage, or a
fraction
 Probability = number of times an event is expected to happen
number of times an event could happen
Predicting Results of
Monohybrid Crosses
 Monohybrid cross: a cross in
which only one characteristic is
tracked
 Biologists use a diagram called
a Punnett square to help
predict the probability of
inherited traits of offspring
How To: Monohybrid Cross
1. The alleles of the first parent will be
placed on the top of the square
2. The alleles of the second parent will
be placed along the left side of the
square
3. The possible gene combinations of
the offspring will be placed inside the
squares
 Letters will represent the alleles
 A capital letter represents a dominant
allele
 A lower case letter represents a
recessive allele
 Example 1:
homozygous x homozygous
 What happens when you cross
a homozygous purple (PP)
flower with a homozygous
white (pp) flower?
 There is a 100% probability that
the offspring will have the
genotype Pp = purple flower
color
Genotypes
Phenotypes
4/4 Pp
4/4 purple
 Example 2:
homozygous x heterozygous
 What happens when you cross a
guinea pig that is homozygous
dominant trait of black coat
color (BB) and a guinea pig that
is heterozygous for this trait (Bb)?
 There is a 2/4 (50%) chance for
genotype BB and 2/4 (50%)
change for Bb
Genotypes
Phenotypes
2/4 BB
2/4 Bb
4/4 black
 You Try! What if we crossed one that
was homozygous for a brown coat (bb)
and heterozygous for black (Bb)?
 2/4 (50%) black & 2/4 (50%) brown
 Example 3:
heterozygous x heterozygous
 What happens when you cross two rabbits
heterozygous for black coats (Bb)?
 There is a 1/4 (25%) chance for genotype
BB, 2/4 (50%) chance for Bb, and 1/4 (25%)
for bb
 = 3/4 (75%) will have black coat &1/4
(25%) will have brown coat
Genotypes
Phenotypes
1/4 BB
2/4 Bb
1/4 bb
3/4 black
1/4 brown
 Genotypic ratio: the ratio of genotypes that
appear in offspring
 Ex: 1 BB : 2 Bb : 1bb
 Phenotypic ratio: the ratio of the offspring’s
phenotypes
 Ex: 3 black : 1 brown
You Try!
 In dogs, the allele for short hair (B) is dominant
over the allele for long hair (b). Two short
haired dogs have a litter of puppies. Some of
the puppies have short hair and some of the
puppies have long hair.
 What are the genotypes of the parents?
 Bb and Bb
 How would the Punnett square look like?
 What is the probability of offspring with short
hair?
 3/4 short hair
 Long hair?
 1/4 long hair
 If the litter of puppies contained 12 pups, how
many would you expect to have short hair?
 3/4 of 12 = 9 pups
 How many would you expect to have long
hair?
 1/4 of 12 = 3 pups
Predicting Results of Dihybrid
Crosses
 Mendel designed an
experiment that helped to
follow two different genes as
they passed from parent to
offspring
 Used a dihybrid cross
 Dihybrid cross: a cross in which
two characteristics are tracked
 Offspring of a dihybrid cross
are called dihybrids
Steps in Setting Up a Dihybrid
Cross
1. Determine the parental genotypes
2. Determine the gametes using “FOIL”
method
3. Set up a large 4x4 Punnet square &
place one set of gametes from one
parent on top & the other set on the
side
4. Write the genotype of the offspring in
each box
Sample Problem
Homozygous x Homozygous
 A pea plant that is homozygous for round, yellow seeds (RRYY) is
crossed with a one that is homozygous for wrinkled, green seeds
(rryy). Determine the offspring.
Sample Problem #2
Heterozygous x Heterozygous
 Determine the results of crossing two pea plants heterozygous
for round, yellow seeds.
Shortcut!
In any case where the parents are
heterozygous for both traits (AaBb x AaBb)
you will always get a 9:3:3:1 ratio
9 is the number for the two dominant traits
3 is the number for the dominant/recessive
combo
1 individual will display both recessive traits
Exceptions to Mendel’s
Principles
 There are some exceptions to
Mendel’s principles
 Not all genes show a pattern of
dominance and recessiveness
 For some genes, there are
more than two alleles
 Many times, traits are
controlled by more than one
gene
Genes & the Environment
 Gene expression is always the
result of the interaction of
genetic potential with the
environment
 The presence of the gene is not
all that is required for the
expression of a trait
 There must be proper
environmental conditions
present
 The phenotype of any organism is
the result of interaction between
genes and the environment
Incomplete Dominance
 All traits are not so clear-cut as
dominant & recessive traits –
some genes seem to blend
together
 Sometimes you have offspring
that show incomplete
dominance
 Incomplete dominance:
offspring have a phenotype
that is a blend of its parents
 Ex: Some flowers will be pink if
you cross a red flower with a
white flower
Codominance
 Codominance: occurs when
both alleles for a gene are
expressed in a heterozygous
offspring
 = both alleles of a trait show
 Neither allele is dominant or
recessive, nor do the alleles
blend in the phenotype
How does this differ from incomplete dominance?