Dihybrid
crosses
a study of inheritance
patterns for organisms
differing in two traits… more
realistic
Dihybrid cross
 Monohybrid:
heterozygous (having 2
different alleles for one gene pair).

Ex: Aa
 So
what is a dihybrid????
 Di: meaning two
 Hybrid: means heterozyous
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instead of one trait we are looking at two
traits at the same time
AaBb represents the genotype of a dihybrid
Remember Mendels laws:
 Law
of segregation.
 Only one allele for each trait can be
passed at a time. Only pass on one ‘A’
AND one ‘B’ and one…
 Law
of independent assortment.
 How each type of letter (allele) sorts or is
passed on to the next generation is
independent

i.e. Which “A” is passed on doesn’t affect which “B” is
passed on
What combinations of alleles
are possible?
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If an organism has a genotype of AaBb
What are all the different combinations of A’s and
B’s?
AB
Ab
aB
ab
These combinations represent the possible
gametes (sperm or egg) that could form during
meiosis.
An example of a dihybrid cross
 Key:
 E = Two Eyes,
 T = Triangular
e = one eye
Head, t = pentagonal head
 So,
lets have two dihybrid parents; therefore
their genotypes are:
 EeTt
x EeTt: What do they look like?
 Phenotype=
 Phenotype:
two eyes and triangular heads.
physical characteristics of an
organism: what you “see”.
 What
 ET,
types of combinations are possible?
Et, eT, et.
Is this all?
Punnett squares
Incomplete Dominance
 Incomplete
Dominance = neither allele is
dominant. Heterozygous individuals show an
intermediate (in between) phenotype.
 Example: petal color in carnations
RR = red
RW = pink
WW = white
Codominance
 Codominance
= both alleles contribute to the
phenotype. Heterozygous individuals show
both traits separately (not blended like in
incomplete dominance).
 Example: coat color in cattle
 RR
= red
 WW = white
 RW = roan
Multiple Alleles
 Multiple
Alleles = genes that have more than
two alleles.
 Example: blood type in humans