Dihybrid Crosses (2 Traits)
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Each side represents possible gametes (egg or sperm) from the parents
Each gamete has one copy of each gene (one of each letter)
Write all the possible combinations of the alleles for genes for parent
4 combinations if parent is heterozygous
For example, if parent is AaBb, gametes possible are AB, Ab, aB and ab
AB
Ab
aB
ab
AB Ab aB
AABB AABb AaBB
AABb AAbb AaBb
AaBB AaBb aaBB
AaBb Aabb aaBb
ab
AaBb
Aabb
aaBb
aabb
• The boxes represent all of the possible offspring for these parents.
• Count how many of each phenotype combination and give the ratio in
the following order- Dominant for both : Dominant for 1st- Recessive for
2nd: Recessive for 1st- Dominant for 2nd : Recessive for both
Dominant for both (A_B_)
Dominant for 1st trait, Recessive for 2nd (A_ bb)
Recessive for 1st trait, Dominant for 2nd (aaB_)
Recessive for both (aabb)
9
3
3
1
If both parents are heterozygous, the phenotype ratio is always 9:3:3:1