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Dihybrid Crosses

The following legends were described for peas by Mendel:

T_
tt

Tall
dwarf

G_
gg

green (pod)
yellow

Purebreeding parents can be crossed to produce a dihybridmeaning
that 2 genes affecting different traits are heterozygous (segregating)
in all the f1 progeny.

Examples:

TT, GG X

tt, gg

Tt, Gg

TT, gg X tt, GG

Tt, Gg

When the F1 is self fertilized (plants) or crossed with another Tt, Gg
individual, the progeny will show the expected 3 dominant : 1
recessive phenotypic ratio for each trait.

If the two traits are independent, the two 3 : 1 ratios will interact to
give a ratio based on 16ths::

#
9
3
3
1

Genotypes
T_, G_
T_, gg
tt, G_
tt, gg

Phenotypes
Tall, Green
Tall, yellow
Dwarf, Green
Dwarf, Yellow

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For the example given the results would be the same as multiplying
the outcome expected for each trait (3 Tall : 1 dwarf) (3Green : 1

yellow) The "classical" 9:3:3:1 ratio of F2 genotypes from a
cross is proof that the 2 traits are independent.

dihybrid

Unless the genes happen to be close together on the
same chromosome, we can expect them to segregate in
an independent fashion. We will also see exceptions to
the expected 9:3:3:1 F2 dihybrid ratio if one or both
genes does not show a simple dominant/recessive
pattern of inheritance, or if both genes affect the same
phenotypic character.

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