### Dihybrid test cross or back cross

```
As in monohybrid crosses, you can’t tell by
looking at the dominant trait whether it is
homozygous or heterozygous.


As in monohybrid crosses, you can’t tell by
looking at the dominant trait whether it is
homozygous or heterozygous.
So in order to find out what the genotype is –
do a test cross with an organism that has
both recessive characteristics.


-
As in monohybrid crosses, you can’t tell by
looking at the dominant trait whether it is
homozygous or heterozygous.
So in order to find out what the genotype is –
do a test cross with an organism that has
both recessive characteristics.
If the parent is heterozygous you get a ratio
of 1:1:1:1


-
As in monohybrid crosses, you can’t tell by
looking at the dominant trait whether it is
homozygous or heterozygous.
So in order to find out what the genotype is –
do a test cross with an organism that has
both recessive characteristics.
If the parent is heterozygous you get a ratio
of 1:1:1:1
If it is homozygous, all the offspring will
show the dominant phenotype for both
characteristics.
Genotype:
Gametes:
AaBb
x
aabb
Genotype:
AaBb
x
Gametes: AB Ab aB ab
aabb
ab
Genotype:
AaBb
x
Gametes: AB Ab aB ab
Punnett Square:
aabb
ab
Genotype:
AaBb
x
Gametes: AB Ab aB ab
Punnett Square:
ab
AB
AaBb
Ab
Aabb
aB
aaBb
ab
aabb
aabb
ab

Now, you have an organism that is
homozygous dominant for one trait, but
heterozygous for the other – the test
cross/back cross still works.


-
Now, you have an organism that is
homozygous dominant for one trait, but
heterozygous for the other – the test
cross/back cross still works.
In the cross:
For the homozygous dominant characteristic,
no recessive trait will show up in the
offspring


-
-
Now, you have an organism that is
homozygous dominant for one trait, but
heterozygous for the other – the test
cross/back cross still works.
In the cross:
For the homozygous dominant characteristic,
no recessive trait will show up in the
offspring
For the heterozygous characteristic the
recessive trait will show up
Genotype:
Gametes:
AABb
x
aabb
Genotype:
Gametes:
AABb
x
AB AB Ab Ab
aabb
ab ab ab ab
Genotype:
Gametes:
Punnett Sq:
AABb
x
AB AB Ab Ab
aabb
ab ab ab ab
Genotype:
Gametes:
AABb
x
AB AB Ab Ab
Punnett Sq:
ab
AB
AaBb
Ab
Aabb
aabb
ab ab ab ab
Genotype:
Gametes:
AABb
AB AB Ab Ab
x
aabb
ab ab ab ab
Punnett Sq:
ab

AB
AaBb
Ab
Aabb
ALL show the dominant trait for A and HALF
show the recessive trait for B. From this, you can
deduce the genotype as AA for trait A and Bb for
trait B
```