Dihybrid crosses, Mendel's 2nd Law and test crosses

As Mendel did his work, he would sometimes observe 2 traits in a plant that differed from the plant it was crossed with.  This is what is meant by "dihybrid" crossing.  Simply, when the cross is carried out, two traits are observed.
For example, Mendel carried out a cross of a pea plant producing round, yellow peas, with a plant that produced wrinkled green peas.  All of the F1 were round and yellow.  When the F1 were crossed he made the following observation:

http://www.biozoomer.com/2011/03/dihybrid-cross.html

Mendel observed a 9:3:3:1 ratio of the different phenotypes, which led him to the conclusion that the genes for the two traits are inherited independently.

Mendel's 2nd Law:  The Law of Independent Assortment

Mendel noticed that the different traits had no impact on each other, so he concluded that genes for different traits assort independently of one another in the formation of gametes.

It is interesting to note that if you consider 1 trait at a time we get the "usual" 3:1 ratio that we saw in the single hybrid cross.  Just compare the colour trait in the offspring - 12 green and 4 yellow or a 3:1 ratio.  The same is seen with seed texture, showing that the traits are inherited independently of each other.

TEST CROSSES

Often when we observe the phenotype of an organism, we don't know from the phenotype whether the individual is homozygous or heterozygous for that trait.  Carrying out a test cross can solve this dilemma.  A test cross is simply when the individual with the unknown phenotype is crossed with a homozygous recessive individual and the ratio of offspring are observed.  If all of the offspring show the dominant phenotype, then the unknown individual must have a homozygous genotype, if half the offspring are recessive, then the unknown individual must be heterozygous.

http://tipstoscore.blogspot.hu/2011/10/test-cross-mendelian-genetics.html