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Why is not Mendel’s second law always valid for two or more phenotypical traits of an individual?

Mendel’s second law, or the law of the independent assortment, is valid for genes located in different chromosomes. These genes during meiosis segregate independently.

Mendel’s second law however is not valid for phenotypical features conditioned by genes located in a same chromosome (genes under linkage), since these genes, known as linked genes, do not separate in meiosis (except for the phenomenon of crossing over).

Why is drosophila a convenient animal for the study of genetic linkage?

The fruit fly drosophila is suitable for the study of Genetics because it presents many distinct traits but only four chromosomes (one sex chromosome and three autosomes).

What is linkage?

Two genes are said to be under linkage, or linked, when they reside in the same chromosome.

For example, the research of the human genome discovered that the factor III of clotting gene and the factor V of clotting gene are located in the same chromosome (the human chromosome 1). The factor VII gene however is not linked to those genes since it is located in the chromosome 13.