Explain the meanings of Mendel’s principle of segregation and principle of independent assortment. Under what circumstances does the principle of independent assortment hold true? Give an example in which this law does not apply.

Mendel's principle of segregation and principle of independent assortment are two fundamental principles in genetics.

The principle of segregation, also known as Mendel's first law, states that during the formation of gametes (sperm and egg cells), the alleles (alternative forms of a gene) for each trait segregate and separate from each other. This means that offspring inherit one allele from each parent and that the alleles separate during the production of gametes. As a result, each gamete receives only one allele for a particular trait.

The principle of independent assortment, also known as Mendel's second law, states that the alleles for different traits segregate independently of each other during the formation of gametes. This means that the inheritance of one trait does not influence the inheritance of another trait. In other words, the alleles for different traits are assorted randomly and independently into gametes.

The principle of independent assortment holds true under the circumstance that the genes for different traits are located on different chromosomes or are far apart on the same chromosome. Chromosomes assort independently during the process of meiosis, which is the cell division that produces gametes.

An example where the principle of independent assortment does not apply is when genes for different traits are located close to each other on the same chromosome. This is known as genetic linkage. When genes are closely linked, they tend to be inherited together as a unit, rather than independently assorting. Geneticists refer to this as linked genes. Linked genes violate the principle of independent assortment since they do not segregate independently during inheritance.

One classic example of linked genes is the inheritance of coat color and eye color in fruit flies. In certain fruit fly species, the genes for these traits are linked on the same chromosome. As a result, specific combinations of coat color and eye color tend to be inherited together rather than independently assorting as expected by the principle of independent assortment.