Detailed explaining test-crosses - the usefulness, purpose and how it is performed

Explain the test - cross. I have to explain the usefullness of the test-cross. I had my paper written then was told I need to incorporate the Punnett square, monohybrid cross, dihybrid cross etc. into the paper. I am now confused - maybe I have just researched so much I cannot make sense of any of it. How is the Punnett Square, test cross, monohybrid cross, dihybrid cross all tied in together?

genes

Highly expressed genes have a much greater tendency to lose introns (in evolutionary time scale) than genes expressed at very low levels. What molecular mechanism explains this difference?

Down syndrome

Down syndrome occurs once in approximately 700 live births. The vast majority of these cases are due to trisomy 21 resulting from a non-disjunction event. In rare cases, Down syndrome is caused by a reciprocal translocation between chromosome 21 and the tip of chromosome 14 (or some other autosome) in the germ line. No genes on chr14 are moved to chr21 and nearly all the genes on chr21 are added to the tip of chr14 (i.e., a Robertsonian translocation). Because Down syndrome females with this translocation are fertile, they can pass the translocation chromosome to their descendents, resulting in a high incidence of Down syndrome in their families. Assuming that the translocation chromoso... click for more

Understanding how inheritance and genetics is involved in evolution?

How is the genetic and inheritance prosess related in evolution?

eukaryotic DNA

Is eukaryotic DNA packaged in the cell in the form of a double helix