What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.

Chromosomes are thread-like structures present in the nucleus of a cell that contain information for the inheritance of features from parents to the next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.

In sexually reproducing organisms, the number of chromosomes in the progeny is maintained through the following mechanism:

1. Meiosis (Reduction Division): Organisms have special lineages of cells in specialized reproductive organs. These cells undergo a specific type of cell division called meiosis, which halves the number of chromosomes and the amount of DNA compared to non-reproductive body cells. This results in the formation of haploid ($n$) germ-cells (gametes).
2. Fertilization: During sexual reproduction, a male germ-cell ($n$) fuses with a female germ-cell ($n$) to form a zygote. This fusion re-establishes the original diploid ($2n$) number of chromosomes and the DNA content in the new generation (progeny).