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

Chromosomes are thread-like structures, made of proteins and DNA, found in the nucleus at the time of cell division.
In sexually reproducing organisms, the gametes undergo meiosis, and hence, each gamete contains only half a set of chromosomes. When two gametes fuse, the zygote formed contains the full set of chromosomes. Hence, the formation of gametes by meiosis helps to maintain the number of chromosomes in the progeny.

Chromosomes are the thread like structures having DNA and associated proteins. Sexual reproduction includes gametogenesis via meiosis and random fertilization of male and female gametes. Meiosis reduces the chromosome number to half in gametes and fertilization restores the somatic chromosome number in the zygote.