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Across 1. is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses 4. depends upon the character of the genes contained in the parents' chromosomes, which in turn depends on the particular genetic code carried by the DNA of which the chromosomes are composed 6. The threadlike structures made of DNA molecules that contain the genes 7. is the study of changes in the expression of genes that do not result from alterations in the sequence of the genetic code 8. are siblings who develop from separate eggs and separate sperm cells, but during the same fertilization period. So they end up being conceived at the same time, developing at the same time, and being born at approximately the same time (2 Words)

Down 2. the process by which such forces as competition, disease, and climate tend to eliminate individuals who are less well adapted to a particular environment and favor the survival and reproduction of better adapted individuals, thereby changing the nature of the population over successive generations (2 Words) 3. are essentially the segments of DNA molecules that contain the code for particular peptides or proteins which then determine who we are (at birth and what we can become) 5. occur when a single fertilized egg splits into two separate embryos which then implant into the uterus (2 Words)