Introduction:
DNA is the genetic material in living organisms that stores information for the development and function of all living organisms. It consists of four nucleotide bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The ratio between these nucleotides varies among different species.

Answer:
The ratio that is generally constant for a given species is the ratio of G and C nucleotides to A and T nucleotides, known as the GC content. This ratio is typically expressed as a percentage of the total number of nucleotides in the DNA sequence. The GC content can vary greatly among different species, ranging from as low as 20% to as high as 80%.

Explanation:
The GC content is generally constant for a given species because it is influenced by several factors such as the environment, mutational bias, and natural selection. These factors can influence the frequency of nucleotide substitutions in the DNA sequence, thus affecting the GC content of the genome. However, the overall GC content remains relatively stable over time due to selective pressures that favor certain GC-rich or GC-poor regions in the genome.

Conclusion:
In conclusion, the GC content is generally constant for a given species and is an important characteristic of the genome that can influence many aspects of gene expression, evolution, and adaptation. Understanding the GC content of different species can provide insights into their biology and help researchers develop new strategies for studying and manipulating genetic information.

According to chargaff's first parity rule both are correct but according to 2nd parity rule both are wrong as the DNA is vary from species to species.