The polar bear, scientifically classified as Ursus maritimus, is a specialized Arctic carnivore linked to the frozen sea ice environment. Understanding the biology of this large mammal begins with its genetic makeup. The information stored within the cells dictates traits like its thick white fur and ability to survive in harsh climates. This genetic blueprint is organized into structures that define the species.
The Specific Chromosome Count
The total number of chromosomes found within the body cells of the Ursus maritimus is a fixed biological characteristic. Polar bears possess a diploid number of 74 chromosomes. This count is consistently found in all non-reproductive cells, such as those that make up the skin, muscle, and organs of the animal.
This number, the 2n number, means the chromosomes exist in 37 distinct pairs. The specific number of chromosomes is a defining feature of the species, maintained across generations since the polar bear diverged from its ancestors.
Defining the Genetic Units
Chromosomes are microscopic thread-like structures composed of DNA and proteins that reside within the nucleus of every cell. They serve as carriers for the genetic instructions, or genes, that determine the polar bear’s traits. The count of 74 refers to the diploid state, where chromosomes are arranged into pairs: one set of 37 inherited from the mother and the other set of 37 inherited from the father.
These paired chromosomes are known as homologous pairs, carrying genes for the same traits in the same sequence. The diploid count is twice the number found in reproductive cells, or gametes. These sex cells contain only one set of chromosomes, giving them a haploid number (n) of 37.
This halving of the chromosome number is achieved through a cell division process called meiosis. When a sperm and egg combine during fertilization, the two haploid sets merge, restoring the full diploid count of 74 chromosomes in the offspring. This process ensures that the species’ characteristic chromosome number is maintained across generations.
Genetic Relationship to Other Species
The specific number of 74 chromosomes is not unique to the polar bear; its closest relative, the brown bear (Ursus arctos), shares the same diploid count. This shared karyotype, or chromosome set, indicates the two species’ recent evolutionary split. The genetic compatibility resulting from this identical chromosome number permits successful interbreeding when their ranges overlap.
When a polar bear and a brown bear mate, they can produce fertile hybrid offspring, commonly known as grolar bears or pizzly bears. The ability of these hybrids to reproduce depends on their parents having the same number of chromosomes, ensuring correct pairing during the hybrid’s reproductive cell formation. For contrast, the offspring of a horse (64 chromosomes) and a donkey (62 chromosomes) is a sterile mule, because the mismatch prevents proper pairing and subsequent reproduction.
While the shared count highlights a close relationship, the number of chromosomes does not determine the complexity of an organism; for instance, humans possess only 46 chromosomes. The identical 74-chromosome count in polar and brown bears underscores their shared lineage and genetic similarity, even though the polar bear has adapted specialized traits for its marine environment.