Sexual Reproduction and Meiosis

Genes and Chromosomes

Chromosomes carry genes on them which are expressed as traits in the organism.
Offspring often show similar traits to their parents because parents pass on genes (packets of information about traits) to their offspring. A gene is a unit of heritable material that codes for a particular trait. There are genes for hair color, eye color, height, and every other trait an organism expresses. There are also genes that cause diseases, such as sickle cell anemia and cystic fibrosis. A gene is a sequence of DNA nucleotides (adenine, thymine, cytosine, and guanine) found on chromosomes. A chromosome is a structure that contains DNA, the genetic material that is passed from one generation to the next. Living things have varying chromosome numbers. For example, humans have 23 pairs of chromosomes. Fruit flies have eight total chromosomes, and the adder's-tongue fern (Ophioglossum) has over 1,000 chromosomes. Chromosome number is not an indicator of how complex an organism is.
Organisms have varying numbers of chromosomes, which are large bundles of genetic material. Humans have 46 chromosomes, while chimpanzees have 48 chromosomes. But chromosome numbers do not relate to organism complexity. A mulberry tree has 308 chromosomes-far more than humans or chimpanzees.

Numbers of Chromosomes in Different Organisms

Organism Chromosome Number (2n)
American Beaver ( Castor canadensis) 40
Buffalo ( Bison bison) 60
Cat ( Felis catus) 38
Chimpanzee ( Pan troglodytes) 48
Roundworm ( Ascaris lumbricoides) 48
Corn ( Zea mays) 40
Sugar Beet ( Beta vulgaris) 18
Black Mulberry ( Morus nigra) 308
*

2n refers to the diploid number, in which there are two copies of each chromosome.

Not all genes are expressed all the time. The traits an organism expresses at any given moment are the result of certain genes being activated. Some genes are activated by extracellular signals, some of which are based on environmental conditions. This may mean that a tail is formed or that fur changes from white to brown during the summer. For example, the life cycle of a frog includes markedly different physical appearances at different stages. The juvenile form (a tadpole) has gills and a tail. As that tadpole becomes a frog, the gills are slowly replaced by lungs, the tail disappears, and legs form. The DNA of the organism remains the same throughout its life—there is no change in the DNA from tadpole to frog; however, the genes of the frog coded by its DNA are active at different times, resulting in the various appearances.
The life cycle of a frog involves the activation and deactivation of certain genes. As genes are activated or deactivated, different traits are expressed.