When organisms reproduce, genetic information is passed to the next generation through DNA. Within DNA are blocks of nucleotides called genes, each of which contains the code needed to produce a specific protein. Genes are responsible for traits, or characteristics, in organisms such as eye color, height, and flower color. The sequence of nucleotides in DNA is called an organism’s genotype, while the resulting physical traits are the organism’s phenotype.
Different versions of the same gene (e.g., one that codes for blue eyes and one for green eyes) are called alleles. During sexual reproduction, the child receives two alleles of each gene—one each on the mother’s chromosomes and the father’s chromosomes. These alleles can be homozygous (identical) or heterozygous (different). If the organism is heterozygous for a particular gene, which allele is expressed is determined by which alleles are dominant and/or recessive. According to the rules of Mendelian heredity, dominant alleles will always be expressed, while recessive alleles are only expressed if the organism has no dominant alleles for that gene.
The genotype, and resulting phenotype, of sexually reproducing organisms can be tracked using Punnett squares, which show the alleles of the parent generation on each of two axes. (Note that dominant alleles are always depicted using capital letters while recessive alleles are written in lower case.) The possible phenotype of the resulting offspring, called the F1 generation, are then shown in the body of the square. The squares do not show the phenotypes of any one offspring; instead, they show the ratio of phenotypes found across the generation. In the figure below, two heterozygous parents for trait R are mated, resulting in a ratio of 1:2:1 for homozygous dominant, heterozygous, and homozygous recessive. Note that this creates a 3:1 ratio of dominant to recessive phenotypes.
Similarly, crossing two parents that are heterozygous for two traits (dihybrids) results in a phenotypic ratio of 9:3:3:1, as shown below. This ratio is known as the dihybrid ratio.
Non-Mendelian inheritance describes patterns in inheritance that do not follow the ratios described above. The patterns can occur for a number of reasons. Alleles might show incomplete dominance, where one allele is not fully expressed over the other, resulting in a third phenotype (for example, a red flower and white flower cross to create a pink flower). Alleles can also be codominant, meaning both are fully expressed (such as the AB blood type).
The expression of genes can also be regulated by mechanisms other than the dominant/recessive relationship. For example, some genes may inhibit the expression of other genes, a process called epistasis. The environment can also impact gene expression. For example, organisms with the same genotype may grow to different sizes depending on the nutrients available to them.
When a person’s genetic code is damaged, that organism may have a genetic disorder. For example, cystic fibrosis, which causes difficulty with basic bodily functions such as breathing and eating, results from damage to the gene which codes for a protein called CFTR. Down syndrome, which causes developmental delays, occurs when a person has three copies of chromosome 21 (meaning they received two copies from a parent as a result of an error in meiosis).
Genes are not static. Over time, mutations, or changes in the genetic code, occur that can affect an organism’s ability to survive. Harmful mutations will appear less often in a population or be removed entirely because those organisms will be less likely to reproduce (and thus will not pass on that trait). Beneficial mutations may help an organism reproduce, and thus that trait will appear more often. Over time, this process, called natural selection, results in the evolution of new species. The theory of evolution was developed by naturalist Charles Darwin based in part on his observations of finches on the Galapagos Islands. These finches had a variety of beak shapes and sizes that allowed them to coexist by using different food sources.
As a result of these processes, all organisms share a distant evolutionary predecessor. As evolution progressed, species subsequently split off as different branches of the phylogenetic (evolutionary) tree of species diversity, leading to the complexity of life seen today. For example, humans share a recent evolutionary ancestor with other primates (but did not evolve directly from any of these species).