Humans have long bred plants and animals to favor certain traits (e.g., dogs that retrieve things, trees that produce a lot of fruit). Thus, we know that these traits are influenced by genes. Such traits (called complex traits) vary continuously, like a bell curve, from less to more.
Many human traits, such as height and blood sugar, show a similar pattern, and these traits can also be inherited (e.g., tall parents tend to have tall children). Diseases such as heart disease, cancer, and diabetes are complex too. Genetically, complex traits are caused by many genes and environmental factors.
The basic laws of inheritance are important because they can reveal how a genetic trait of interest or a disorder can be passed from generation to generation.
Each person has 22 pairs of chromosomes. For each pair of chromosomes, one comes from the mother and one comes from the father. Thus, because genes are on chromosomes, there are actually two copies of each gene, one paternal in origin and the other maternal. We also get one sex chromosome from each parent, for a total of 46 chromosomes.Karyotype: a picture of a person's chromosomes arranged into pairs by size from 1-22. The X and Y chromosomes determined gender (female: XX; male: XY). Picture courtesy of NHGRI.
The specific degree of a complex trait is very difficult to predict from one generation to the next (e.g., the exact height of offspring). This is because the precise combination of genes contributing to the trait cannot be predicted either (or, perhaps, even known).
When geneticists look for evidence of genetic influence on a disease, such as heart disease or mental illness, they look for families that have many affected over several generations. However, for complex traits there is not obvious pattern from one generation to the next.
Unlike common complex traits, certain rare traits do show clear patterns of inheritance. In these traits, single genes strongly influence a trait in an 'either/or' manner.
Dominant traits require only one copy of a gene to express the trait (e.g., ability to roll your tongue, Huntington disease).
Recessive traits require two copies of a gene to express the trait (e.g., having a straight thumb, cystic fibrosis). A 'carrier' has only one copy of the gene for the recessive trait, so the carrier does not have the trait.