Geneticists use pedigree analysis to study inheritance of traits in human families. A pedigree is a diagram that uses different shapes to represent individuals, and lines connecting them to represent genetic relationships between individuals. Pedigrees in laboratory exercises help students visualize the inheritance of traits that follow the rules of Mendelian inheritance. For example, autosomal recessive, autosomal dominant, and sex-linked traits are easy to follow on a pedigree, by shading in the symbols representing the individuals with the particular traits. In this way, the pedigree demonstrates the inheritance of a specific trait as it passes from parents to offspring.
Karyotype analysis refers to studies of the appearance of chromosomes in the nuclei of eukaryotic cells. As humans are eukaryotes, human genetic studies often involve karyotype analysis. Typically, the experimenter arrests the cells during mitosis, when the chromosomes condense. Then, the experimenter stains the chromosomes and observes them under a light microscope. Abnormalities in chromosome length, centromere position, banding patterns, or differences in sex chromosomes are associated with phenotypic abnormalities, such as disease. If cell staining and microscopy is not possible in the laboratory, digital imaging will illustrate the process. Experimenters often diagnose human diseases through observations of chromosome abnormalities during karyotyping.
Single genes in the genome of an organism influence Mendelian traits. While many genes in the genome influence most human traits, a few traits follow the rules of Mendelian inheritance. For example, the ability to taste the chemical phenylthiocarbamide is an autosomal dominant trait, while albinism is an autosomal recessive trait. Discussions of Mendelian traits help illustrate the differences between dominant and recessive traits.
More than one, and often many genes in the genome influence quantitative, or metric genetic traits. Environmental variation also affects these traits or phenotypes. Quantitative traits follow a continuous distribution in human populations. For example, human height and weight are quantitative traits. Students measure the A quantitative traits of each student in the classroom, and construct a histogram to show the continuous distribution of such traits. This exercise helps explain the difference between these traits and Mendelian traits.