Geneticists, scientists who study heredity, use family trees (called pedigrees), karyotyping, and molecular techniques to learn about human inheritance patterns.

1. A pedigree is a chart that shows how a trait, and the genes that control it, are inherited in a family. Pedegree analysis is the study of certain traits over several generations.
1. a circle represents a female and a square represents a male,
2. a line between a male and a female represents a marriage,
3. children are indicated by a line extending down from the marriage line,
4. a shaded circle or square indicates a person has the trait being considered,
5. a half-shaded circle or square indicates a person carries the allele for the trait but does not exhibit the trait,
6. the generations in a pedigree are numbered with Roman numerals (I, II, III, IV, etc.), and individuals are numbered with Arabic numerals (1, 2, 3, 4, etc.)
2. A karyotype is a photograph that shows an individual's chromosomes in homologous pairs and allows geneticists to study chromosomes directly. A karyotype can be used to find any chromosomal abnormality that is visible, like too many or too few chromosomes.

One pair of an individual's chromosomes, the sex chromosomes, determine the sex of the individual. Females have two identical sex chromosomes, termed "X" chromosomes, while males have two different sex chromosomes, an "X" and a "Y" chromosome. The other twenty-two pairs of chromosomes (44 total in humans ) are called autosomal chromosomes, or autosomes.

A mutation is a spontaneous change in a gene or chromosome. Some types of mutations are described below.

1. Nondisjunction

Occasionally, one or more chromosome pairs fail to separate during meiosis. This failure to separate during meiosis is called nondisjunction. Gametes (egg or sperm) will have either an extra chromosome or one too few. Then, if one of these gametes is involved in fertilization, instead of having the normal two chromosomes (one from mom and one from dad) for each of the 23 sets, the zygote and the resulting person will have for the affected chromosome either three copies (two from one parent and one from the other) or only one (one from one parent and none from the other). The condition in which an individual lacks a chromosome from one parent is called monosomy (mono means "one"); the condition in which an individual has an extra chromosome is called trisomy (tri means "three").

Some disorders that are the result of nondisjuction are:

1. Down syndrome occurs when an individual has three chromosome #21s ("trisomy 21").
2. Klinefelter syndrome occurs when a male has an extra X chromosome (XXY).
3. Turner syndrome is a disorder in which a person has only one X chromosome and no other sex chromosome. Affected persons are females (XO) fail to develop sexually and remain sterile.

1. Defective chromosome structure

1. Inversion, in which a section of a chromosome breaks off and reattaches upside-down. The code of bases, which determine the amino acids composition of a protein, will be read incorrectly.
2. Deletion, in which part of a chromosome breaks off. Cri-du-chat syndrome is an example of a disorder caused by a large deletion from chromosome 5.
3. Duplication, in which a delected section joins its homologous chromosome, thus duplicating the genes on that chromosome.
4. Translocation, in which a delected section joins a distant chromosome.

1. Gene Mutations: a single base is added, deleted, or exchanged.

1. Substitution, in which one base is replaced by another. This is also called a point mutation because only one amino acid will be affected.
2. Addition, in which one base is inserted. All the code down the DNA line will be offset by one and all the resulting transcribed amino acids will be wrong.
3. Deletion, in which one base is deleted. Like addition, all the codes down the DNA strand from the deletion will be mis-translated.