Chromosomes divide and replicate through the process of mitosis. The cellular process of mitosis replicates chromosomes and prepares two identical nuclei as the fertilized ovum, known as the zygote, undergoes a kind of cell division. During this process, the double helix of DNA separates into two chains; each chain serves as a template for constructing a new chain. Individual DNA nucleotides are linked into new strands with bases complementary to those in the originals. In this way, two identical double helices are formed, each containing one of the original strands and a newly formed complementary strand. These double helices are duplicates of the original DNA chain.

Mitotic cell division occurs in five phases: interphase, prophase, metaphase, anaphase, and telophase. At interphase, one diploid cell of 46 chromosomes (2n) begins DNA replication of chromosomes as two sister chromatids attached at the centromere of the chromosome. At prophase, the nuclear membrane and nucleolus begin to break down as DNA condenses to form chromosomes while the mitotic proteins called microtubules form and attach to the kinetochores of each chromosome. In metaphase, DNA condenses the chromosomes that align at the center of the cell through the action of the microtubules fibers with DNA. At anaphase, the microtubules fibers shorten, pulling the chromatids of each chromosome to opposite poles of the cell while unattached fibers further elongate the cell. During telophase, the chromatids (called chromosomes) reach the poles of the cell as nuclear membranes reform as nucleoli, as chromosomes relax and DNA unwinds. Cytokinesis then occurs as division of the cytoplasm forms two daughter cells with 46 (2n) chromosomes in each of these cells genetically identical to the original and to each other.

Meiosis is the process of formation of the egg cell found in the ovary for the female and for the sperm cell found in the testis for the male. As germ cells, these cells are diploid (2n) having two sets of chromosomes that will undergo meiosis to become a haploid cell (1n) or one set of chromosomes. During fertilization, haploid cells fuse to form a diploid offspring (female or male). Meiosis involves two divisions, as Meiosis I and Meiosis II, which result in four haploid cells (1n). In Meiosis I, beginning at Prophase I, identical sister chromatids are joined at their centromeres including two homologous chromosomes lined up next to each other that undergo crossing-over at the site of the centromere to exchange DNA. After crossing over, the sister chromatids of one chromosome are no longer identical to one another. In Metaphase I, homologous chromosomes line up randomly along the equator of the cell, known as the process of independent assortment where gametes have different combinations of parental chromosomes with spindle fibers that attach to each centromere. In Anaphase I, chromosomes move apart from one another along the spindle fibers to the opposite end of the cell, with each chromosome double-stranded with two sister chromatids while separating each homologous pair as haploid for the formation of two different new cells. In Telophase I, cytokinesis occurs with cell division to form two cells created with half the number of chromosomes of the original cell. Meiosis II starts similar to division in mitosis at Prophase II where two cells with two sets of chromosomes each include spindle fibers with formation at the poles of the cell. In Metaphase II, the chromosomes line up along the equator, as each cell has only one of each homologous chromosome. Then in Anaphase II, sister chromatids move away from each other. Cytokinesis occurs in Telophase II forming four genetically different (1n) haploid cells.

Nondisjunction occurs with chromosomes that fail to disjoin during mitosis or meiosis. There are three forms of nondisjunction that include failure of sister chromatids separating during mitosis, failure of a pair of homologous chromosomes to separate in Meiosis I, and failure of sister chromatids to separate in Anaphase II in Meiosis II. Monosomy refers to a nondisjunction in which an individual is missing one of a pair of chromosomes, as 2n – 1. An example of monosomy is Turner’s syndrome where the female has only one X chromosome. Trisomy refers to a nondisjunction in which an individual has an extra chromosome, as 2n + 1. Down’s syndrome, or trisomy 21, refers to an individual with 3 chromosomes found at chromosome 21 instead of having one pair of chromosomes.

Alteration of the structure of the chromosome can occur through a deletion, duplication, translocation (to another chromosome), inversion, or ring formation in the
chromosome. A de novo abnormality is a noninherited chromosomal condition, while maternal age and environment can also be factors for chromosomal abnormalities. Mosaicism is another abnormality referred to as an abnormal cell division in two or more cells creating different numbers of chromosomes.

Genetic alterations related to cancer include aneuploidy, which refers to an abnormal chromosome number due to malignancy. Amplification occurs when overexpression of a gene results from an increase number of genes found in certain cancers. The four active stages in the cell cycle (G1, S, G2, and M) can also be altered when loss of checkpoints in one or more of these stages can also lead to genetic instability and cell damage.

Each parent contributes one set of chromosomes (and therefore one set of genes) so that every offspring has two genes for every locus on the autosomes. Some characteristics, or traits, such as eye color, are determined by one gene that may have many variants (alleles). Others, called polygenic traits, require the interaction of two or more genes. In addition, environmental factors may affect how genes are expressed.

Variations in a particular gene — such as brown, blue, or green eye color — are called alleles. A person who has identical genes on each member of the chromosome pair is homozygous for that gene; if the alleles are different, the person is said to be heterozygous.

On autosomes, one allele may be more influential than another in determining a specific trait. The more powerful, or dominant, gene product is more likely to be exhibited in the offspring than the less influential, or recessive, gene product. Offspring will exhibit a dominant allele when one or both chromosomes in a pair carry it. A recessive allele will not be exhibited unless both chromosomes carry identical copies of the allele. For example, a child may receive a gene for brown eyes from one parent and a gene for blue eyes from the other parent. The gene for brown eyes is dominant, and the gene for blue eyes is recessive. Because the dominant allele is more likely to mask the recessive allele, the child is more likely to have brown eyes.