Chromatid

Either of the two longitudinally adjacent threads formed when a eukaryotic chromosome replicates prior to mitosis. The chromatids are held together at the centromere. Sister chromatids are derived from the same chromosome.

Chromosome

One of the threadlike “packages” of genes and other DNA in the nucleus of a cell. Different kinds of organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes, 46 in all: 44 autosomes and two sex chromosomes. Each parent contributes one chromosome to each pair, so children get half of their chromosomes from their mothers and half from their fathers.

Cognition

The mental process of thinking, learning, remembering, being aware of surroundings, and using judgment.

Gene

The functional and physical unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.

Genetic Map

A listing of the relative positions of genes on the chromosomes and the distances between them. In addition to its value in Down syndrome research, genetic mapping has been used successfully to find the single gene responsible for relatively rare inherited disorders, like cystic fibrosis and muscular dystrophy. Maps have also become useful in guiding scientists to the many genes that are believed to interact to bring about more common disorders, such as asthma, heart disease, diabetes, cancer and psychiatric conditions.

Genotype

The genetic identity of an individual that does not show as outward characteristics. The genotype specific to Down syndrome is chromosomal content with an extra 21st chromosome.

Meiosis

A form of cell division happening in sexually reproducing organisms by which two consecutive nuclear divisions (meiosis I and meiosis II) occur without the chromosomal replication in between, leading to the production of four haploid gametes (sex cells) each containing one of every pair of homologous chromosomes (that is, with the maternal and paternal chromosomes being distributed randomly between the cells).

Mosaicism

Mosaicism is a condition in which cells within the same person have a different genetic makeup. In Down syndrome mosaicism, there is a mixture of cells containing either 46 or 47 chromosomes. This results from a random event during cell division early in fetal development.

Neuron

A type of cell that receives and sends messages from the body to the brain and back to the body. The messages are sent by a weak electrical current. Also called nerve cell.

Non-disjunction

The failure of homologous chromosome or chromatids to segregate during mitosis or meiosis with the result that one daughter cell has both of a pair of parental chromosomes or chromatids, and the other has none.

Phenotype

The observable traits or characteristics of an organism, for example, hair color, weight, or the presence or absence of a disease. Phenotypic traits are not necessarily genetic.

Synapse

The space between the end of a nerve cell and another cell. Nerve impulses are usually carried to the neighboring cell by chemicals called neurotransmitters, which are released by the nerve cell and are taken up by another cell on the other side of the synapse. The neighboring cell may be another nerve cell, a muscle cell, or a gland cell.

Translocation

Breakage and removal of a large segment of DNA from one chromosome, followed by the segment’s attachment to a different chromosome.

Trisomy 21

The presence of a single extra 21st chromosome, yielding a total of three chromosomes instead of a pair.  Partial trisomy refers to the presence of an extra copy of a segment of the 21st chromosome, which results from translocation.

(Source, U.S. National Institutes of Health)