Birth Definitions

Chromosome Definition


A chromosome is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells. The word “chromosome” comes from the Greek words chroma, meaning “color,” and soma, meaning “body.” 

The function of chromosomes is to store and transmit hereditary information. This information is encoded in the sequence of base pairs that make up each chromosome’s DNA. During cell division, chromosomes are copied so that each new cell gets a complete set. When sperm and eggs fuse during fertilization, they form a zygote with 23 pairs of chromosomes, half from each parent. Chromosomes are very important because they carry the genes that determine an individual’s traits and physical characteristics, such as hair and eye color, height, and build.

Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. One set of 23 chromosomes comes from the mother’s egg, and the other set comes from the father’s sperm. When a sperm fertilizes an egg, the union results in a zygote with 46 chromosomes—half from each parent. The 22 pairs of autosomes are numbered roughly in order of decreasing size—the largest pair being number 1 and the smallest being number 22. In addition to these 22 pairs of autosomes, there is also a pair of sex chromosomes. In males, these are designated XY; in females they are designated XX.

During pregnancy, the chromosomes from the father’s sperm combine with the mother’s egg to form the baby’s unique DNA. This process is called fertilization. The baby’s DNA will determine its sex, as well as other physical characteristics. When changes occur in the structure or number of chromosome pairs, it is called a chromosomal aberration or abnormality.

Chromosomal Abnormality

Most chromosome abnormalities happen by chance as the embryo forms and grows. These abnormalities can cause problems with growth, development, and function of the body’s systems. Some babies with chromosomal abnormalities do not survive to birth (stillbirth), while others who make it to term often have significant health problems requiring special care.

Examples of chromosomal abnormalities include:

• Down syndrome (trisomy 21): Extra chromosome 21. Children with Down syndrome have a wide range of developmental delays, intellectual disability, and other physical abnormalities

• Edwards syndrome (trisomy 18): Extra chromosome 18. Infants with Edwards syndrome often have heart defects, low birth weight, small head size, and clenched hands. The majority of infants with this abnormality do not survive the first year of life.

• Patau syndrome (trisomy 13): Extra chromosome 13. Infants with Patau syndrome often have brain and heart defects, cleft lip or palate, extra fingers or toes (polydactyly), and low birth weight. The majority of infants with this abnormality do not survive the first week of life.

This image shows how often the term ‘Chromosome’ is used in relation to other, similar birth terms:

Some terms closely associated with the term chromosome are: DNA, genes, heredity, inheritance, and traits.

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