Non-Disjunction Causes and Effects

Non-Disjunction Causes and Effects

Overview

This paper shows the basis of non-disjunction of chromosome, a failure of chromosomes to separate properly during cell division. It emphasizes particularly on how the reproductive cells or gametes is created during reproduction. The gametes of women are eggs and of men are sperms. When the egg and the sperm combine during fertilization at conception, they create a normal offspring with the proper number of chromosomes of 46. Nondisjunction occurs when these chromosome fail to segregate and can cause common abnormality of having an extra chromosome or missing a chromosome in its cells thus creating an offspring with chromosome abnormalities of 45 or 47.

Chromosome is a structure located inside the nucleus (center) of a cell that has long DNA molecule and proteins that present in every cell of the body. It also carries portions of the hereditary information needed for that cell to develop. Homologous chromosomes are chromosomes that occur in pairs. Normal humans have twenty-three homologous pairs.

A female's egg contains only one representative from each of the 23 chromosome pairs and the male's sperm contains the other half of the 23 chromosome pairs. Eggs and sperm are formed following meiosis to create the total count of 46 chromosomes in a cell of a normal offspring. A normal human normally has a situation of genetic balance. Each gene is working properly and contributing ideally towards that person's development. However, if a chromosome is either removed from or added, it creates a genetic imbalance (nondisjunction) and compromises that individual's development. This means that nondisjunction occurs when it produces the wrong number of chromosomes in a cell, results in severe genetic defects. Nondisjunction is one of the most common genetic disorders in humans, occurring in human pregnancies. It affects hundreds thousands of people in the world.

Specifically, nondisjunction is the failure of two members of a homologous pair of chromosomes to separate properly in cell division and create the unequal segregation of chromosomes during meiosis. This can happen in one of two ways: homologues chromosomes fail to separate during anaphase I of meiosis, or sister chromatids not pulled apart during anaphase meiosis II. If the segregation process is running normally during meiosis, homologous chromosomes pair up and each gamete will contain one copy of every chromosome, which produces normal genetic. If the segregation process is running into error, the two members of a homologous pair of chromosomes fail to separate and this condition gives two types of gamete. The first gamete has two copies of one chromosome, and the second has no copy of that chromosome. This event is called aneuploidy - an abnormal chromosome number condition involving the loose or gain of a single chromosome. If the cell has one missing chromosome, it is called monosomic. It is a deficiency in number of chromosomes, which contains one less chromosome. When fertilized, the cell posses 45 chromosomes in total and it has less chance to survive since it does not make copies. When a cell has gained a chromosome, it is referred to as trisomy because it forms a zygote with 3 copies of one of the chromosomes. If this nondisjunction occurs in chromosome 21 of a human egg cell, it will produce a disease known as Down's syndrome (also referred to as trisomy 21). This is because their cells possess 47 chromosomes as opposed to the normal chromosome compliment in humans of 46.

A nondisjunction error during meiosis produces the wrong number of chromosomes in every cell of individual's body, can cause severe genetic defects and diseases such as Down's syndrome. Down syndrome happens more than ninety percent of nondisjunction cases.

Down Syndrome: It occurs due to a chromosome error (nondisjunction), causing both the embryo to have three chromosomes at chromosome #21. Individuals with Down syndrome have 47 chromosomes in a cell instead of the usual 46. In general, people with Down syndrome are usually having some distinctive physical features such as flattened face, slanting eyes, a short nose, hand, and their tongues are frequently sticking out. They normally have a lower Intelligence quotient (IQ) than the normal average person but some are smart enough to have education at college level. People with Down syndrome are born with many diseases such as leukemia and heart disease, hearing loss, infections disease and the dementia - a symptom associated with the loss of brain function as a side affect of mental or physical illness, which includes memory loss, lack of problem solving ability, and confusion. The incidence of Down syndrome is approximately 1 in 1500 for woman of 24 years olds. Approximately 350,000 people in the United States have Down syndrome. It affects people of all ages and races.

Nondisjunction also caused abnormalities in sex-chromosome and it affects the individual's secondary sexual characteristics and fertility. Males, who are born with more than one X chromosomes (XXY or XXXY), are known to have Klinefelter syndrome. People affected by this disorder usually have below-average intelligence. They have small male organ, tall stature and with long legs. They are actually male, but present some female secondary sexual characteristics. They may develop breasts, and they have little facial hair, and are sterile. Some of them can be treated with testosterone therapy to increase their strength, muscles, energy and sex drive. Klinefelter syndrome is found in about 1 out of every 500-1,000 newborn males.

Turner's syndrome: Females who are born with a single X chromosome (XO) due the absence of a Y chromosome. They are sterile and