CAUSES FOR DISEASES LIKE DOWN SYNDROME

 

                       CAUSES of genetic diseases LIKE DOWN SYNDR

About 1% of all live births produce those who suffer from some genetic disorder. Genetic disorders are medical conditions caused by an error in a person’s genetic material.

What is genetic material? Genetic material is the nucleic acid DNA, which lies coiled and bonded with proteins in the form of chromosomes inside the nucleus. Genetic disorders may be the result of changes in the number or structure of chromosomes.

All living beings are made up of trillions of microscopic cells. Chromosomes can be seen inside the nucleus. Every human living cell has a nucleus. Therefore, every cell contains chromosomes. The DNA molecules of chromosomes are responsible for the function of those cells. Further DNA molecules of reproductive cells (gametes) are responsible for the transmission of hereditary information from one generation to the next. When we think biologically, we can see that one generation is connected with its own next generation only via two cells. That is through sperm (from the father) and ovum (from the mother). These two cells fuse with each other (fertilization) and give rise to a zygote that develops into an individual. Therefore, we can say the connecting cells of the two generations are sperm and an ovum, and the connecting material is DNA, which lies inside the nuclei of sperm and ovum as chromosomes. This DNA controls the characteristics of an individual. Characteristic features of human beings will remain as they are throughout human history because those features are controlled by DNA and transmitted via DNA from one generation to the next. 

Cell nuclei of sperm and ovum contain hereditary material (chromosomes), which will be passed to the next generation by giving the characteristics of mother and father. The DNA in these chromosomes controls the characteristics. Therefore, chromosomes control the observable characteristics of an individual (the phenotype of an individual), and chromosomes are responsible for the transmission of hereditary material. However, some characteristics are developed due to environmental factors.

Each species has its own characteristic number of chromosomes in each cell of its individuals. In humans, this number is 46. That means every living cell (somatic cell) of human beings contains 46 chromosomes in their nuclei. These 46 chromosomes can be arranged into two groups/sets. In each group/ set, we can find 23 chromosomes. If we numbered these chromosomes of one set from 1-23, we can see that each and every chromosome of that set has a chromosome that exactly resembles it from the other set. Therefore, we have two sets of identical chromosomes in our body cells. Only one set of these two sets will go to the sperm when a male produces sperm. The same thing happens when an egg is produced by a female. Therefore, in a fertilized (fused) egg, we can see two sets of chromosomes. Sometimes abnormalities may occur in gamete (sperm and ovum) formation. 

When meiosis is not done correctly, abnormal gametes may form. Normally in the human ovum or sperm, there should be 23 chromosomes. In abnormal gamete formation, one set will miss one chromosome or will gain one chromosome. That means if the set misses one chromosome it will be 22 chromosomes in sperm or ova and if the set gains one it will be 24 chromosomes in sperm or ova. Abnormal gamete formation is rare in males, and sometimes it occurs in women over forty. When an abnormal ovum, which has 24 chromosomes, fuses with a normal sperm with 23 chromosomes, the newborn will have 47 chromosomes in each cell. But in normal conditions, there are only 46 chromosomes. This is the cause of most genetic disorders, such as

 1. Down’s syndrome,

 2. Klinefelter syndrome

3. Turner's syndrome.

Now we can consider the characteristic features of the above syndromes.

1. Down’s syndrome: affecting about 1 in 800 newborns. These patients have 3 chromosomes for the number 21 chromosome instead of 2. The disease is known as

Trisomy 21.

Characteristics of the disease: 

• Mental retardation                                                       
• Short stature, relatively small skull,
• Extra folds of skin at the inner corner of each eye
• Flattened nose,  coarse straight hair, heart defects

1. Klinefelter syndrome: This involves the abnormality of sex chromosomes. Normal males have two sex chromosomes XY. Normal females have two sex chromosomes XX. Abnormal males with Klinefelter syndrome have three chromosomes as XXY instead of two as XY. 1 in 500 males has Klinefelter syndrome.

Characteristics of the disease:

• Feminized males
• Sperm are never produced; erection and ejaculation are possible
• Some breast development
• Eunuch-like appearance
• Little facial hair
• Voice pitched higher than the normal
• Usually taller than the average
• Minor problems in learning
• Turner’s syndrome:

•  Misses one X chromosome, i.e. instead of XX, XO is present
• Sterile, ovaries are absent,
• Shortness of structure
• Small uterus
• Webbed neck may occur
• The hairline at the back of the head is lower than normal