Down Syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21.This additional genetic material alters the course of development and results in the physical and intellectual characteristics associated with the condition.
Down Syndrome is one of the most common chromosomal abnormalities, affecting approximately 1 in every 700 babies born worldwide. Individuals with Down Syndrome often exhibit distinct facial features, such as a flat facial profile, upward-slanting eyes, small ears, and a protruding tongue.
Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21, also known as trisomy 21. This condition leads to a range of developmental and physical characteristics.
Genetics:
Down syndrome occurs when an individual has three copies of chromosome 21 instead of the usual two. This extra genetic material affects development.
Physical Characteristics:
Common physical traits include a flat facial profile, upward-slanting eyes, a small nose, and a single deep crease across the center of the palm. These traits can vary in severity among individuals.
Symptoms of Down syndrome:
Physical Symptoms:
Facial Features:
Flat facial profile.
It is upward slanting eyes with a fold of skin on the inner corner (epicanthal fold).
Down syndrome affects individuals of all races and economic levels.
Global Prevalence:
Down syndrome occurs in about 1 in 700 live births worldwide. The condition is not limited to any specific population or geographic region.
Gender:
Both males and females are equally likely to be born with Down syndrome. The condition does not favor one gender over another.
Age of Parents:
The likelihood of having a baby with Down syndrome increases with the mother’s age. Women over the age of 35 have a higher risk, though younger women can also have children with Down syndrome.
Advanced paternal age (over 40) has also been associated with a slightly increased risk, but the mother’s age is a more significant factor.
Family History:
Down syndrome is not typically inherited. However, in a rare form known as translocation Down syndrome, a portion of chromosome 21 becomes attached (translocated) to another chromosome. This form can be inherited from a parent who carries the translocated chromosome but does not have Down syndrome.
Siblings and Other Relatives:
Siblings of a person with Down syndrome have a slightly increased chance of having a child with Down syndrome, especially if there is a family history of the condition.
No Socioeconomic Boundaries:
Down syndrome affects individuals regardless of socioeconomic status. It is a genetic condition that does not discriminate based on income, education, or access to healthcare.
What is trisomy 21?
Trisomy 21, also known as Down syndrome, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Instead of the typical two copies, individuals with trisomy 21 have three copies of this chromosome.
This extra genetic material affects the development of the individual, leading to the characteristic physical and cognitive features associated with Down syndrome.
Genetic Basis:
Humans normally have 46 chromosomes in each cell, arranged in 23 pairs. In trisomy 21, there are 47 chromosomes in each cell, with three copies of chromosome 21 instead of two.
Types of Trisomy 21:
Full Trisomy 21:
The most common form is where every cell in the body has an extra chromosome 21.
Mosaic Trisomy 21:
Some cells have the extra chromosome 21, while others do not. This can result in milder symptoms.
Translocation Trisomy 21:
A translocation has attached part of chromosome 21 to another chromosome. This form can be inherited.
Causes:
Trisomy 21 usually occurs due to a random error in cell division called nondisjunction, which results in an egg or sperm cell with an extra chromosome 21.
Scientists do not know the exact cause of nondisjunction, but they associate it with maternal age, as the risk increases with the mother’s age.
Physical Characteristics:
Individuals with trisomy 21 often have distinct facial features, such as a flat facial profile, upward-slanting eyes, a small nose, and a single deep crease across the palm.
Other physical traits include short stature, poor muscle tone, and broad, short hands.
Cognitive and Developmental Features:
People with trisomy 21 typically have mild to moderate intellectual disability and developmental delays.
They may have speech and language delays, learning difficulties, and slower progress in acquiring new skills.
How do doctors diagnose Down syndrome before birth?
Doctors can diagnose Down syndrome before birth through various prenatal screening and diagnostic tests.
Prenatal Screening Tests:
First-Trimester Combined Screening:
This typically includes a blood test to measure levels of certain proteins and hormones in the mother’s blood (such as PAPP-A and free beta-hCG) combined with an ultrasound measurement of the nuchal translucency (thickness of the back of the baby’s neck). Abnormal levels or measurements may indicate a higher risk for Down syndrome.
Second-Trimester Quadruple or Triple Screen:
Also known as the multiple marker test, this blood test measures levels of AFP (alpha-fetoprotein), hCG (human chorionic gonadotropin), estriol, and sometimes inhibin A. Abnormal levels may suggest an increased risk of Down syndrome.
Cell-Free DNA Testing (cfDNA):
This non-invasive test analyzes fetal DNA that circulates in the mother’s blood. It can detect the presence of extra chromosome 21 (trisomy 21) with high accuracy. While highly sensitive, healthcare providers typically offer cfDNA testing to women at higher risk based on age or screening results.
Prenatal Diagnostic Tests:
Chorionic Villus Sampling (CVS)
This invasive procedure involves taking a small tissue sample from the placenta (chorionic villi) for genetic testing. CVS is usually performed between weeks 10 and 13 of pregnancy and can detect chromosomal abnormalities, including Down syndrome, with high accuracy.
Amniocentesis:
Another invasive procedure, amniocentesis involves inserting a thin needle through the abdomen into the amniotic sac to collect a small amount of amniotic fluid. Fetal cells in the fluid are tested for chromosomal abnormalities, typically between weeks 15 and 20 of pregnancy. Amniocentesis can provide a definitive diagnosis of Down syndrome.
Diagnostic Accuracy:
Both CVS and amniocentesis accurately detect Down syndrome and other chromosomal abnormalities are considered diagnostic tests.The tests provide definitive results, but they have a small risk of causing a miscarriage. This risk is estimated to be about 1 in 300 to 1 in 500 for amniocentesis and slightly higher for CVS.
Counseling and Decision-Making:
Before undergoing any prenatal testing, expectant parents need to receive genetic counseling to understand the benefits, risks, and limitations of each test. This allows families to make informed decisions based on their circumstances and preferences.
Prenatal diagnosis of Down syndrome allows families to prepare emotionally and medically for the birth of a child with special needs, access early intervention services, and make decisions about pregnancy management based on their personal beliefs and values.
Conclusion
Down Syndrome results from an extra copy of chromosome 21, causing a variety of physical and intellectual characteristics. While individuals with Down Syndrome may face developmental delays and health challenges, advancements in medical care, early intervention, and inclusive practices have greatly enhanced their quality of life.
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