Trisomy 21

What is Trisomy 21?

Karyotype of human with Trisomy 21 (Down Syndrome)

Trisomy 21, also known as Down Syndrome, is a genetic disorder that occurs when a person has 47 chromosomes instead of the 46 normal chromosomes. In a normal human cell there are 23 pairs of chromosomes that contain genetic information. In the case of Down Syndrome, chromosome 21 usually has a third copy. This third copy of chromosome 21 is whats responsible of most of the malformation in a human being. The malformation or birth defects range from body and brain development.

It has been suggested that advanced maternal age can pose a greater risk for bearing a child with Trisomy 21. For a woman that is under the age of 35 the chances of having a child with Trisomy 21 are 1 in 1ooo. On the other hand, the chances for women older than 35 changes dramatically to 1 in 200. This means that women that 35 years of age or older bear 15% of all the babies born and have 50% of babies that are born with Down Syndrome.

History of Trisomy 21

Trisomy 21 is known today as Down Syndrome in the honor of John Langdon Down. Most would agree that in 1866 Down was one of the first physicians to observe the similar traits among his mentally retarded patients. In the 1930’s physicians started to suggest that Down Syndrome was due to a chromosomal abnormality. It wasn’t until 1959 that researchers confirmed that down syndrome was due to the presence of three copies of chromosome 21.


The symptoms of Trisomy 21 vary from person to person and can be present at different degrees.

This image provides a broad description of physical traits among children with Down Syndrome.

Physical Development:

  • Broad & Flat Face
  • Thick tongue
  • Small nose
  • Oval shapes eyes
  • Excess skin at nape of neck
  • Single crease in palm of hand
  • White spots on colored part of eye (Brushfield Spots)
  • Small ears
  • Decreased muscle tone
  • Extremely flexible joints

Mental and Social Development:

  • Short attention span
  • Slow learning
  • Impulsive behavior
  • Poor Judgment

Medical Conditions:

  • Congenital Heart Disease ( atrial and/or ventricular septal defect)
  • Gastrointestinal blockage
  • Hearing loss
  • Eye disorders
  • Leukemia
  • Thyroid Disorders


Testing whether a baby is born with Trisomy 21 can be done after birth or while the fetus is in the womb. During the first few months of pregnancy there are three tests that can be performed to screen for Trisomy 21:

  1. Amniocentesis

    Amniocentesis– occurs when a small amount of amniotic fluid is removed fromthe sac around the baby to test for birth defects or chromosomal abnormalities.

  2. Chorionic villus sampling– a sample of chorionic villi is removed from the placenta to test for chromosomal abnormalities.
  3. Nuchal translucency ultrasound– as the name suggests this scan looks at the amount of fliud around the neck of the fetus. The risk of having Trisomy 21 increases as the amount of fluid behind the neck increases.

After the baby is born the simplest way of determining whether the baby has Trisomy 21 is by simply observing how the baby looks. If indeed possesses the traits of a baby with down syndrome tests need to be performed to check for complications including heart defects gastrointestinal blockage.


Unfortunately, Trisomy 21 is permanent and cannot be reversed since it is a chromosomal abnormality. Initially after birth medications can be administered to the baby to improve the function of the heart and blood flow. If nothing improves or worsens, surgical intervention may be necessary to correct heart and or gastrointestinal problems.

Sexual Development

It is unclear whether to say that people with Trisomy 21 can be considered infertile. There have been cases of males and females with Down syndrome reproducing successfuly.

  • Males: Males with Trisomy 21 usually go through puberty in the normal time range compared to people with a normal karyotype. The levels of FSH (follicle stimulating hormone) and LH (luteinizing hormone) are normal during the beginning phase of buberty, but are elevated post puberty. The levels of testosterone appear normal throughout puberty. The size of penile length and the size of the testis in people with Trisomy 21 are below the average with those without the chromosomal abnormality. This suggests that males with Trisomy 21 present partial gonadal deficiency leading to infertility. But this is not a universal rule because there are cases of males with Trisomy 21 conceiving a child succesfully.
  • Females: In recent studies females with Trisomy 21 females have been observed to be normal in regards with reproductive organs. There usually no abnormalities in both external genitalia and internal reproductive organs. Females go through puberty in the normal time frame and have regular menstrual cycles. FSH and LH hormone levels are in normal range potentially leading to successful reproduction.

An explanation for the infertility of people with Trisomy 21 can be the lack of knowledge on sexual intercourse not because there are physiological problems present.


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