Down syndrome, or trisomy 21, is a genetic disorder caused by the presence of all or part of an extra copy of the 21st chromosome. Having this additional genetic material changes the course of development and results in the characteristics associated with Down syndrome.
Down syndrome varies in severity among individuals, causing lifelong intellectual disability and developmental delays. It’s the most common genetic chromosomal disorder and cause of learning disabilities in children. It also commonly causes other medical abnormalities, including heart and gastrointestinal disorders.
A better understanding of Down syndrome and early interventions can significantly increase the quality of life for children and adults with this disorder and help them live fulfilling lives. While it is usually not inherited, understanding how it is passed on can be helpful for those affected by the condition.
Common Characteristics of Individuals with Down Syndrome
Individuals with Down syndrome commonly have distinct physical features, unique health issues, and changes in cognitive development.
Physical features
Some common physical characteristics of Down syndrome can include:
- eyes that slant upward
- skin folds on the inner corner of the upper eyelid
- white spots on the iris
- low muscle tone
- small stature and a short neck
- a flat nasal bridge
- single, deep creases across the centre of the palms
- a protruding tongue
- small hands and feet
Developmental delays
People with Down syndrome usually have cognitive development profiles that suggest mild to moderate intellectual disability. However, cognitive development and intellectual ability are highly variable.
People with Down syndrome also experience learning difficulties that lead to developmental delays. A person with Down syndrome has a specific pattern of cognitive and behavioural features. These differ from those seen in typically developing children and children with other causes of intellectual disability.
Children with Down syndrome often reach developmental milestones a little later than their peers. They may be slow to sit, turn over, and stand.
There may also be a delay in coordination and fine motor skills (movements using small muscles in the hands and wrists). These skills can take time to develop after the child acquires gross motor skills, which involve movement of the whole body.
Development of speaking and grasping a language may also take longer than expected. With this said, people with Down syndrome eventually meet many of these milestones.
People with Down syndrome may also experience the following:
- difficulties with attention
- a tendency to make poor judgments
- impulsive behaviour
With engagement and regular therapy, most people with Down syndrome can attend school and become active community members.
Health issues
Sometimes, general health problems can affect any organ system or bodily function. About 40–60% of all people with Down syndrome have a congenital heart defect.
There may also be a higher risk of:
- congenital hypothyroidism
- hearing loss
- seizures
- vision disorders such as cataracts
- decreased muscle tone
Children with Down syndrome are also more likely to develop some infections, such as:
- respiratory infections
- middle ear infections
- tonsillitis
- pneumonia
There seems to be a lower risk of hardening in the arteries, diabetic retinopathy, and cancer.
Types of Inheritance Patterns
Down syndrome can be inherited in three ways: nondisjunction, mosaicism, and translocation.
Nondisjunction occurs when an error takes place during meiosis when chromosomes fail to separate properly during cell division. Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Before or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. The extra chromosome is replicated in every body cell as the embryo develops. This type of Down syndrome, which accounts for 95% of cases, is called trisomy 21.
Mosaicism refers to cases where some cells contain three copies of chromosome 21 while others have two. This is diagnosed when there is a mixture of two cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. Mosaicism is the least common form of Down syndrome and accounts for only about 2% of all cases of Down syndrome.
In translocation, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.
Risk Factors
The risk factors associated with Down syndrome vary depending on the type of inheritance pattern involved. For nondisjunction, age plays a role, as older mothers are more likely to conceive a child with Down syndrome due to increased chances for errors during meiosis. With translocation, there are no known risk factors; however, there may be a higher chance if you have family members who have had children with the disorder due to possible hereditary involvement. Finally, mosaicism does not have any known risk factors either; it just happens randomly and spontaneously due to errors occurring during cell division after conception.
Regardless of the type of Down syndrome a person may have, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells.
Mother’s age
The cause of the extra full or partial chromosome is still unknown. Maternal age is the only factor linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism. However, due to higher birth rates in younger women, 51% of children with Down syndrome are born to women under 35 years of age.
Pregnancy
No definitive scientific research indicates that Down syndrome is caused by environmental factors or the parents’ activities before or during pregnancy. The additional partial or full copy of the 21st chromosome, which causes Down syndrome, can originate from either the father or the mother. Approximately 5% of the cases have been traced to the father.
Life with Down Syndrome
A person with Down syndrome can do many things that other people do. Children may take longer to acquire skills such as walking and talking, but they can develop at their own pace and attend school with early stimulation and treatment. Depending on how the condition affects a person, they may work and live semi-independently with Down syndrome.
People with Down syndrome need friendships and relationships. Some will live with a partner or get married, having an independent life.
The Centers for Disease Control and Prevention (CDC) note that the life expectancy of a person with Down syndrome has increased significantly with modern advancements in medicine and treatment.
In 1960, a person with Down syndrome lived an average of 10 years. By 2007, a person with Down syndrome lived an average of 47 years.
A person born with Down syndrome today has the best chance at leading a long, engaging life due to modern advancements in healthcare, early treatments, and successfully managing congenital issues such as heart conditions.
Conclusion
Down syndrome is a lifelong condition. Services early in life will often help babies and children with Down syndrome to improve their physical and intellectual abilities. Most of these services focus on helping children with Down syndrome develop to their full potential. These services include speech, occupational, and physical therapy, typically offered through early intervention programs in each state. Children with Down syndrome may also need extra help or attention in school, although many are included in regular classes.
While Down syndrome cannot be cured or prevented at this time, understanding how it is inherited can help those affected manage their expectations and plan accordingly for their future needs and care options. If you are concerned that you may be at risk because you or someone in your family has been diagnosed with Down syndrome, speak with your doctor or a genetics counsellor who can provide more information on testing options and risks associated with each type of inheritance pattern discussed here today.
References:
https://www.mayoclinic.org/diseases-conditions/down-syndrome/symptoms-causes/syc-20355977
https://www.nichd.nih.gov/health/topics/down/conditioninfo/causes
https://www.medicalnewstoday.com/articles/145554
