1. Chromosomal Variations
Dawn Adams
Cytogenetics
CDC’s 2003 Science Ambassador Program
Now that we know what a karyotype is, how it can be obtained, and what its results will look like, it is important to investigate what these results mean. In a typical human karyotypes we would expect to see two sets of 23 chromosomes each. However, sometimes we might see variations in this number. These variations can cause different conditions or syndromes depending on the type of variation. In this presentation, we will explore different types of chromosomal variations and the syndromes that can occur from these variations.

2. Overview I. Chromosomal Variations A. Polyploidy B. Aneuploidy
II. Syndromes resulting from Aneuploidy
A. Trisomy 13
B. Trisomy 18
C. Down Syndrome
D. Turner Syndrome
E. Klinefelter Syndrome
F. XYY Syndrome
In particular, we will be investigating polyploidy and aneuploidy, two types of chromosomal variation. We will then look at five syndromes that result from aneuploidy, including occurrence and characteristics of each syndrome.

3. Variations in Chromosomal Number
Euploidy – the usual number and sets of chromosomes
Polyploidy – the presence of three or more complete sets of chromosomes
Aneuploidy – the presence of additional chromosomes or missing individual chromosomes
Euploidy is the presence of the usual number of chromosomes in a cell. In the case of humans, there are generally two sets of 23 chromosomes or 46 total chromosomes in cells. However, in variations of chromosomal number, there can be differences. For example, polyploidy is the presence of three or more complete sets of chromosomes in a cell. Another example of a chromosomal alteration is aneuploidy, the presence of additional chromosomes or missing individual chromosome. (1)

4. Types of Polyploidy Triploidy – three sets of chromosomes 23 x 3 = 69
Tetraploidy – four sets of chromosomes
23 x 4 = 92
There are two main types of polyploidy, triploidy and tetraploidy. Triploidy is the presence of three sets of chromosomes. This means that the total number of chromosomes in a triploidy cell would be 23 chromosomes per set multiplied by 3 sets or 69 total chromosomes. Tetraploidy is the presence of four sets of chromosomes or 92 chromosomes total. (1)
In a cell, each chromosome can have 100 to 1000 genes. Cells with triploidy can have from 6900 to genes which is significantly more than the 4600 to genes in the usual cell. Cells with tetraploidy can have from 9200 to genes, double the genes found in typical cells. (1)
Because of such an extreme difference in the amount of genetic material as compared to the usual amount, cases of triploidy and tetraploidy have severe effects. Babies with triploidy (referred to as triploidy syndrome) are usually lost through miscarriage. In rare cases, infants have survived up to five months with multiple birth defects present. Those that survive usually have some cells with a typical number of chromosomes (46 chromosomes) and some cells with the extra set (69 chromosomes). Babies with tetraploidy have an even rarer chance of surviving birth. (2)

5. Types of Aneuploidy Monosomy – one less chromosome (23 x 2) – 1 = 45
Trisomy – one additional chromosome
(23 x 2) + 1 = 47
There are also two main types of aneuploidy, namely monosomy and trisomy. Monosomy is the absence of only one chromosome, leaving the affected individual with 45 chromosomes instead of 46. Trisomy is the presence of an additional chromosome, or 47 chromosomes instead of the typical 46. (1)

6. Aneuploidy
When aneuploidy occurs in humans, syndromes can result. Examples include the following:
1. Trisomy 13
2. Trisomy 18
3. Down Syndrome
3. Turner Syndrome
4. Klinefelter Syndrome
5. XYY Syndrome
It is possible for babies with monosomy or trisomy to survive. In many cases, there are certain characteristics associated with aneuploidy, depending on which chromosome(s) are affected and the severity of the affected chromosome(s). Characteristics also vary depending on the number of cells that have the chromosomal variation. If very few cells have the variation, severity will be minimal. If most have the variation, severity will be much greater. (3)
When an individual has a particular karyotype as well as specified characteristics, their condition is defined as a syndrome. Some examples of aneuploidy associated syndromes include Trisomy 13, Trisomy 18, Down Syndrome, Turner syndrome, Klinefelter Syndrome, and XYY syndrome. The remainder of the presentation will discuss these syndromes and their features. (1)

7. Trisomy 13 Chromosomal Variation – Trisomy 13
(three copies of chromosome 13)
Occurrence – 1 in 5000
Trisomy 13 occurs when cells have three copies of chromosome 13 instead of two. Trisomy 13 affects about 1 in 5000 babies. (4)

8. Features of Trisomy 13 Severe developmental delays
Head and facial abnormalities
Extra fingers and toes
Kidney malformations
Heart defects
Early death
Two commonly associated characteristics of Trisomy 13 are developmental delays sometimes stemming from incomplete brain development, and head and facial abnormalities including unusually small eyes, an abnormal groove in the upper lip (cleft lip), and incomplete closure of the roof of the mouth (cleft palate). Less common head and facial abnormalities include a relatively small head with sloping forehead, a broad flat nose, wide-set eyes, vertical skin folds covering the eyes, scalp defects, and malformed, low-set ears. (4)
Those with Trisomy 13 might also have extra fingers or toes, kidney malformations, and structural heart defects. Generally, many infants with Trisomy 13 do not grow and gain weight at the expected rate and life threatening complications can develop during infancy or early childhood. (4)

9. Trisomy 18
Chromosomal Variation – Trisomy 18 (three copies of chromosome 18)
Occurrence – 1 in 5000
Trisomy 18 is the presence of three copies of chromosome 18 instead of just two. Like Trisomy 13, the occurrence of Trisomy 18 is 1 affected baby out of (5)

10. Features of Trisomy 18 Severe developmental delays
Head and facial malformations
Malformations of the hands and feet
Skeletal malformations
Kidney malformations
Structural heart defects
Early death
Common symptoms of Trisomy 18 include developmental delays and head and facial malformations including a prominent back portion of the head, malformed low-set ears, a small jaw, a narrow roof of the mouth, an upturned nose, widely spaced eyes, narrow eyelid folds, and drooping eye lids. Overlapped, flexed fingers, webbing of the toes, and other foot malformations may also occur. Skeletal malformations include a small pelvis and short breastbone. Kidney and heart problems can also result and, as with Trisomy 13, early death is associated with Trisomy 18. (5)

11. Down Syndrome
Chromosomal Variation – Trisomy 21 (three copies of chromosome 21)
Occurrence – 1 in
Down syndrome is the presence of three copies of chromosome 21 instead of just 2. Down syndrome occurs in one out of 800 to 1000 babies. This is a significantly higher rate than Trisomy 13 and Trisomy 18. (6)

12. Features of Down Syndrome
Low muscle tone
Head and facial malformations
Abnormalities of the extremities
Developmental delays
Heart malformations
Increased risk of infectious disease
Early death
Features include low muscle tone, a tendency to keep mouth open with a protrusion of the tongue, head and facial malformations such as a short small head with upwardly slanting eyelid folds, a depressed nasal bridge, a small nose, misshapen ears, narrow roof of the mouth, dental abnormalities, excessive skin on the back of the neck, and a relatively flat facial profile. People with Down syndrome also have abnormalities of the extremities including unusually short arms, legs, and fingers, as well as unusual skin patterns. Affected individuals might also be short in stature, have poor coordination, developmental delays, as well as hearing impairment. (7)
Sometimes Down syndrome can be associated with structural malformations of the heart at birth and an increased chance of infectious disease including leukemia. These problems can potentially be life-threatening. Early death is also associated with Down syndrome. (7)

13. Turner Syndrome Monosomy of sex chromosome
(only one X chromosome present)
Occurrence – 1 in 2500 live female births
Turner syndrome is normally found in woman and is caused by the absence or partial absence of an X chromosome. There is about one case of Turner syndrome in 2500 live female births. Approximately 60,000 girls and women are affected in the United States with around 800 new cases diagnosed every year. (8,9)

14. Features of Turner Syndrome
Short stature
Lack of ovarian development
Neck abnormalities
Skeletal disorders
Increased risk of osteoporosis, cardiovascular constriction, diabetes, and kidney and thyroid problems
Short stature and lack of ovarian development are among the most common characteristics of Turner syndrome. Additionally, neck abnormalities including a webbed neck and low hairline can also occur. Skeletal disorders including scoliosis (curvature of the spine), dislocated hips, and elbows that turn out can also be characteristics. People with Turner syndrome are more likely to have certain health problems such as osteoporosis, cardiovascular problems including constriction of the aorta and high blood pressure, and diabetes. Kidney and thyroid problems result in about 1/3 of the cases. It is also important to note that unlike Trisomy 13, Trisomy 18, and Down syndrome, developmental delays are not associated with Turner Syndrome. In fact, there is no association between developmental delays and any sex chromosomal abnormality. This indicates that genes affecting mental development and ability lie only on autosomal chromosomes. Early death is also not associated with any of the sex linked chromosomal abnormalities. (8,9)
Turner syndrome is commonly treated with growth hormones, and estrogen replacement therapy. Additionally, reproductive technology can help women with Turner syndrome become pregnant. (4)

15. Klinefelter Syndrome Trisomy of sex chromosome - XXY
(An additional X chromosome in males)
Occurrence – 1 in males
Kilinefelter syndrome can occur as the result of an individual having one Y and two X chromosomes instead of one of each. The occurrence of Klinefelter’s is estimated to be one in 500 to 1000 males, making it one of the most common sex chromosome disorders. (10,11)

16. Features of Klinefelter Syndrome
Tall
Sexually underdeveloped
Infertility
Sparse facial and body hair
Developmental delays
Increased risk of autoimmune disorders, breast cancer, osteoporosis, leg ulcers, depression, and dental problems
Characteristics normally associated with Klinefelter syndrome include being tall, sexually underdeveloped, and infertile, though in some case testicular function is preserved. Sparse facial and body hair is also a common characteristic. Klinefelter’s can also cause delays in speech and motor skills as well as deficits in attention, auditory processing and social skills. Learning disabilities, anxiety, and depression can also result. Treatment for these problems includes testosterone therapy and assisted learning. Finally, people with Klinefelter’s have an increased chance of certain health problems including autoimmune disorders such as type II diabetes, breast cancer, osteoporosis, leg ulcers, depression, and dental problems. (10,11)

17. XYY Syndrome Trisomy of sex chromosome – XYY
(An additional Y chromosome in males)
Occurrence – 1 in men
XYY syndrome is the presence of an additional Y chromosome in males. Reports of occurrence range from 1 out of 1000 men to 1 out of 2000 men. (13,14) There is no increased likelihood of fetal death as a result of this syndrome. (13,14)

18. Features of XYY Syndrome
Taller
Comparatively low weight relative to stature
Larger craniofacial dimensions
Severe acne in adolescence
Behavior problems
Learning disabilities
Slightly lower IQ than normal
Through the first decade of life and as adults, people with XYY syndrome might not show outward signs of the disorder, although final height is approximately 7cm above average and there is also comparatively low weight relative to stature. Other common symptoms include larger craniofacial dimensions than those without the condition, severe acne in adolescence, increased physical activity, and behavior problems. Learning disabilities can also result, though developmental delays are not considered a symptom of XYY syndrome. While intelligence is usually within the typical range, IQ is 10 to 15 points lower than siblings’. (12, 13)
Physical sexual development is standard and despite a certain level of decreased sperm quality, males are fertile. Additionally, it is very rare that fathers with XYY pass the syndrome on to their sons. (12, 13)
Treatment includes appropriate channeling of physical activity as well as monitoring for learning problems. (13)

19. Conclusions
Variations of chromosomal number involving a set of chromosomes (polyploidy) or an individual chromosome (aneuploidy) can occur
Aneuploidy results in syndromes with distinct characteristics
In conclusion, variations of chromosomal number can occur. These variations can either be polyploidy, in which extra chromosomal sets are present, or aneuploidy, in which there is an additional chromosomes or missing individual chromosome. While polyploidy usually results in fetal death, aneuploidy often results in characteristics that make up particular syndromes. Examples of these syndromes include Trisomy 13, Trisomy 18, Down syndrome, Turner syndrome, Klinefelter syndrome, and XYY syndrome. The characteristics of these syndromes can vary depending on severity of the chromosomal abnormality and the number of cells affected.

20. References
Fairbanks, D. J. and W. R. Anderson. Genetics: The Continuity of Life. Pacific Grove (CA): Brooks/Cole Publishing Company; 1999.
National Organization for Rare Disorders. Triploid Syndrome [online] [cited 2004 Feb 6]. Available at URL: disname=Triploid%20Syndrome.
March of Dimes. Chromosomal Abnormalities [online] [cited 2004 Feb 6]. Available from URL:
National Organization for Rare Disorders. Trisomy 13 [online] [cited 2004 Feb 6]. Available at URL: search/rdbdetail_abstract.html?disname=Trisomy%2013%20Syndrome.

21. References (continued)
National Organization for Rare Disorders. Trisomy 18 [online] [cited 2004 Feb 6]. Available at URL: search/rdbdetail_abstract.html?disname=Trisomy%2018%20Syndrome.
March of Dimes. Down Sydrome [online] [cited 2004 Feb 6]. Available from URL: 681_1214.asp.
National Organization for Rare Disorders. Down Syndrome [online] [cited 2004 Feb 6]. Available at URL:
Turner Syndrome Society of the United States. Resources & Research: FAQs [online] [cited 2004 Feb 6]. Available from URL:

22. References (continued 2)
National Institutes of Health. Turners Syndrome: Clinical Information [online] [cited 2004 Feb 6]. Available from URL:
American Association for Kilnefelter Syndrome Information and Support. Understanding Klinefelter Syndrome [online] [cited 2004 Feb 6]. Available from URL:
Hambley, Vaughn. What is XXY? [online] [cited 2002 Feb 6]. Available from URL:
Texas Department of Health. Birth Defect Risk Factor Series: 47, XYY [online] [cited 2004 Feb 6]. Available from URL:

23. References (continued 3)
California Center for Health Improvement. XYY Syndrome [online] [cited 2004 Feb 6]. Available from URL: