Module 5 Lesson 4 - 4

Chromosomal Disorders

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Any modification to the number of chromosomes can have a dramatic effect or can even be fatal to the affected individuals. All somatic cells in a human body have two copies of 22 autosomal chromosomes and two sex chromosomes for a total of 46 chromosomes. Nondisjunction can result in gametes with an extra copy of the chromosome or lacking a copy of the chromosome. When these abnormal gametes are fertilized, the result can be chromosomal disorders.

Chromosomal abnormalities resulting from nondisjunction can be categorized into two situations: monosomy and trisomy. Monosomy is the condition of an individual lacking one copy of the chromosome. Trisomy is the condition of an individual having an extra copy of the chromosome.

For example, Down Syndrome results from three copies of chromosome 21. It is also called trisomy 21. Down Syndrome is marked by intellectual and learning disability and characteristic facial features. Turner Syndrome is a condition resulting from having only one copy of the X chromosome. This condition is monosomy because this individual has only one copy of the sex chromosome. The affected individual is always female, and the condition is marked by short stature and inability to have children.

The table below shows the chromosomal disorders involving nondisjunction. Most of these conditions are very rare in human populations because most monosomic and trisomic zygotes are aborted spontaneously during embryonic development. In general, numerical abnormalities of the sex chromosomes have milder effect than trisomy of autosomal chromosomes.

Syndrome	Sex	Chromosome	Frequency	Description
Patau	M/F	Trisomy 13	1 in 16000	Multiple, complex organ problems  Most infants do not survive beyond first year of life.
Edward	M/F	Trisomy 18	1 in 5000	Most infants do not survive beyond first year of life.  Multiple life-threatening medical problems  Severe intellectual disability
Down	M/F	Trisomy 21	1 in 1000	Cognitive, physical, and learning disabilities  Characteristic facial features
Turner	F	X_	1 in 5000	Short stature  Unable to have children
Triplo-X	F	XXX	1 in 1000	No noticeable physical features  Increased risk of developmental and learning delays
Klinefelter	M	XXY	1 in 1000	Unable to have children  Tend to be very tall, but with poor muscle tone
Jacobs	M	XYY	1 in 1000	Tend to be very tall but without any other noticeable physical features  Increased risk of developmental and learning delays

Self-Check

Check your understanding. Complete the following questions on nondisjunction and abnormal meiosis; then, check your answers.

1. 1. What has happened that is not normal? What is this process called?
   2. Sketch a normal gamete that should arise from the mother cell.


2. Name three genetic disorders that are the result of nondisjunction.



Use the flowchart showing the results of nondisjunction in meiosis I to answer the next question. 



3. 1. In which division has nondisjunction occurred?
   2. Why is the egg cell shown abnormal?
   3. If the cell with twenty-two chromosomes were to give rise to the egg, would the egg be abnormal? Explain.


4. Zygotes having an abnormal number of chromosomes usually are described by the terms monosomy or trisomy. What do these terms mean?



Assume the eggs that could be produced from the division shown in question 3 are fertilized as illustrated.








5. Label each illustrated situation as either trisomy or monosomy.


6. Identify the syndrome produced by each of the abnormal sex chromosome combinations shown in the chart.







7. Some individuals may have the combination of sex chromosomes XXXY. What is the sex of this person and why?