A. Change in number of chromosomes
Sometimes, mistake may happen in the separation of the chromosome pairs during the formation of the egg or sperm, resulting in either an addition or deletion of chromosomes. When a sperm or egg with the usual 23 chromosomes combine with this egg or sperm with a changed number of chromosomes, it will result in an embryo with chromosomes of number more than or less than the usual number of 46.
Addition and deletion of chromosome have strict rules:
. deletion of any autosome (chromosome 1 - 22) is usually incompatible with life, resulting in miscarriage (流產) or stillborn (胎死腹中) in the first 3 months of pregnancy.
. addition of 2 or more autosomes is also usually incompatible with life in the first 3 months of pregnancy.
. addition of autosome other than chromosome 21 usually results in non-survival in infancy or early childhood (usually within the first year).
. addition or deletion of sex chromosomes (X or Y) is possible except that in the resultant chromosome, there cannot be Y-only chromosomes (i.e. Y chromosome must have an X chromosome in order for survival).
As such, the possible common changes in number of chromosomes and their related effect on the health and development of the child are listed as follows:
| Chromosome condition | Description | Syndrome | Symptoms |
| 47,XX+21 47,XY+21 | 47 chromosomes with extra chromosome 21 in female or male | Down’s syndrome (唐氏綜合症) | Mental retardation, smaller growth parameters including height, neck, head circumference etc., flat face. Usually infertility in male. High risk of miscarriage in female with 50% chance of conceiving babies with Down’s syndrome. |
| 45,X | 45 chromosomes with single X chromosome in female | Turner syndrome | Normal intellectual performance, slower growth and lower height, infertility |
| 47,XXY
| 47 chromosomes with extra X chromosome in male
| Klinefelter syndrome | Learning difficulty, tendency to grow at quicker rate, small testcles/penis, infertility |
| 48,XXXY 49,XXXXY | rare extensions with additional X chromosomes) of 47,XXY (male) |
| More exaggerated features of 47,XXY syndrome |
| 47,XXX
| 47 chromosomes with extra X chromosome in female
| XXX syndrome | ‘super-female’, usually no observable physical and medical anomalies apart from being taller than average, normal sexual development, risk of learning difficulty
|
| 48,XXXX 49,XXXXX | rare extensions with additional X chromosomes) of 47,XXX (female) |
| Decreased mental and physical ability, tall texture |
| 47,XYY | 47 chromosomes with extra Y chromosome in male | XYY syndrome | ‘super-male’, usually no observable physical and medical anomalies apart from being taller than average, normal sexual development., risk of learning difficulty |
Sometimes, a portion of the chromosome is duplicated or deleted during the formation of the egg or sperm, resulting in extra or loss of genetic material in the chromosome. Some common examples and their related effect on the health and development of the child are listed as follows:
| Chromosome condition | Description | Syndrome | Symptoms |
| 46,XX,dup(17) (p12) | Duplication of p arm, band 1 and sub band 2 of chromosome 17 in female | Charcot-Marie-Tooth disease (腓骨萎縮症) | Loss of muscle tissues in feet and legs, high arch, claw toes. |
| 46,XX,5p- | Deletion of part of p arm of chromosome 5 in female | Cri-du-chat syndrome (貓哭症) | Intellectual disability, delayed development, small head size, high-pitched cry sound like cat. |
C.1 Inversion
Sometimes a segment of the chromosome breaks off, rotates 180 degrees and reconnects to the original chromosome during the formation of the egg or sperm. (e.g. 46,XY,inv(12) (p11.2q24.3) refers to a male with inversion in chromosome 12 from p arm band 1, sub-band 1, sub-sub-band 2 to q arm band 2, sub-band 4, sub-sub-band 3.)
C.2 Translocation
Sometimes, segments of 2 chromosomes break off and exchanged during the formation of the egg or sperm. (e.g. 46,XX, t(4,20) (q31,q12) refers to a female with translocation between chromosome 4, long (q) arm band 3, sub-band1 and chromosome 20, long (q) arm band 1, sub-band 2.)
Translocation is much more complicated than inversion as there is an exchange of chromosome segments (i.e. genetic material) between different chromosomes. It may or may not cause problems to the health and development of the child as depending on type of chromosomes involved (e.g. exchange of genetic material between autosome and sex chromosome) and the portion of chromosomes involved.The above gives some simplistic views on how changes in chromosomes may affect the health and development of the child. It is recommended that affected individuals should seek professional advice from genetic counseling on details of medical issues, and implications on any possible inheritance issues when considering births of offspring ...................
