The chromosomal abnormalities are caused by errors in the number or structure of chromosomes. Many children with chromosomal abnormalities have mental and/ or physical birth defects. Some chromosomal abnormalities result in miscarriage and stillbirth.
What are chromosomes?
Chromosomes are tiny, string-like structures in cells of the body that contain the genes. Humans have about 20,000 to 25,000 genes that determine traits like eye and hair color; genes also direct the growth and development of every part of the body. Each person normally has 23 pairs of chromosomes or 46 in total. We inherit one chromosome per pair from our mother and one from our father.
Causes of chromosomal abnormalities:
Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops. It is not known why these errors occur:
i. Abnormality due to chromosomal number:
Egg and sperm cells each contain 23 chromosomes. When they combine, they form a fertilized with 46 chromosomes. But sometimes, something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes.
When this cell with the wrong number of chromosomes combine with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality (C.A).
Example: A common type of C.A is called trisomy. This means that an individual has three copies of a specific chromosome, instead of two. For example, individual with Down syndrome generally have three copies of syndrome 21.
ii. Abnormality due to chromosomal structure:
In this form of abnormality, the errors before fertilization can alter the structure of one or more chromosomes. Individuals with structural chromosomal abnormalities usually have normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be deleted, duplicated, inverted, misplaced or exchanged with part of another chromosome.
iii. Abnormality due to Mosacism:
Errors in cell division can occur after fertilization. This can result in mosacism, a condition in which an individual has cells with different genetic make-up. For example, individuals in the mosaic form of turner syndrome are missing an X chromosome in some, but not all, of their cells. Some individuals with chromosomal mosaicism may be mildly affected but the severity depends largely on the percentage of abnormal cells.
4. Consequences of chromosomal abnormalities:
In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the pregnant woman has a miscarriage. This happens very early in pregnancy, before a woman may realize that she is pregnant.
The structural chromosomal abnormalities may have no effect on a person if all of the chromosome is there but just rearranged. In other cases, the rearrangements may result in pregnancy loss or birth defects. Some of the most common chromosomal abnormalities are as follows.
Autosomes Chromosome Abnormalities:
Down syndrome is caused by extra genetic material from chromosome 21. Individuals in down syndrome have varying degree of intellectual disability characteristic facial features and often breast defects and other problems.
ii. Trisomies 13 & 18:
Trisomics 13 and 18 are caused by extra genetic material from chromosome 13 and 18 respectively. These trisomies are usually more severe them down syndrome but are less common. Babies suffering with these abnormalities have severe intellectual disablities and many physical birth defects. Most affected babies die before their first birthday.
Sex chromosome abnormalities:
The last pair of the chromosomes is the sex chromosomes. Generally females have two X chromosomes and males have one X and one Y chromosome. Sex chromosome abnormalities may cause infertility, growth abnormalities and, in some cases, behavioral and learning problems. However, most affected individuals live fairly normal lives. Common sex chromosomal abnormalities include.
iv. Turner Syndrome:
Girls with Turner syndrome have one X chromosome and are missing all or part of the other X chromosome.
They usually are infertile and do not undergo normal puberty changes unless they are treated with sex hormones. Affected girls are short but treatment with growth hormone can help increase height. Some have other health problems, including heart and kidney defects. They have normal intelligence, though some have learning difficulties, particularly with mathematics and spatial concepts.
v. Triple X:
Girls with Triple X have an extra X chromosome. Affected girls are tall, usually have no physical birth defects, experience normal puberty and are futile. They have normal intelligence, though many have learning problems.
vi. Klinefelter Syndrome:
Boys with klineflter syndrome have two, or occasionally more, X chromosomes along with their Y chromosome. They have normal intelligence, though many have learning problems. As adults, they produce lower-than-normal amounts of the male hormone testosterone and are infertile.
Boys with XYY syndrome have an extra Y chromosome. Affected males are sometimes taller than average, have normal sexual development and are fertile. Most have normal intelligence, though some have learning, behavioral and speech problems.
Structural Chromosomal abnormalities:
Chromosomal deletions: A portion of chromosome is missing or deleted.
viii. Cri-du-chat (cat cry) syndrome:
It is caused by deletion on chromosomes. Affected children have a cat-like, pitched cry during infancy, intellectual disabilities & physical abnormalities, poor muscle tone; a small heard six and 1000 birth weight have normal lifespan unless born with some other serious organ defects.
ix. Prader-Will Syndrome:
Caused by deletion of chromosome 15. Affected children have intellectual disabilities, behavioral problems and short stature. They may also develop extreme obesity.
Chromosome is duplicated resulting in extra senltic material.
x. Pallister Killian syndrome:
Result of an extra no. 12 chromosome material.
Severe mental retardation, poor muscle tone, “coarse” facial features and a prominent forehead, poor feeding, stiff joints cataracts in adulthood, hearing loss and heart defect have shortened lifespan but can live into their 40’s.
Of all the structural chromosome rearrangements, the most clinically significant is a translocation. Translocation involves two non-homologous chromosomes (e.g chromosome 2 & chromosome 6). Following a break in each of the chromosomes, and subsequent reunion, a segment of chromosome 2 becomes attached to chromosome 6 vice versa.
xi. Robertsonian translocation:
Reciprocal transfer of the long arms of the two acrocentric chromosomes (13,14,15 12 or 22). A relatively common R.T is between chromosome 14 and 21.
Theoretically, a person who carries a 14;21 translocation has a
i. 1/3 chance of normal child.
ii. 1/3 chance of child with normal translocation.
iii. 1/3 chance of Down syndrome.
Involves only one chromosome in which two breaks occur and in the process of repair, the interfering segment is rejoined in an inverted manner.
Inversions usually do not cause any abnormalities in carriers as long as the rearrangement is balanced with no extra or missing DNA. The most common inversion seen in human is on chromosome 9. Generally no deletions affects but there is some suspicion it could — to an increased risk for miscarriage or infertility for some affected individuals.