Biology Dictionary


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Monosomy Definition

Monosomy is an example of aneuploidy, which is an imbalance in chromosome numbers. This means that we can either find extra or missing chromosomes in the cells as a result of mistakes that occur during cell division. In a monosomy, one chromosome of a homologous pair is missing. In the case of human beings, we normally have two copies of each chromosomes; one from each parent, resulting in 46 total chromosomes found in the nucleus of cells. When one pair has a missing chromosome, the resulting number becomes 45 chromosomes instead of 46, which leads to abnormalities. Keep in mind that monosomy is very rare in humans since in most cases the embryos lacking a chromosome die. Interestingly, many instances of monosomy have only part of the chromosome missing, like in cri du chat syndrome.

Types of Monosomies

Complete Monosomy

Meiosis is a process that results in four reproductive cells (eggs or sperm), each ready for fertilization and containing half the number of chromosomes found in a normal cell. Complete monosomy comes about due to unsuccessful separation of a pair of chromosomes during the process of meiosis, called nondisjunction . Once a cell that has an extra or a missing chromosome goes through fertilization, the embryo will then have an incorrect number of chromosomes. The diagram below shows how this can happen, and for simplicity purposes only one pair of chromosomes is shown at the start of meiosis. In the diagram on the left hand side, the sister chromatids don’t separate during meiosis II as they should, leading to an abnormal number of cells in two of the resulting eggs. On the other hand, the fault occurs in the first division, meiosis I, in the diagram on the right hand side. The homologous pair fails to separate at this stage, leading to the eggs either having three or a single chromosome, instead of two. As shown, in the case of the diagram on the left, where unsuccessful separation happens in a later step, there are eggs that are normal, which leads to embryos with a normal number of chromosomes after fertilization. Therefore, it is not as detrimental as the other case since it allows for the possibility of a normal human being forming. Note that complete monosomy is not inherited.

Nondisjunction Diagrams

Nondisjunction Diagrams

Partial Monosomy

A partial monosomy is where only some of the chromosome is missing. This can either happen when a portion of the chromosome is deleted in meiosis or at some point during the early stages of the development of the embryo. Like complete monosomy, this case of partial monosomy can’t be inherited. On the other hand, partial monosomy caused by translocation is often inherited from the parents.

Translocation is a type of mutation where a section is detached from a chromosome and joined to a different, nonhomologous chromosome. This means that the first chromosome has lost important genetic information. When this occurs in people, they will still have the genetic information within their cells and so usually have no abnormalities. However, if they are to have offspring, then there is a possibility of them passing on a chromosome that has missing information, leading to a partial monosomy. Of course, the parent can also pass on the chromosome carrying the extra information, and that would then lead to a partial trisomy. Trisomy is the presence of a third, extra copy of a chromosome, like in the case of Down’s syndrome. Therefore, partial monosomy refers to an extra copy of part of a chromosome.

Example of Monosomy

Turner’s Syndrome

Also called monosomy X, it is a genetic disorder where an individual ends up with only one X chromosome and nothing else in their final pair of chromosomes. Remember that females have two X chromosomes as their sex chromosomes , and males have an X and a Y. Therefore, individuals with Turner’s syndrome happen to be females with only one X chromosome. Having only one chromosome from the pair, these females do not develop the same way as the average female, but rather display some physical and hormonal abnormalities. All girls with Turner’s syndrome are infertile, but some have more symptoms than others. For example, they may not undergo puberty, develop breasts, or have a menstrual cycle. Also, nearly all girls are shorter than average, and often have wide necks or puffy hands and feet. Other problems can occur, too, including hearing or heart problems.

Turner’s syndrome occurs in about 1 in every 2,500 girls that are born and is not inherited from parents. Interestingly, Turner’s Syndrome is the only case of monosomy that someone can live out their life with, though most fetuses with this syndrome die before birth. Nowadays, treatments like hormone therapy help girls with this disorder live healthy lives.

Related Biology Terms

  • Down’s syndrome – A genetic disorder characterized by mental retardation and caused by trisomy 21.
  • Mitosis – The division of the nucleus that leads to two identical daughter cells in living organisms.
  • Mosaicism – Cells within the same organism having different genetic information.
  • Polyploidy – The presence of more than two sets of each chromosome in the cell.


1. Which of the following is the cause of complete monosomy?
A. Partial deletions
B. Partial monosomy
C. Translocations
D. Nondisjunction

Answer to Question #1
D is correct. Complete monosmy is a result of nondisjunction, where a pair of homologous chromosomes fails to separate during meiosis.

2. What do most monosomies lead to in humans?
A. Small feet
B. Death
C. Most aren’t dangerous and lead to no problems
D. Infertility

Answer to Question #2
B is correct. Most cases of monosomy lead to death before birth.

3. In humans, Turner’s and Down’s syndromes are examples of:
A. Monosomy
B. A monsomy and a trisomy, respectively
C. A trisomy and a monosomy, respectively
D. An aneuploidy and a monosomy, respectively

Answer to Question #3
B is correct. Turner’s syndrome is monosomy of the X chromosome, while Down’s syndrome is trisomy 21.

4. Which of the following is not a symptom of Turner’s syndrome?
A. Infertility
B. Short stature
C. Swollen ears
D. Swollen hands and feet

Answer to Question #4
C is correct. Turner’s syndrome can lead to infertility, short stature, swollen hands and feet, as well as other different symptoms. However, swollen ears are not a symptom of Turner’s syndrome.

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    HumanChromosomal Disorders
    April 30, 2003

    Readings: Web Notes Only!

    I. Karyotype Analysis ofHuman Chromosomes

    1. Karyotype preparation and analysis

    Cells (from blood, amniotic fluid, etc) aregrown in vitro (in a cell culture dish) to increase theirnumber

    Cell division is then arrested in metaphasewith colchicine (prevents mitotic spindle from forming)

    Cells are centrifuged and lysed to releasechromosomes

    Chromosomes are stained, photographed, andgrouped by size and banding patterns

    This is a photograph of the 46 human chromosomes in a somatic cell, arrested in metphase. Can you see that they are duplicated sister chromatids?

    2. Normal male karyotype (a Cytogeneticist has lined these chromosomes up, matching homologues up and arranging them by size)

    3. Normal female karyotype

    II. Alterations inchromosome number:

    Nondisjunction occurs when either homologues fail to separate during anaphase I of meiosis, or sister chromatids fail to separate during anaphase II. The result is that one gamete has 2 copies of one chromosome and the other has no copy of that chromosome. (The other chromosomes are distributed normally.)

    If either of these gametes unites with anotherduring fertilization, the result is aneuploidy (abnormalchromosome number)

    • A trisomic cell has one extra chromosome (2n +1) = example: trisomy 21. (Polyploidy refers to the condition of having three homologous chromosomes rather then two)
    • A monosomic cell has one missing chromosome (2n – 1) = usually lethal except for one known in humans: Turner’s syndrome (monosomy XO).

    The frequency of nondisjunction is quite highin humans, but the results are usually so devastating to thegrowing zygote that miscarriage occurs very early in thepregnancy.

    If the individual survives, he or she usuallyhas a set of symptoms – a syndrome – caused by theabnormal dose of each gene product from that chromosome.

    1. Human disorders due to chromosome alterations in autosomes (Chromosomes 1-22). There only 3 trisomies that result in a baby that can survive for a time after birth; the others are too devastating and the baby usually dies in utero.

    A. Downsyndrome (trisomy 21): Theresult of an extra copy of chromosome 21. People with Downsyndrome are 47, 21+. Down syndrome affects 1:700 children andalters the child’s phenotype either moderately or severely:

    • characteristic facial features, short stature; heart defects
    • susceptibility to respiratory disease, shorter lifespan
    • prone to developing early Alzheimer’s and leukemia
    • often sexually underdeveloped and sterile, usually some degree of mental retardation.
    • Down Syndrome is correlated with age of mother but can also be the result of nondisjunction of the father’s chromosome 21.



    Karyotype of a boy with Down Syndrome:

    B. Patau syndrome (trisomy 13) : serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months.

    C. Edward’ssyndrome (trisomy 18) :almost every organ system affected 1:10,000 live births. Childrenwith full Trisomy 18 generally do not live more than a fewmonths.

    2. Nondisjunction of the sex chromosomes (X or Y chromosome): Can be fatal, but many people have these karyotypes and are just fine!

    A. Klinefelter syndrome: 47, XXY males. Male sex organs; unusually small testes, sterile. Breast enlargement and other feminine body characteristics. Normal intelligence.

    B. 47, XYYmales : Individuals are somewhattaller than average and often have below normal intelligence. Atone time (~1970s), it was thought that these men were likely tobe criminally aggressive, but this hypothesis has been disprovenover time.

    3. Trisomy X: 47, XXX females. 1:1000 live births – healthy and fertile – usuallycannot be distinguished from normal female except by karyotype

    4. Monosomy X (Turner’s syndrome) : 1:5000 live births; the only viable monosomy in humans – women with Turner’s have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)

    III. Alterations in chromosome structure:

    Sometimes, chromosomes break,leading to 4 types of changes in chromosome structure:

    1. Deletion : a portion of onechromosome is lost during cell division. That chromosomeis now missing certain genes. When this chromosome is passed onto offspring the result is usually lethal due to missing genes.

    Example – Cri du chat (cry of the cat) : A specific deletion of a small portion of chromosome 5; these children have severe mental retardation, a small head with unusual facial features, and a cry that sounds like a distressed cat.

    2. Duplication: if the fragment joins the homologous chromosome, thenthat region is repeated

    Example – Fragile X : the most common form of mental retardation. The X chromosome of some people is unusually fragile at one tip – seen "hanging by a thread" under a microscope. Most people have 29 "repeats" at this end of their X-chromosome, those with Fragile X have over 700 repeats due to duplications. Affects 1:1500 males, 1:2500 females.

    3. Translocation: a fragment of a chromosome is moved ("trans-located") from one chromosome to another – joins a non-homologous chromosome. The balance of genes is still normal (nothing has been gained or lost) but can alter phenotype as it places genes in a new environment. Can also cause difficulties in egg or sperm development and normal development of a zygote. Acute Myelogenous Leukemia is caused by this translocation:

    Interested in finding out more about Chromosomal Conditions ? UKMC website
    Interested in finding out more about
    GeneticCounseling or Cytogenetics as aprofession (at IUPUI)?


    1. What is a karyotype, and how is thisanalysis performed?
    2. What is nondisjunction?
    3. Distinguish between the chromosomal alterationsthat are involved in the following human disorders:

    • Down syndrome, Patau’s syndrome, Edwards syndrome
    • Klinefelters syndrome, XYY syndrome
    • Trisomy X, Monosomy X

    5. Distinguish between the 3 different alterations in chromosome structure, and how Cri du Chat, Fragile X, and Acute myelogenous leukemia occur.