Did you know that seedless watermelons weren't always around? They were actually created by breeders who introduced a chromosomal abnormality. By breeding a watermelon with four sets of chromosomes with one that has two sets, they created a watermelon with three sets of chromosomes. This causes the watermelon to be sterile, meaning it can't produce mature black seeds. These seedless watermelons are actually triploid hybrids, and they're perfect for eating on hot summer days. This is just one example of chromosomal disorders affecting the world around us.

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Introduction to Chromosomes

Chromosomes are like tiny packages of DNA found in the nucleus of a cell. Most humans have 46 chromosomes - that's 23 pairs of chromosomes - in every cell in their body. But sometimes chromosomes don't separate properly, or other abnormalities happen, which can lead to big changes in the genome. These changes are called chromosomal disorders.

Some organisms, like humans, have two sets of chromosomes - we're called diploid. Others, like seedless watermelons, have three sets of chromosomes, which makes them triploid. And salmon? They have four sets, or tetraploid (Fig. 2).

But the number of chromosome sets doesn't determine how complex an organism is. For example, humans are diploid even though we're pretty complex. Chromosomal disorders can be caused by missing, rearranged or extra large pieces of chromosomes.

Chromosome Disorders

Chromosomes are like tiny packages of DNA, and we can describe one complete set of chromosomes using the variable N. In most cells, humans have two complete sets of chromosomes, which we can write as 2N. But in gametes - that's sperm and egg cells - there's only one complete set of chromosomes, or N.

When an organism has more or less than its normal set of chromosomes, we call it euploidy. For example, a diploid organism with three or more full sets of chromosomes is called a polyploid, which we can write as 3N. But if a diploid organism only has one complete set of chromosomes, it's called a monoploid, or N. Monoploids and triploids are usually lethal or sterile.

Sometimes, there can be changes to the number of a specific chromosome, which we call aneuploidy. For example, trisomy is when there's an extra copy of a specific chromosome - that's 2N + 1. Monosomy is when there's a missing copy of a specific chromosome - that's 2N - 1. In rare cases, an organism can lose or gain two chromosomes, called nullisomy (2N - 2) or tetrasomy (2N + 2). We don't know the cause of aneuploidy, but it's more common in women who try to have children later in life.

Extra Chromosome Disorders

Extra chromosome disorders occur when there's an additional chromosome - this is called aneuploidy. This can happen during meiosis or mitosis when chromosomes don't separate properly. This is called non-disjunction, and it can lead to one daughter cell having an extra chromosome and the other having none (Fig. 3). If a gamete with an extra chromosome is fertilized, the fetus can end up with 47, instead of 46, chromosomes in some or all cells.

When there's an extra chromosome, it's called trisomy, which we can write as 2N + 1. Most trisomies will result in spontaneous abortion during pregnancy, but there are three instances where the fetus can survive until birth. These are trisomy 21, also known Down syndrome, trisomy 13, or Patau syndrome, and trisomy 18, or Edward syndrome.

Down Syndrome

Trisomy 21, or Down syndrome, is when there's an extra copy of chromosome 21 in some or all cells. Down syndrome occurs in around 1 in every 1000 live births worldwide. This condition is associated with both physical and mental challenges due to changes in development. While the physical characteristics of Down syndrome can vary between individuals, some common features include upward slanting eyes, a flattened bridge of the nose, and short stature. Despite the associated health complications, such as congenital heart defects and memory loss in adulthood, improved intervention, de-stigmatization, and care have led to better survival rates and quality of life for individuals with Down syndrome.

Patau Syndrome

Trisomy 13, also known as Patau syndrome, is when there's an extra copy of chromosome 13 in some or all cells. This condition is much rarer than trisomy 21, affecting around 1 in every 10,000 to 22,000 live births. The most serious complications associated with trisomy 13 are in the nervous and cardiovascular systems, which makes survival in the first few weeks after birth extremely difficult. Although there have been some reports of longer survival with improved surgical procedures and aggressive medical treatments, severe disability is expected in these patients.

Edward Syndrome

Trisomy 18, also known as Edward syndrome, is the addition of chromosome 18 to some or all cells.

It affects around 1 in every 8600 live births. However, the occurrence has increased due to a trend of women having children later in life. Children born with trisomy 18 often have feeding and breathing difficulties, smaller head sizes, delayed psychomotor development, and growth impediments. Although the survival of females with Edward syndrome is higher than males, the underlying reason for this is unclear.

Missing Chromosome Disorders

Monosomy is when there's a missing chromosome in some or all cells of the body, resulting in 45 chromosomes instead of the normal 46. This can occur due to non-disjunction during meiosis, where a sperm or egg cell receives no copies of a specific chromosome. Monosomy X, also known as Turner's syndrome, is the most common example of monosomy and only affects females, as males without an X chromosome do not survive. Turner's syndrome affects around 1 in every 2500 live female births. Females with monosomy X tend to be shorter in stature, do not naturally go through puberty, and may have other kidney or heart co-morbidities. Although females with Turner's syndrome can still have children, fertility treatments are often necessary due to their rapid loss of eggs.

Sex Chromosome Disorders

It's true that most biological females have XX chromosomes, while most biological males have XY chromosomes. However, as we discussed earlier, aneuploidies can also occur in sex chromosomes. There are six common classifications of sex chromosome disorders, which include:

1. XXX females: This is a condition where females have three X chromosomes instead of the typical two.
2. XXY males (Klinefelter's syndrome): This is a condition where males have an extra X chromosome, resulting in XXY instead of the typical XY.
3. XX males: This is a condition where males have two X chromosomes instead of the typical XY.
4. XYY males: This is a condition where males have an extra Y chromosome, resulting in XYY instead of the typical XY.
5. XO (Turner's syndrome): This is a condition where females have only one X chromosome instead of the typical two.
6. XY females: This is a condition where females have two X chromosomes but also have the male Y chromosome, resulting in XY instead of the typical XX.

XXX Chromosome Disorder

XXX females, also known as triple X syndrome, is a condition where a female is born with an extra X chromosome in some or all of their cells. It occurs in approximately 1 in every 1000 women, but due to the mild or absence of symptoms, it's estimated that only 10% of women with triple X syndrome are aware of their condition. This may lead to an underestimation of its prevalence. Females with triple X syndrome go through puberty and can conceive children naturally. However, triple X syndrome has been associated with behavioral problems, delayed development of psychomotor skills, and learning disabilities in some cases.

XXY Chromosome Disorder

You have a good understanding of the different sex chromosome disorders. XXY males, or Klinefelter's syndrome, occur when a male is born with an extra X chromosome in some or all cells of their body. It occurs in approximately 1 in every 500 males and is often not diagnosed until the person is trying to have children. While symptoms are mild, males with Klinefelter's syndrome are sterile. Symptoms associated with Klinefelter's syndrome include larger stature, less body hair, breast growth, and lower sex drive.

XX males can occur when a specific gene called the SRY gene from the Y-chromosome crosses over onto the X-chromosome. XX males are often smaller in height, with smaller testes, and many are sterile.

XYY males have inherited an additional Y chromosome in some or all of their cells. It is a rare condition, and XYY males are often undiagnosed or misdiagnosed. XYY males are often taller in stature but tend to develop acne during puberty. XYY males often go on to live healthy lives.

XO females, also known as Turner's syndrome, are females that have an absent X chromosome. Characteristics of XO females can vary, but often include infertility and short stature.

XY females, also called Swyer syndrome, are females who develop biological female reproductive structures, even though they possess XY chromosomes, which are indicative of biological males. XY females often have insufficiently developed female gonads, and hormone replacement therapy is needed to trigger puberty.

Chromosomal Disorders - Key Takeaways Chromosome disorders are changes to the number of chromosomes or chromosome sets leading to large-scale changes in the genome. Humans are diploids meaning that we have two complete sets of chromosomes. A complete set of chromosomes can be denoted by variable, N. Euploidy is a change in the number of sets of chromosomes, while aneuploidy is a change in the number of specific chromosomes. Examples of aneuploidy are trisomy (an extra chromosome) or monosomy (one less chromosome).Chromosome disorders can also affect sex chromosomes such as XXX females and XXY males.

Chromosomal Disorders

What is chromosomal disorder?

Chromosome disorders are changes to the number of chromosomes or chromosome sets leading to large-scale changes in the genome.

Is autism a chromosomal disorder? 

While autism certainly has a genetic component, it is not caused by a single chromosome change. Rather, autism is caused by mutations in multiple different genes. There is some evidence that genomic segment of chromosome 15 and sex chromosome are involved in autism spectrum disorder.

Is Down syndrome a chromosomal disorder?

Yes, Down syndrome is caused by trisomy (an extra copy) of chromosome 21 in some or all cells of the body. 

How do chromosomal abnormalities lead to genetic disorders?

Chromosomes are large segments of DNA that are composed of genes. If gene expression is not properly regulated, as is the case of chromosomal abnormalities,  it can lead to genetic disorders. 

What is a defective chromosome disorder?

Chromosome disorders are changes to the number of chromosomes or chromosome sets leading to large-scale changes in the genome.

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