Chromosomes: Facts about our genetic storerooms

Chromosomes are defined as thread-like structures of deoxyribonucleic acid DNA  that carry hereditary information for everything from height to eye color. They are found in the nucleus of a cell and help ensure DNA is replicated and divided evenly during cell division, according to genomics website Healio.com. Chromosomes consist of a protein and a single molecule of DNA. Chromosomes make us who we are.

Your DNA blueprint (genome) doesn't exist as one continuous scroll tightly rolled up in the nucleus of each cell. Instead, your DNA is divided into 46 'chapters' called chromosomes — 23 from each parent. These 23 chromosomes from each parent pair up and form the 23 chromosome pairs in each cell. 

Chromosomes are packaging masters, without them DNA would simply not fit inside cells. Chromosomes keep DNA coiled around spool-like proteins known as histones, according to the National Human Genome Research Institute. If you unraveled all the DNA from a single human cell and placed it end-to-end, it would stretch 6 feet (1.8 meters)!

Not all chromosomes are created equal. Some contain a lot of genetic material whilst others carry comparably little. Chromosome 1 for example is the biggest and contains eight percent of your total DNA according to Medline Plus. An individual's collection of chromosomes is called a karyotype. The term chromosome comes from the Greek words for color (chroma) and body (soma) due to their ability to be strongly stained by dyes used in research according to the National Human Genome Research Institute. 

Number of chromosomes

The number of chromosomes an organism possesses varies considerably but has little to do with the complexity of the organism or the quantity of DNA it possesses. Most humans have 46 chromosomes whilst a potato has 48! According to the National Human Genome Research Institute, most bacteria have just one or two circular chromosomes, a fruit fly has eight chromosomes, a rice plant 24, and a dog 78. 

But some organisms have chromosome counts in the thousands! The world record holder for the highest chromosome number recorded is the adder's tongue fern Ophioglossum reticulatum, with an estimated 1,440 chromosomes (or 720 pairs!), according to the Guinness World Records. 

In humans, if the number of chromosomes changes, it can lead to changes in growth and development, according to Medline Plus. When an organism is missing chromosomes or has extra chromosomes it is known as aneuploidy. Trisomy, refers to the presence of an extra chromosome in cells, an example of a condition caused by trisomy is Down's Syndrome. People with Down's Syndrome have three copies of chromosome 21, instead of two copies. 

The only human cells that do not contain a pair of chromosomes are reproductive cells (gametes). Egg and sperm cells carry just one copy of each chromosome so that when they unite they become a single cell containing a pair of chromosomes. 

X and Y chromosomes

Human sex chromosomes are referred to as X and Y, and their combination determines a person's sex. Typically, human females have two X chromosomes while males possess an XY pairing. This XY sex-determination system is found in most mammals as well as some reptiles and plants.

Whether a person has XX or XY chromosomes is determined when a sperm fertilizes an egg. Unlike the body's other cells, the cells in the egg and sperm — called gametes or sex cells — possess only one chromosome, known as haploid. Gametes are produced by meiosis cell division, which results in the divided cells having half the number of chromosomes as the parent, or progenitor, cells. In the case of humans, this means that parent cells have two chromosomes, known as diploid and gametes have one.

All of the gametes in the mother's eggs possess X chromosomes. The father's sperm contains about half X and half Y chromosomes. Sperm is the variable factor in determining the sex of the baby. If the sperm carries an X chromosome, it will combine with the egg's X chromosome to form a female zygote. If the sperm carries a Y chromosome, it will result in a male.

During fertilization, gametes from the sperm combine with gametes from the egg to form a zygote. The zygote contains two sets of 23 chromosomes, for the required 46. 

Although, in general, females carry two X sex chromosomes in each body cell and males carry one X and one Y, there is some natural variation in the number of sex chromosomes people carry. Sometimes, there may be extra sex chromosomes, or one might be missing, so other patterns, such as X, XXX, XXY and XXYY, can also occur. People can carry extra chromosomes without knowing it, but sometimes, they can cause health conditions.

It is important to remember that sex and gender have two separate definitions and many cultures include more labels than simply "male" and "female" to identify others.

The structure of X and Y chromosomes

While the chromosomes for other parts of the body are the same size and shape — forming an identical pairing — the X and Y chromosomes have different structures.

The X chromosome is significantly longer than the Y chromosome and contains hundreds more genes. Because the additional genes in the X chromosome have no counterpart in the Y chromosome, the X genes are dominant. This means that almost any gene on the X, even if it is recessive in the female, will be expressed in males. These are referred to as X-linked genes. Genes found only on the Y chromosome are referred to as Y-linked genes, and expressed only in males. Genes on either sex chromosome can be called sex-linked genes.

There are approximately 1,098 X-linked genes, though most of them are not for female anatomical characteristics. Many are linked to disorders such as hemophilia, Duchenne muscular dystrophy, fragile-X syndrome and several others. They are responsible for red-green color blindness, considered the most common genetic disorder and found most often in males. The non-sex feature X-linked genes are also responsible for male pattern baldness.

According to the National Human Genome Research Institute, the Y chromosome is about one-third the size of the X chromosome. The X chromosome has about 900 genes while the Y chromosome has about 55.  The Y chromosome contains a 'male-determining gene' called the SRY gene. This gene is what triggers testes to form in the embryo, if there is a mutation in this gene the embryo will develop female genitalia despite having XY chromosomes.

Sex chromosome abnormalities

Abnormalities in the sex chromosome combination can result in a variety of gender-specific conditions that are rarely lethal. These abnormalities can be caused by full or partial chromosome deletions or duplicated sex chromosomes, according to MSD Manual.

Turner syndrome occurs when females are missing or partially missing an X chromosome, according to the Mayo Clinic. Turner Syndrome can cause a variety of medical and developmental problems including short stature, failure in the development of ovaries and heart defects. 

Trisomy X syndrome — also known as Triple X syndrome — is caused by three X chromosomes instead of two and affects approximately 1 in 1000 females according to the Mayo Clinic. Most people who have Trisomy X do not experience any symptoms or have mild symptoms. Occasionally, more significant symptoms may occur which may delay the development of speech and language skills, behavioral problems, seizures and kidney problems. 

According to Medicine Plus, one of the most common sex chromosome disorders is Klinefelter syndrome which affects approximately one in 650 males and is caused by an extra X chromosome. In some cases, the symptoms can be so mild that many go undiagnosed until they reach puberty, and some researchers believe up to 75% of affected males may never know they have the condition. Symptoms of Klinefelter syndrome include small testes producing lower amounts of testosterone which can lead to delayed puberty, breast development, infertility and decreased muscle mass. 

Additional resources

To find out more about chromosomes check out this material from Your Genome. Learn more about gender identity with Planned Parenthood. The Intersex Society of North America has an insightful article on whether having a Y chromosome makes someone a man. Discover more about modes of inheritance and genetic basics in animals with this article from VCA animal hospitals. 

Bibliography

• Y Chromosome Infographic, The National Human Genome Research Institute.
• What are trisomies and monosomies? Boston's Children Hospital.  
• Klinefelter syndrome, Medline Plus.  
• Triple X syndrome, Mayo Clinic. 
• Down's Syndrome Association
• Gardner, RJ MKinlay, Grant R. Sutherland, and Lisa G. Shaffer. Chromosome abnormalities and genetic counseling. No. 61. OUP USA, 2011. 
• Schmidt, Carla, Patrick Schindele, and Holger Puchta. "From gene editing to genome engineering: restructuring plant chromosomes via CRISPR/Cas." aBIOTECH 1.1 (2020): 21-31.
• Sharma, Arun Kumar, and Archana Sharma. Plant chromosomes: Analysis, manipulation and engineering. CRC Press, 2019. 
• Hedrick, Philip W. "Virgin birth, genetic variation and inbreeding." Biology Letters 3.6 (2007): 715.