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A threadlike structure of nucleic acids and proteins found in the nucleus of most living cells, carrying genetic information in the form of genes

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Humans have 22 chromosome pairs and two sex chromosomes. Females have two X chromosomes; males have an X chromosome and a Y chromosome.

Credit: U.S. National Library of Medicine

Chromosomes are thread-like molecules that carry hereditary information for everything from height to eye color. They are made of protein and one molecule of DNA, which contains an organism's genetic instructions, passed down from parents. In humans, animals, and plants, most chromosomes are arranged in pairs within the nucleus of a cell. Humans have 22 of these chromosome pairs, called autosomes.

Humans have an additional pair of sex chromosomes for a total of 46 chromosomes. The 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. 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 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. The sperm are 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. Most women are 46XX and most men are 46XY, according to the World Health Organization.

There are some variations, though. Recent research has found that a person can have a variety of different combinations of sex chromosomes and genes, particularly those who identify as LGBT. For example, a certain X chromosome called Xq28 and a gene on chromosome 8 seem to be found in higher prevalence in men who are gay, according to a 2014 study in the journal Psychological Medicine.

A few births out of a thousand of babies are born with a single sex chromosome (45X or 45Y) and are referred to as sex monosomies. Others are born with three or more sex chromosomes (47XXX, 47XYY or 47XXY, etc.) and are called sex polysomies. "In addition, some males are born 46XX due to the translocation of a tiny section of the sex determining region of the Y chromosome," said WHO. "Similarly some females are also born 46XY due to mutations in the Y chromosome. Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex."

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.

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. In fact, 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.

In contrast to the large X chromosome, the Y chromosome contains only 26 genes. Sixteen of these genes are responsible for cell maintenance. Nine are involved in sperm production, and if some are missing or defective, low sperm counts or infertility may occur. One gene, called the SRY gene, is responsible for male sexual traits. The SRY gene triggers the activation and regulation of another gene, found on a non-sex chromosome, called the Sox9. The Sox9 triggers the development of non-sexed gonads into testes instead of ovaries.

Abnormalities in the sex chromosome combination can result in a variety of gender-specific conditions that are rarely lethal.

Female abnormalities result in Turner syndrome or Trisomy X, according to the U.S. National Library of Medicine. Turner syndrome occurs when females have only one X chromosome instead of two. Symptoms include failure of the sex organs to normally mature, which may lead to infertility, small breasts and no menstruation; short stature; a wide, shield-shaped chest; and a wide, webbed neck.

Trisomy X syndrome is caused by three X chromosomes instead of two. Symptoms include tall stature, speech delays, premature ovarian failure or ovarian abnormalities, and weak muscle tone - although many girls and women exhibit no symptoms.

Males can be affected by Klinefelter syndrome. Symptoms include breast development, abnormal proportions such as large hips, tall height, infertility, and small testicles.

Additional resources

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Chromosomes are thread-like molecules that carry hereditary information for everything from height to eye color. They are made of protein and one molecule of DNA, which contains an organism's genetic instructions, passed down from parents. In humans, animals, and plants, most chromosomes are arranged in pairs within the nucleus of a cell.

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RNA stores information and conducts chemical reactions, and also plays a role in controlling gene activity

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Read and learn for free about the following article: RNA: The basics

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Telomeres in relation to aging and cancer

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Telomeres in relation to aging and cancer

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Telomeres are the ends of chromosomes. They may be related to cellular aging, and possibly other diseases like cancer

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Are Telomeres the Key to Aging and Cancer

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Yet, each time a cell divides, the telomeres get shorter. When they get too short, the cell can no longer divide; it becomes inactive or "senescent" or it dies. This shortening process is associated with aging, cancer, and a higher risk of death. So telomeres also have been compared with a bomb fuse.

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Scientists are learning of another way to customize proteins by RNA editing. RNA editing is a molecular process through which some cells can make discrete changes to specific nucleotide sequences within a RNA molecule after it has been generated by RNA polymerase.

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RNA Interference arose as a way to reduce the production of a gene's encoded protein for purposes of growth or self defense. Also believed to eliminate unwanted viral RNA

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RNA interference (RNAi): by Nature Video

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Gene expression is a very important process in dna. Gene expression is when the instructions in our dna is made into a product such as protein.

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Inherited factors that do not change the DNA sequence of nucleotides are called epigenetic changes.

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Epigenetics, as a simplified definition, is the study of biological mechanisms that will switch genes on and off. What does that mean? Well, if you are new to this whole thing, we first need a quick crash course in biochemistry and genetics:

Cells are fundamental working units of every human being. All the instructions required to direct their activities are contained within the chemical deoxyribonucleic acid, also known as DNA.

DNA from humans is made up of approximately 3 billion nucleotide bases. There are four fundamental types of bases that comprise DNA - adenine, cytosine, guanine, and thymine, commonly abbreviated as A, C, G, and T, respectively.

The sequence, or the order, of the bases is what determines our life instructions. Interestingly enough, our DNA sequence is mostly similar to that of a chimpanzee. Only a fraction of distinctively different sequences makes us human.

Within the 3 billion bases, there are about 20,000+ genes. Genes are specific sequences of bases that provide instructions on how to make important proteins - complex molecules that trigger various biological actions to carry out life functions.

Now that you understand genetics, let's learn about epi genetics. Epigenetics, essentially, affects how genes are read by cells, and subsequently how they produce proteins. Here are a few important points about epigenetics:

Epigenetics Controls Genes. Certain circumstances in life can cause genes to be silenced or expressed over time. In other words, they can be turned off (becoming dormant) or turned on (becoming active).

Epigenetics Is Everywhere. What you eat, where you live, who you interact with, when you sleep, how you exercise, even aging - all of these can eventually cause chemical modifications around the genes that will turn those genes on or off over time. Additionally, in certain diseases such as cancer or Alzheimer's, various genes will be switched into the opposite state, away from the normal/healthy state.

Epigenetics Makes Us Unique. Even though we are all human, why do some of us have blonde hair or darker skin? Why do some of us hate the taste of mushrooms or eggplants? Why are some of us more sociable than others? The different combinations of genes that are turned on or off is what makes each one of us unique. Furthermore, there have been indications that some epigenetic changes can be inherited.

Epigenetics Is Reversible. With 20,000+ genes, what will be the result of the different combinations of genes being turned on or off? The possible arrangements are enormous! But if we could map every single cause and effect of the different combinations, and if we could reverse the gene's state to keep the good while eliminating the bad... then we could theoretically* cure cancer, slow aging, stop obesity, and so much more.

Curious to know how different foods may epigenetically influence health? Check out our e-book Epigenetics in Life: What We Eat, which cites over 20+ epigenetic studies.

Here's an analogy that might further help you to understand epigenetics. Think of the human lifespan as a very long movie. The cells would be the actors and actresses, essential units that make up the movie. DNA, in turn, would be the script - instructions for all the participants of the movie to perform their roles. Subsequently, the DNA sequence would be the words on the script, and certain blocks of these words that instruct key actions or events to take place would be the genes. The concept of genetics would be like screenwriting. Follow the analogy so far? Okay. The concept of epi genetics, then, would be like directing. The script can be the same, but the director can choose to eliminate or tweak certain scenes or dialogue, altering the movie for better or worse. After all, Steven Spielberg's finished product would be drastically different than Woody Allen's for the same movie script, wouldn't it?
Next Step... Explore popular categories in epigenetics: Want to learn about epigenetics in further detail? Read on: Fundamentals of Epigenetics *Editor's Note: Be wary of self-help claims that exploit epigenetics and seem too good to be true. We recommend you read about the abuse of epigenetics and pseudoscience.

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A Super Brief and Basic Explanation of Epigenetics for Total Beginners | What is Epigenetics?

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Epigenetics, as a simplified definition, is the study of biological mechanisms that will switch genes on and off. What does that mean? Well, if you are new to this whole thing, we first need a quick crash course in biochemistry and genetics: Cells are fundamental working units of every human being.