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A chromosome is a thread-like molecule of DNA that contains all of the genetic information of a particular organism. Chromosomes are made from proteins and a single molecule of deoxyribonucleic acid (DNA). This information contains specific instructions that will be passed on from parents to their children.

What is a chromosome?

A chromosome is a long, thread-like structure that contains molecules of DNA — our genetic information — and protein. Chromosomes are found in every living thing, from humans to plants to bacteria. They determine our physical traits, like eye color, blood type, and reproductive organs. Humans have 23 pairs of chromosomes, which means 46 chromosomes in total. Half are inherited from the parent with ovaries, and half are inherited from the parent with testes.

The 23rd set is made up of X-chromosomes and/or Y chromosomes, called sex chromosomes, which can determine a person’s biological sex. People born with testes typically have an XY chromosome pair, and people born with ovaries typically have an XX chromosome pair. It is also possible to be born with another combination of genes or ambiguous external genitalia, which is a condition known as intersex.

Chromosomes and male fertility

Since chromosomes determine our sexual reproductive development, there is a link between chromosomes and male fertility. However, it is important to note that genetic causes for infertility are much rarer than other causes of male-factor infertility.

Y-chromosome microdeletions are one example of how chromosomes impact male fertility. This is a chromosomal abnormality that entails small deletions of genes on the Y-chromosome within a man's sperm. When a person with testes inherits a chromosome with Y-chromosome microdeletions, they typically have impaired sperm production and are at greater risk of developing azoospermia, a condition where sperm is absent in the semen.

Another example is Klinefelter syndrome, where a person with testes is born with an extra X chromosome (making their 23rd chromosome XXY). Upwards of 95% of patients with Klinefelter syndrome experience infertility.

History of chromosomes

Nineteenth-century scientists Walther Flemming, Theodo Boveri, and Walter Sutton made great strides in genetic research with a series of discoveries involving chromosomes. Previously, Gregor Mendel and Charles Darwin laid the groundwork with genetic theory, the idea that cells could carry traits from one generation to the next.Once chromosomes and their link to heredity were discovered, scientists were able to dig deeper and understand the events that take place in the nucleus, chromosome behavior during cell division, and gamete formation. These findings and more led to the development of the chromosome theory and paved a path towards modern genetics as we know it today.

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