The second half of the nineteenth century was a time of remarkable advances in genetic research: Scientists were able to identify chromosomes under the microscope.
But how did they figure out how important chromosomes really are?
Considerable evidence of what was to happen in biology was already present:
Darwin had delineated the evolution of animal species, Mendel had discovered some basic rules about inheritance and Weissman, Roux, Dries, de Vries and other embryologists were beginning to decipher how an organism develops from a single cell.
Then, over the next few decades, Walther Flemming, Theodor Boveri and Walter Sutton made a series of significant discoveries involving chromosomes and what role they likely play in the transmission of genetic characteristics.
It is following the work of Thomas Hunt Morgan in the early twentieth century, that researchers were finally able to directly link the inheritance of genetic traits to the behaviour of chromosomes, thereby providing concrete evidence for what became known as the chromosome theory of heredity.
The theory stated that inheritance patterns may be generally explained by assuming that genes are located in specific sites of chromosomes.
In 1910, Thomas Hunt Morgan was the one to truly bring Mendel’s Laws and the Chromosome Theory together into a revolutionary idea.
The idea that genes are located on chromosomes was proposed based on experiments using Drosophila melanogaster, or more commonly known as a fruit fly.
Morgan chose fruit flies because they can be cultured easily, are present in large numbers, have a short life cycle and only four pairs of chromosomes that can be easily identified under the microscope.
In Drosophila, normal flies have red eyes. Red eye colour is dominant. Morgan discovered a recessive mutation that caused white eyes.
When he mated a red-eyed female to a white-eyed male, all the progeny had red eyes. This result made perfect sense with a dominant inheritance pattern.
But Morgan got a surprising answer when he made the reciprocal cross, mating white-eyed females to red-eyed males. Instead of all red-eyed progeny, he saw that all the females had red eyes and all the males had white eyes.
From these observations, Morgan conclude that the allele-producing eye colour must lie on the X chromosome that governs sex.
This provided the first correlation between a specific trait and a specific chromosome.
Chromosomes are indeed the physical carriers of hereditary information.
Chromosomes are made of DNA and genes are located within these chromosomes. Genes are locuses on the DNA itself.
DNA contains the instructions for building proteins.
And proteins control the structure and function of all cells that make up your body.
Each human cell has 46 chromosomes. All the DNA is organized into two sets of 23 chromosomes.
We all get genetic material from both of our parents.
Are chromosomes linked to cancer?
German embryologist, Theodor Bovery linked chromosomes and heredity. He was the first to argue that cancer is caused by defects in chromosomes and that these can be caused by a failure of cells to divide properly.
His theory was based on the views that cancer is a cellular problem: cancers originate from a single cell, this cell has an abnormality on its chromosomal constitution and the chromosomal abnormality which is passed on to all descendants of the cell of origin, is the cause of rapid cancerous cell proliferation.
Indeed, when cells multiply, each new cell usually gets an exact copy of all 46 chromosomes.
But in a cancer cell, this genetic copying process often goes out of control. Pieces in the chromosomes are often missing, duplicated or rearranged caused by genomics instabilities.
Breast cancer is the most common cancer that affects women.
There are two major genes whose mutation leads to breast and/or ovarian cancer. They are called BRCA1 and BRCA2. Their acronym stands for: BReast CAncer.
Dr. Mary-Claire King was the first to demonstrate in 1990 that a single gene on chromosome 17 which she named BRCA1 was responsible for breast and ovarian cancer.
BRCA1 and BRCA2 genes are located on chromosome 17 and 13 respectively. Their function is to ensure the stability of the cell’s DNA repair mechanisms.
Yet, when either of these genes suffers a mutation, DNA damage may not be repaired properly and cells are more likely to develop additional genetic alterations resulting in cancer.
Harmful variants in BRCA genes induce breast and/or ovarian cancer and can provoke an early onset, even before 30. This is why in a recent publication Dr. Mary-Claire King has suggested that the current screening guidelines on BRCA1 and 2 should be thoroughly revised worldwide.
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