Two DNA Profiles, One Body: The Fascinating Science of Chimerism

October, 1953. Northern England.

33-year-old Mrs. McK (a pseudonym) visited a nearby blood clinic with her husband to donate blood for the first time. After donating, they both finished some little work and returned home. By then, the blood donated at the clinic had reached the laboratory for screening tests. As soon as the lab technician received the report and glanced at it, he was shocked! To his surprise, he whispered  – What is this? The same person seems to have two different blood groups flowing through her body! Something biologically impossible.

Preparations for a re-test began.

Thinking there had been an error in the initial report, he drew another sample and set out to test it once more. This time, he proceeded slowly, carrying out each step with focused attention. Yet, the result was the same! In the stillness of the night, strange thoughts began to creep into the lab technician’s mind. He wondered – what mystery lay hidden behind this unusual blood?

To unravel this puzzle, he carefully secured the blood bag and stored it in a safe place. The following day, he personally took it to England’s renowned blood research lab, the “Medical Research Council Blood Group Unit.” At the time, two eminent researchers were working there, Robert Race and Ruth Sanger. After reviewing the reported results and hearing the lab technician’s narrative, they both delved into the mystery.

Setting aside the medical test reports, the researchers chose a different approach. They explored Mrs. McK’s family history to uncover any unusual events from her past. They soon discovered that she had been born a twin, alongside a brother. Tragically, he passed away at the age of three—not due to any accident, but because of a rare illness.

Old medical records revealed that, while still in their mother’s womb, Mrs. McK and her brother underwent a unique Hematopoietic Stem Cell Infusion. This exchange of cells would leave an indelible mark on both of them. The cellular fusion had lasting effects. Just three years after birth, her brother succumbed to a rare condition known as Intrinsic Hemolytic Anemia. Although Mrs. McK was affected, she survived with some physical complications, as her exposure had been less severe. Thus, Mrs. McK became the first documented case of a chimeric human—someone who carried within her the cellular legacy of her twin. Yet at that time, the true causes and mystery of chimerism remained unsolved, adding to the intrigue surrounding her remarkable case.

The First Clue in Solving the Mystery

Nearly fifty years after this incident, in early 2002, Lydia Fairchild was living in a small American town with her husband and two children. One day, a minor dispute created distance between them, and the relationship began to be fragile. Eventually, separation became inevitable. At the time, Lydia was seven months pregnant. Under these difficult circumstances, their marriage came to an end. Lydia was forced to leave her home, seeking refuge with her father.

Some weeks later, Lydia applied for custody of her children in court. In response, the court ordered DNA testing for Lydia and the children she claimed as her own. The results arrived, and Lydia attended her court hearing.

But she was shocked and felt as if the sky had fallen. The DNA test revealed no genetic match between her and her children! Lydia was overwhelmed—how could this possibly be true? Her former husband was equally stunned, yet, hoping to benefit from the case, he kept his surprise carefully in secret.

Naturally, Lydia was found in the role of a false mother in court. She was proven to be a fraud. Yet, her attorney made a heartfelt plea, requesting the court’s patience until the birth of her third child. The court agreed to postpone the verdict, but with one condition—a magistrate would attend Lydia’s delivery, bearing witness to the birth and verifying her DNA against that of the newborn.

At last, the awaited moment arrived. Lydia, enduring intense labor pains, was taken to the operating room. As per the court’s condition, a female magistrate accompanied her. Lydia safely delivered a beautiful baby girl. The newborn’s DNA was immediately tested. After seeing the report, the magistrate was utterly shocked—there was no DNA match between Lydia and this child either!

Dark clouds now loomed over Lydia Fairchild’s fate. She found herself utterly trapped. The accusation? That she had deceived her husband by secretly implanting another couple’s embryo in exchange for money. Consequently, the court ordered her to prepare for imprisonment. Yet, she was given a brief reprieve to allow her newborn child time for healthy development.

Time passed and most people  forgot Lydia Fairchild’s case. But one day, Lydia’s lawyer noticed a small article in the newspaper that caught his attention. It described another mother who, just like Lydia, couldn’t find a DNA match with her own child. Intrigued by this case, a team of researchers decided to investigate and soon got to work. In time, they uncovered the mystery behind this strange phenomenon—the mother was a person with the rare condition known as chimerism.

Chimerism is a remarkable biological phenomenon where tissues from different zygotes converge to create a single organism, known as a chimeric entity. In humans, this means that an individual may possess genetic traits from two or more distinct individuals, resulting in a unique organism that harbors multiple sets of DNA within one body.

This intriguing concept may seem almost surreal—could it really be possible? Indeed, it is! Chimerism is not limited to humans; it also occurs in the plant kingdom, where certain plants and flowers display their own fascinating chimeric traits. This phenomenon highlights the incredible complexities of life, where the boundaries of individuality and unity intertwine in the most unexpected ways.

In typical humans, the DNA profile is uniform across all cells, meaning that the DNA found in a person’s saliva, thyroid gland, and reproductive cells is identical. However, this is not the case for chimeric individuals. For them, both saliva and reproductive cells can reveal two or more distinct DNA profiles. This fascinating characteristic allows chimeric beings to harbor multiple genetic identities within their bodies, showcasing a unique complexity not found in the average human.

Reevaluation of the Case

Let’s revisit the case of Lydia Fairchild. Following a review of the research findings, Lydia’s lawyer promptly filed an appeal, arguing that she, too, was a chimeric individual with two distinct types of DNA. As a result, the court ordered a retest of her DNA. The new analysis confirmed the presence of two different DNA profiles within Lydia’s body, which matched perfectly with the DNA of her children, her mother, and even her grandmother. This established the validity of her claims to motherhood, leading to her acquittal and the restoration of her parental rights.

One might question why the initial DNA test did not reveal this duality. The first test was conducted using a biological sample from Lydia’s saliva cells, identified as having DNA set A. In contrast, the DNA inherited by her children originated from Lydia’s germ cells, classified as DNA set B. Because Lydia is a chimeric individual, these two types of cells exhibited different DNA profiles.

Causes of Chimerism

• Chimerism can occur for various reasons, but one of the most common involves twin embryos. When two zygotes develop close together in the womb, sometimes one zygote partially or fully absorbs the other. Partial absorption can lead to a situation where the absorbed zygote is incomplete, resulting in the birth of a vanishing or underdeveloped twin, as seen in the case of Miss Mac and her brother.

• In cases where one zygote fully absorbs the other, only one baby is born. However, this baby will carry the genetic material from both original zygotes, inheriting traits from each. This was the case for Lydia Fairchild, who carried the DNA of her absorbed twin. Thus, chimeric individuals can emerge when distinct zygotes merge during the earliest stages of development.

There are four types of chimerism

• Twin Chimerism

This type of chimerism typically results from one zygote partially or completely absorbing another during the early embryonic stage. In such cases, a twin chimeric baby is born with two sets of DNA—one from their own genetic material and another from their absorbed sibling.

• Tetra-Gametic Chimerism

Under normal circumstances, a single sperm fertilizes a single egg, forming one zygote. However, if two sperm fertilize two separate eggs, which later fuse into a single embryo, tetra-gametic chimerism occurs. The resulting child has a prominent presence of two distinct sets of DNA. 

• Microchimerism

While in the mother’s womb, a fetus may unknowingly absorb some cells from the mother, or vice versa, where the fetus absorbs some of the mother’s cells. These cells can circulate through the bloodstream, potentially settling in an organ permanently, leading to a microchimeric condition.

• Artificial Chimerism

Artificial chimerism can occur during complex medical procedures like blood transfusions, stem cell transplants, or bone marrow transplants. In these cases, some cells from a donor may remain in the recipient’s body and continue to grow, leading to the presence of two DNA sets within one individual. However, with modern medical advancements, this type of chimerism is less common and typically only affects the blood, without influencing other physical traits.

Chimerism in Plants

In plants, chimerism doesn’t occur through the absorption of different zygotes, as in animals. Instead, chimeric plants develop when different types of tissues form from a single type of zygote, often due to mutations. Additionally, grafting—a common technique where different plant sections are combined to produce unique flowers or fruits—is an example of creating an artificial chimeric plant.

Distinctive Traits of Chimeric Individuals

Chimeric individuals often exhibit a fascinating blend of characteristics, such as:

Dual DNA Profiles: The presence of two unique sets of DNA in one body.

Mixed Blood Types: Two types of blood flow coexisting within the bloodstream.

Heterochromia: Eyes that display two different colors, creating a striking visual contrast.

Varied Skin Tones: Skin with subtle variations in color across different areas.

Dual Immune Systems: Two independent immune systems working within the same body.

Asymmetrical Shoulders: One shoulder may sit slightly higher or lower than the other, adding to the physical uniqueness.

Varied Effects of Chimerism

Chimeric individuals possess a remarkable uniqueness but may also experience certain structural and physiological irregularities, including: 

• Severe headaches
• A predisposition to obesity
• Physical fatigue or weakness
• Irregular menstrual cycles in women

In cases where one zygote fully absorbs another:

• If the twin zygotes in the womb are of the same sex, the child will share that sex but contain two distinct tissue types.

• If the absorbed zygotes are of opposite sexes, the child may develop as a hermaphroditic chimeric, with a combination of both male and female characteristics, creating a truly rare and distinctive profile.

The History Behind the Name

In ancient Greek mythology, the Chimera is a legendary creature that embodies the essence of multiple beings—its head and mane are those of a lion, its body resembles a goat, its legs are akin to those of a bull, and its tail takes the form of a serpent. This mythical beast, a fusion of various animal traits, symbolizes the merging of diverse life forces within a single body.

A similar concept is found in Indian mythology, where the figure of Narasimha—a lion-headed human—blends the traits of both man and lion. It is from these powerful mythological beings that the term “Chimerism” originated, representing the union of different forms or identities within a single entity.

Chimerism is a captivating mystery of creation, one that cannot be completely removed from the human body. However, experts believe that by helping patients develop a positive outlook on their condition and providing regular treatment to manage symptoms, their lives can become easier, more vibrant, and more comfortable.

References:

1. Lydia Fairchild- Alchetron
2. The Chimera: The Hybrid Monster of Greek Mythology – Mythology Source
3. Human and Non-Human Chimeras – Stanford Encyclopedia of Philosophy
4. The Case of Lydia Fairchild and Her Chimerism (2002) – Embryo Project Encyclopedia
5. Human Chimerism: A Guide to Becoming Two People at Once – Interesting Engineering
6. Chimerism in the Immunohematology Laboratory in the Molecular Biology Era – Transfusion Medicine Reviews
7. Natural History of Mixed Chimerism After Bone Marrow Transplantation with CD6-Depleted Allogeneic Marrow: A Stable Equilibrium – Science Direct.