Geneticists are Partial to Chimeras

In 2002, Lydia Fairchild had recently separated with her now ex-partner Jamie Townsend, the father of Lydia’s two children and the third she was currently pregnant with. She applied for welfare support and provided the required DNA evidence, but the results she received were surprising to say the least. DNA testing had proven that Jamie was indeed the father; but they also claimed that Lydia wasn’t the mother.

Lydia was taken to court for fraud and suspicion of being involved in a surrogacy scam, and the judge ordered a witness to be present at the birth of Lydia’s third child to ensure the proper blood samples were taken from both Fairchild and her newborn infant. Two weeks later, Lydia gave birth to her third child, and DNA tests yet again indicated that she was not the mother, despite the witness having observed the birth.

It was eventually discovered that Lydia was what is known as a chimera, which in genetics is a single organism composed of genetically distinct cells. The effects of such a phenomenon can range anywhere from subtle morphological variations to multiple blood types to even gonads from both sexes. For Lydia, it meant that different organs in her body could have different genomes; unfortunately for her, the genome found in her children was not found in the organs she was initially tested at. Her name was eventually cleared when a cervical smear test found a match for her children’s DNA.

Lydia had a specific type of chimerism known as tetragametic chimerism, in which two separate ova are fertilized by two separate sperm, creating two genetically unique fertilized zygotes that proceeded to fuse at the blastocyst or zygote stage. In other words, two nonidentical twins are fused into one individual with intermingling cell lines. As this chimera zygote develops, certain organs can possess different sets of chromosomes as they did in Lydia’s case.

There are other types of chimeras found in animals, some of which are specific to certain organisms or groups of organisms.

  • Hemopoietic twin chimerism, in which connections between the blood-vessels of nonidentical twins in the womb allows some blood to flow between them, allowing for two blood types to exist in the same individual. This type of chimerism is actually not too uncommon in nonidentical human twins – a study of 600 twin pairs found that almost half exhibited this phenomenon.
  • Parasitic chimerism, found in Ceratioid anglerfish, is a natural and essential part of the organism’s life cycle. Males begin their search for females early on in their lives, and upon finding one bites into her skin and releases enzymes that digests skin of both individuals near the bite, fusing their circulatory systems. Once fused, the male will reach sexual maturity and develop large testes; at the same time, the male’s other organs atrophy and it is consumed by the female, creating a single hermaphroditic individual with a constant sperm supply for the eggs.
  • Germline chimerism, found in marmosets, occurs when an organism’s germ cells are genetically different from the rest of the organism. This is extremely common in marmosets (who frequently give birth to fraternal twins), where placental fusion during development allows individuals to carry the reproductive cells of their fraternal twin siblings.

About Mr. Mohn

Biology Teacher

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