Fraternal twin in the context of "Doppelgänger"

Hamnet Shakespeare (baptised 2 February 1585 – buried 11 August 1596) was the only son of William Shakespeare and Anne Hathaway, and the fraternal twin of Judith Shakespeare. The twins were probably named after Hamnet Sadler, a baker who witnessed Shakespeare's will, and the baker's wife, Judith. Hamnet died at the age of 11. Some Shakespearean scholars speculate on the relationship between Hamnet and his father's later play Hamlet, as well as on possible connections between Hamnet's death and the writing of King John, Romeo and Juliet, Julius Caesar, and Twelfth Night.

Hamnet found cultural representation in 21st-century works such as Neil Gaiman's comic book The Sandman, the 2018 film All Is True, Maggie O'Farrell's 2020 book Hamnet and its 2025 film adaptation, as well as the comedy drama series Upstart Crow (2016–2020).

Judith Quiney (baptised 2 February 1585 – 9 February 1662), née Shakespeare, was the younger daughter of William Shakespeare and Anne Hathaway and the fraternal twin of their only son, Hamnet Shakespeare. She married Thomas Quiney, a vintner of Stratford-upon-Avon. The circumstances of the marriage, including Quiney's misconduct, may have prompted the rewriting of Shakespeare's will. Thomas was struck out, while Judith's inheritance was attached with provisions to safeguard it from her husband. The bulk of Shakespeare's estate was left, in an elaborate fee tail, to his elder daughter, Susanna, and her male heirs.

Judith and Thomas Quiney had three children. By the time of Judith Quiney's death, she had outlived her children by many years. She has been depicted in several works of fiction as part of an attempt to piece together unknown portions of her father's life.

Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics and in related fields, from biology to psychology. Twin studies are part of the broader methodology used in behavior genetics, which uses all data that are genetically informative – siblings studies, adoption studies, pedigree, etc. These studies have been used to track traits ranging from personal behavior to the presentation of severe mental illnesses such as schizophrenia.

Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: "identical" or monozygotic (MZ) twins share essentially 100% of their genes, which means that most differences between the twins (such as height, susceptibility to boredom, intelligence, depression, etc.) are due to experiences that one twin has but not the other twin. "Fraternal" or dizygotic (DZ) twins share only about 50% of their genes, the same as any other sibling. Twins also share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) because they are born into the same family. The presence of a given genetic or phenotypic trait in only one member of a pair of identical twins (called discordance) provides a powerful window into environmental effects on such a trait.