Slide showing a pedigree diagram of X and Y chromosones. Slide shows a black and white pedigree diagram of X and Y chromosones. The table is labelled with the following key: ' |x+ = normal X chromosome, |x = X chromosome with xeroderma allelomorph. Yx+ = normal Y chromosome, Yx = Y chromosome with xeroderma allelomorph.' The image is labelled 'Fig. 2.' Late 19th - early 20th century