Vie CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. II

CYTOGENETIC AND REPRODUCTIVE STUDIES
OF XX/XY CHIMERIC TWIN BULLS

H.O0., Dunn and R.M. Kenney, Cornell University, Ithaca, New York
W.H. Stone and Stephanie Bendel, U. of Wis., Madison, Wisconsin

Vascular anastomosis between bovine twin embryos of unlike sex
produces the sterile freemartin heifer and an apparently normal twin
bull, each having a 60,XX:60,XY chimeric karyotype and a chimeric
mixture of red cell antigens (1,2). Freemartins have been extensive-
ly studied, but their male twins have not. These bulls are not
esteemed by breeders and few are allowed to mature (1), presumably
because of the belief that they show poor reproductive performance or
that they transmit undesirable genes. To our knowledge, there is no
documentation that twins of freemartins are less fit than non-twin
bulls. However, XX-spermatogonia have been found in newborn bull -
testis (1) and the presence of XX bivalents in diakinesis (ca. 5-10%)
established the capacity of female germ cells in chimeric bulls to
enter meiosis (3). The present paper reports the cytogenetic, repro-
ductive and serologic studies on five chimeric bulls.

MATERIAL AND METHODS. The five chimeric bulls were located in two
A.I. centers. Semen samples and records were available on four bulls.
Biopsy and necropsy specimens were available on three of them.

Blood and tissue cultures were performed by methods previously
described (4). Bone marrow specimens from the sternum were taken
during anesthesia and metaphase spreads were prepared for chromo-
somal study (5). Testes were obtained at necropsy or while bulls
were anesthetized. Sections of testes were taken for tissue culture,
squash preparations (6,7) and histopathological examinations.

1. Lymphocyte chimerism. The XX:XY ratios of cultured lymphocytes,
estimated by examining a minimum of 50 metaphases for each of the
five bulls, were 85:15, 60:40, 33:67, et Yandiid3: 8 s heneis
centages of XX cells in the five ratios are used as identifying
codes, i.e., B-85, B-60, B-33, B-29, and B-13.

2. Progeny tests for germ cell chimerism. If functional XX-spermato-
gonia are present in the testes in a significant number, the sex
ratio would be affected by increasing the number of female progeny.
Furthermore, X-bearing sperm derived from the donor germ cells would
carry the haploid genome representing the twin sister's genotype.
Therefore, sex ratios were obtained on three samples of single born
calves sired by B-85, B-33 and B-29. The sex ratio was 45 males:55
females in a sample of 266 progeny of B-85 and 52:48 in 126 progeny
of B-29. Neither ratio was significantly different from a 50:50
ratio (P>.05). A report (8) on 48 progeny of bull B-33 gave a ratio
of 29:71 which was a significant deviation (P<.05). Blood samples
from sire B-85 were typed and further analyzed by differential hemo-
lysis tests (2) to determine the proportions of the two antigenic
cell types. It was found that 10-15% of the blood represented

877