TBX1 is a member of the T-box family of genes and encodes for a transcription factor involved in a complex and highly regulated genetic pathway especially active in early vertebrate development. This specific pathway governs the embryonic pharyngeal system that contributes to the formation and morphogenesis of many different structures including the heart, certain endocrine glands, facial bones, and structures of the inner ear . The exact function of the protein product of the gene is not entirely known, but it is evident that its proper functioning is vital to normal development as the absence of just one of the copies of the gene leads to a range of physical and mental abnormalities. Due to the wide array of symptoms, multiple different disorders are thought to be caused by deletion at this gene including DiGeorge syndrome and velocardiofacial syndrome, and are now grouped together under the name 22q11 deletion syndrome (22q11DS) after the location of the mutation on the q arm of the 22nd chromosome. Although 22q11DS is caused by the deletion of a 1.5-3 Mbp region that contains multiple genes, current research indicates the TBX1 is a major genetic determinant of the disorder. 22q11DS impacts approximately 1 in 4000 live births and its symptoms include anomalies in the cardiac outflow tract, hypoplasia of the thymus and parathyroid, cleft palate and other facial dysmorphogenesis, and certain learning and psychiatric disorders .
Araucana are a breed of domestic chicken originally sought after for their blue-shelled eggs, unique ear tufts and short tails. Analysis of the Araucana genome has shown that individuals that have ear tufts are heterozygous for a mutation in a .58 Mbp region on the 15th chromosome, which contains the chicken TBX1 gene which is homologous to the human version. Chickens that possess this mutation have a relatively high mortality rate in their eggs as well as directly after hatching, indicating that there are developmental issues associated with this breed . The aim of my investigation was to analyze the heart and facial structures of Araucana chickens that died before hatching using histology. By sectioning the tissue into thin slices that could be stained on microscope slides and photographed, I was able to identify some key abnormalities found in the chickens development that shed some light on the different processes that TBX1 governs.