1.1 Congenital Birth DefectsBirth defects, congenital anomalies are terms used to describe developmental defects present at birth. There are many genetic and environmental interactions in the complex transition from a fertilized egg to a fully formed human being. The developmental stage at which interactions are affected influences the type of birth defects. The causes of birth defects can be divided into two types, genetic and environmental causes. There are also congenital defects caused by multifactorial inheritance (George, 2007). Genetic Causes of Birth Defects It has been estimated that 25% of all birth defects are caused by genetic factors and of these 85% are known (Brent, 2004). Birth defects can be caused by chromosomal aberrations, mutations in individual genes, and interactions of environmental and genetic factors. Chromosome aberrations are frequently observed as complex mechanisms such as meiosis or mitosis are prone to errors. Another frequent causative factor of multiple birth defects is chromosomal aneuploidy (loss or gain of an entire chromosome). Multiple congenital anomaly syndromes are caused by defects in a single gene (Prescott, 2001). Chromosomal anomalies They can be numerical or structural and are important causes of approximately 5-6% of all birth defects (Young, 2005). These abnormalities may occur recently or perhaps be inherited from a parent with the same chromosomal aberration. Numerical anomalies are caused by any deviation from the human diploid number of 46 chromosomes, called chromosomal aneuploidy. This leads to multiple birth defects as it disturbs the action of multiple genes. Usually, numerical chromosomal abnormalities are res... halfway through the paper ......Synthetic lethality indicates that when two genes have the same role and a mutation occurs in both of them making them non-functional, only then lethality occurs . Furthermore, when both genes have the same function in the pathway, each mutation in the genes decreases the function of the pathway. Although the sequencing of the mouse and human genomes has been completed, the function of most genomes is still unknown. An effective way to discover the function of genes is to determine the consequences of mutations in organisms. This study focuses on congenital birth defects by studying mutations in the mouse genome that cause developmental anomalies that serve as models for human congenital birth defects. This study attempts to identify genes that cause developmental defects and lethality, thus aiding the study of congenital birth defects..