1.1 Congenital malformationsCongenital malformations are terms used to describe congenital malformations 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 to which the 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 malformations due to multifactorial transmission (George, 2007). Genetic Causes of Birth Defects It has been estimated that 25% of all birth defects are due to genetic factors and of these, 85% of the causes are known (Brent , 2004). Birth defects can be caused by chromosomal aberrations, mutations in single genes, and interactions of environmental and genetic factors. Chromosomal aberrations are frequently observed because complex mechanisms such as meiosis or mitosis are error-prone. Chromosomal aneuploidy (loss or gain of an entire chromosome) is another common causative factor in multiple birth defects. Multiple birth defect syndromes are caused by single genetic abnormalities (Prescott, 2001). Chromosomal abnormalities These 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 be inherited from a parent with the same chromosomal aberration. Numerical abnormalities are due to any deviation from the human diploid number of 46 chromosomes, called chromosomal aneuploidy. This leads to multiple birth defects because it disrupts the action of several genes. Usually, numerical chromosomal abnormalities are the result...... middle of article...... Synthetic lethality states that when two genes have the same role and a mutation occurs in both, the rendering them non-functional, this only results in lethality. Additionally, when both genes have the same function in the pathway, each mutation in the genes decreases the function of the pathway. Even though the sequencing of the mouse and human genomes has been completed, the function of a large portion of the genomes still remains unknown. An effective way to discover the function of genes is to determine the consequences of mutations in organisms. This study focuses on birth defects by investigating mutations in the mouse genome causing developmental abnormalities that serve as models for human birth defects. In this study, an attempt is made to identify genes responsible for developmental defects and lethality, thereby facilitating the study of birth defects..