<P> Mutations in tumour suppressor genes or proto - oncogenes can predispose an individual to developing tumours . These mutations make a person susceptible to tumour development if the other copy of the oncogene is randomly mutated . These mutations can occur in germ cells, allowing them to be heritable . Individuals who inherit germline mutations in tumour suppressor genes are therefore predisposed to certain cancer variants . Examples of this include mutations in the BRCA1 and BRCA2 genes which predispose to breast and ovarian cancer, or mutations in MLH1 which predispose to hereditary non-polyposis colorectal cancer . </P> <P> Trisomy 21 (also known as Down Syndrome) results from a child having 3 copies of chromosome 21 . This chromosome duplication occurs during germ cell formation, when both copies of chromosome 21 end up in the same daughter cell in either the mother or father, and this mutant germ cell participates in fertilization of the zygote . Another, more common way this can occur is during the first cell division event after the formation of the zygote . </P> <P> Cystic Fibrosis is an autosomal recessive disorder that causes a variety of symptoms and complications, the most common of which is a thick mucus lining in lung epithelial tissue due to improper salt exchange, but can also affect the pancreas, intestines, liver, and kidneys . Many bodily processes can be affected due to the hereditary nature of this disease; if the disease is present in the DNA of both the sperm and the egg, then it will be present in essentially every cell and organ in the body; these mutations can occur initially in the germline cells, or be present in all parental cells . The most common mutation seen in this disease is ΔF508, which means a deletion of the amino acid at the 508 position . If both parents have a mutated CFTR (cystic fibrosis transmembrane conductance regulator) protein, then their children have a 25% of inheriting the disease . If a child has 1 mutated copy of CFTR, they will not develop the disease, but will become a carrier of the disease . </P> <P> Many Mendelian disorders stem from dominant point mutations within genes, for example Cystic Fibrosis, B - Thalassemia, Sickle - Cell Anemia, and Tay Sachs Disease . By inducing a double stranded break in sequences surrounding the disease - causing point mutation, a dividing cell can use the non-mutated strand as a template to repair the newly broken DNA strand, getting rid of the disease - causing mutation . Many different genome editing techniques have been used for genome editing, and especially germline mutation editing in germ cells and developing zygotes; however, while these therapies have been extensively studied, their use in human germline editing is limited . </P>

What can happen if there is a mutation to the zygote