Final answer:
Familial breast cancer is dominant in a pedigree because one mutated gene, like BRCA1 or BRCA2, is enough to increase cancer risk, though at the cellular level the mutations are recessive. Incomplete penetrance and variable expressivity also affect whether an individual will develop cancer. Gene mutation prevalence in ethnic groups can result from the founder effect and shared ancestry.
Step-by-step explanation:
Familial breast cancer shows a dominant pattern of inheritance in a pedigree because a single mutated copy of a gene, such as BRCA1 or BRCA2, is sufficient to increase a person's risk significantly. Although these mutations are dominant on the organismal level, at the cellular level, they are recessive; both copies of a tumor suppressor gene must be mutated or lost for a cell to proceed to cancer, which is known as the two-hit hypothesis. However, having one mutated gene greatly increases the likelihood of losing the second copy, leading to a dominant inheritance pattern in families.
This does not mean that everyone who inherits the mutation will develop breast cancer. There is incomplete penetrance and variable expressivity, which means some individuals with the mutation may never develop cancer, and the severity of the disease can vary widely among those who do. Additionally, environmental factors and other genetic variations can influence the likelihood of cancer developing, even among carriers of BRCA gene mutations.
It is possible that Rebecca's mother does not have one of the BRCA gene mutations even if her grandmother, aunt, and uncle did. This can occur if the mutation is not passed down through every generation, or if the mother inherited a non-mutated copy from her parents. In this situation, the absence of cancer in Rebecca's mother might be due to her not carrying the mutation rather than an environmental factor or a lack of penetrance. As for the prevalence of certain gene mutations in specific ethnic groups, it is often due to the founder effect or genetic drift, where certain alleles became more common in a population isolated from others, or simply through inheritance from a common ancestor who carried the mutation.