A nice question here but it is not correct that if the parents have 4 childrens then one out of them will surely receive, Actually it is frequency or chances that if both the parents have recessive gene then there are 25% chance or 1 out of 4 that offspring will receive, And also it is not entirely true that only dominant genes will express itself solely, there are many cases of co Dominance , And about evolution, during gametogenesis there is a stage in miosis where the crossing over occurs that means a part of arm of chromosomes of homologous non sister chromatids got exchanged causes the variation in chromosome