"Unexpected" genetic ratios are typically associated with sampling error and an unconscious tendency to bias counts towards one of the phenotypes. Students should be carefully instructed in the proper ways of data collection, including a very thorough explanation of appropriate random sampling. One common problem is the tendency to 'select' individuals for counting rather than counting ALL of the individuals within a given (randomly selected) area. Other factors, of course, can also come into play, such as incomplete fertilization is due to underwatering the cultures and/or less than ideal growing conditions.