If a couple (married man and woman) wanted to know what the probability of the traits and characteristics their children would have, eye color, hereditary disease, etc. they would have to analyze each side of their family history. By accounting for what type of trait was common in their family they could create a family pedigree, or tree, and track which traits came from who and if they were dominant or not. Then they could arrange the information on a punnet square after having done intensive research to see what characteristics their children would most likely have.
Some traits and characteristics are not inherited genetically. Sometimes during meiosis and reproduction complications can occur that can negatively affect the zygote, the offspring of the couple. Sometimes instead of the zygote inheriting a normal gamete with one chromosome from each parent the zygote could end up having three chromosomes (instead of two). This occurrence is called Trisomy, categorized under nondisjunction and aneuploidy. Often times a person who has had trisomy occur has Down’s syndrome. There are many other mishaps that are not genetically inherited that can happen in the formation of a zygote that can affect how a person will function.
To conclude, a curious father and mother who want to figure out the probability of what traits their children might be born with may not always be able to predict what characteristic their child will be born with because some things just can’t be predicted.