When a husband and wife join in one to have children, it is possible to predict the probability of the child having certain characteristics from the father and mother.
How can this be done?
The answer lies in genetics. We humans and any other organism hold a specific DNA. DNA is the hereditary material that we hold. In DNA there are alleles, and alleles are the different types of genetic material that code a specific protein or gene. Each parent contributes one version of each character; a single version is an allele.
Every organism has two alleles, one from the father and the other from the mother. One allele is dominant and the other is recessive, and the organism will exhibit the trait of the dominant allele.
The two alleles for a given character separate from each other during gamete formation and end up in different gametes. A gamete is a reproductive cell having the haploid or half number of chromosomes, and a mature sperm or egg fuse with a gamete of the opposite sex to produce the fertilized egg.
Human gametes contain an X chromosome (egg/mom) or an X or Y chromosome (sperm/dad). If the gamete of the father contains X chromosomes, it will match up with the mother making it XX; this means the child will be a female. If the gamete of the father contains Y chromosomes, it will unite with the mother making it YX; this means the child will be a male.
Now that we know all this, we can move on to Punnett Squares. A Punnett Square is a tabular summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross. They give the correct probabilities for the genotype outcomes of independent crosses where the probability of inheriting copies of each parental allele is independent.
In conclusion, we use Punnett Squares to predict the possible characteristics of the child having a trait from the father or the mother or combined.